SRS Labs SR400 User Manual

MODEL SR400

GATED PHOTON COUNTER

Phone: [phone] • Fax: [phone]

email: [e-mail] • www.thinkSRS.com

1290-D Reamwood Avenue

Sunnyvale, California 94089

Copyright © 1987 – 2018 by SRS, Inc.

All Rights Reserved.

Revision 2.7 (11/2018)

Table of Contents

TABLE OF CONTENTS

CONDENSED INFORMATION

Safety and Preparation for Use 3
Specifications 5
Menu Display List 7
Abridged Command List 9

INSTRUMENT OVERVIEW

Signal Inputs / Discriminators 11
Counting 12
Gating 13
Count Modes 14
Outputs 14
Measurement Examples 17

OPERATION

Front Panel Summary 21
Power 21
Liquid Crystal Display 21
Menu Keys 21
Spin Knob 21
Start, Stop, and Reset 21
Instrument Status 22
Counter LED's 22
Inputs 22
Outputs 22
Rear Panel Summary 24
Power Entry Module 24
IEEE-488 Port 24
RS-232 Connector 24
Input 24
Output 24
Front Panel Menus 25
Mode Menu 25
Level Menu 27
Gate Menus 30
Setup Menu 31
Com Menu 31

INSTRUMENT SETUPS

Default Setup / Power On Clear 33
Quick Check 33
2 Photon Pile Up Correction 34
Gated Photon Counting 34
Synchronous or Chopped
Photon Counting 34

REMOTE PROGRAMMING

Communications 37

Command Syntax 37
Status LED's 38
RS-232 Echo 38
Try-out with ASCII Terminal 38
Detailed Command List 39
Mode 39
Levels 40
Gates 41
Front Panel 42
Interface 44
Store/Recall 45
Polled Data 45
Continuous Data 45
Examine Data 46
Status Byte 46
Secondary Status Byte 47
Errors / Data Window 47
Trouble Shooting Interface Problems 47
Common Hardware Problems 47
Common Software Problems 48

RS-232 INTERFACE

Data Communications Equipment 49
Echo 49
Wait Command 49
Termination Sequences 49
Example Programs 49
Microsoft BASIC 50
Microsoft FORTRAN 51

GPIB (IEEE-488) INTERFACE

GPIB Capabilities 53
Serial Polls and Service Requests 53
Example Programs 53
Microsoft BASIC 54
Microsoft C 56

TROUBLESHOOTING

LCD Contrast 59
Fan 59
Front Panel Test 59
Power On Reset 59
Quick Test 59
Counting 60

CALIBRATION AND REPAIR

Signal Input Offset 61
Signal Input Repair 61

1

Table of Contents

USING PHOTOMULTIPLIER TUBES

PMT Selection for Photon Counting 63
Geometry 63
Spectral Response 63
Gain and Risetime 63
Dark Counts 64
PMT Base Design 64
Dynode Biasing 64
Snubbing 66
PMT Base Conclusions 67
Cathode Shielding 67
Preamplifiers 67
Gain Requirement 67
Plateauing a PMT 67
The 'Correct' Way 68
The 'Fast and Pretty Good' Way 68

TYPICAL EXPERIMENT

Energy Levels of Ruby 69
Apparatus 69
Operation 69
Instrument Configuration 70
Computer Data Acquisition 70

CIRCUIT DESCRIPTION

Microprocessor Control 73
Microprocessor System 73
RS-232 Interface 73
GPIB Interfac e 73
Input Ports 73
Slow Counters 73
Output Ports 73
Front Panel 73
D/A Converter 74
Fast Counters 74
Signal Inputs 74

Discriminators 74
A and B Counters 74
T Counter 75
Start / Stop and Dwell 75
Gate Generators 76
Digital Delay 76
Analog Delay 76
Delay Reset 77
Gates 77
Power Supply 77

PARTS LISTS

Main Assembly 79
Power Supply 93
Miscellaneous 95

SCHEMATIC DIAGRAMS

Signal Amplifiers SR400-1
Discriminators SR400-2
Fast Counters A and B Prescallers SR400-3
Fast Counters T Prescaller SR400-4
Slow Counters (A, B, T) SR400-5
Dwell Timer: Start/Stop SR400-6
Fast Gate Delay Ramps SR400-7
Slow Gate Delay Counters SR400-8
Fast Gate Ramps SR400-9
Width Oscillators SR400-10
Slow Gate Width Counters SR400-11
Microprocessor System SR400-12
Counters and Timers:
Digital Interface 1 SR400-13
Output Ports:
Digital Interface 2 SR400-14
Digital to Analog Converter SR400-15
Power Supply

2

Safety and Preparation for Use

SAFETY AND PREPARATION FOR USE

**** CAUTION ****

This instrument may be damaged if operated with
the LINE VOLTAGE SELECTOR set for the
wrong ac line voltage or if the wrong fuse is
installed.

LINE VOLTAGE SELECTION

The SR400 operates from a 100V, 120V, 220V, or
240V nominal ac power source having a line
frequency of 50 or 60 Hz. Before connecting the
power cord to a power source, verify that the
LINE VOLTAGE SELECTOR card, located in the
rear panel fuse holder, is set so that the correct ac
input voltage value is visible.

Conversion to other ac input voltages requires a
change in the fuse holder voltage card position and
fuse value. Disconnect the power cord, open the
fuse holder cover door and rotate the fuse-pull
lever to remove the fuse. Remove the small
printed circuit board and select the operating
voltage by orienting the printed circuit board to
position the desired voltage to be visible when
pushed firmly into its slot. Rotate the fuse-pull
lever back into its normal position and insert the
correct fuse into the fuse holder.

LINE FUSE

Use 1 Amp slow blow fuse for 100V/120V as well
as 220V/240V.

LINE CORD

The SR400 has a detachable, three-wire power
cord for connection to the power source and to a
protective ground. The exposed metal parts of the
instrument are connected to the outlet ground to
protect against electrical shock. Always use an
outlet which has a properly connected protective
ground.

WARNING REGARDING USE WITH
PHOTOMULTIPLIERS

The signal inputs may be damaged if a
photomultiplier is used improperly with the photon
counter. When left completely unterminated, a
PMT will charge a cable to a few hundred Volts in
a very short time. If this cable is connected to a
signal input, the stored charge may damage the
front-end transistors. To avoid this problem,

provide a leakage path of about 100 KΩ to ground

inside the base of the PMT to prevent charge
accumulation.

CONNECTION TO OTHER INSTRUMENTS

All front panel BNC shields are connected to the
chassis ground and to the power outlet ground via
the power cord. Do not apply any voltage to either
the shields or to the outputs. The outputs are not
protected against connection to any potential other
than ground.

3

SRS Symbols

Symbols that may be found on SRS products

4

SR400 Specifications

SR400 SPECIFICATIONS

OVERVIEW

There are three complete counters identified as A,
B, and T. All counters count up to 200 MHz.

Counter A can count INPUT 1 or the 10 MHz
internal clock. Counter A is gated by AGATE, and
has a count capacity of 10

9

.

Counter B can count INPUT 1 or INPUT 2.
Counter B is gated by BGATE, has a count
capacity of 10

9

and may be used as the preset

counter to determine the count interval.

Counter T can count the internal 10 MHz clock,
INPUT 2, or the gate trigger. This counter may be
preset to determine the count period for up to 25
hours or 9 x 10

11

counts, gates, or triggers. The

timer has an accuracy or 25 ppm from 0 to 50∞C.

SIGNAL INPUTS

Bandwidth: dc to 300 MHz
Input impedance: 50 Ohms
Linear range: ± 300 mV (at input)
Input protection: ± 5 Vdc, 50 V for 1 µs
Overload recovery: 5 ns for <10 µs duration

overload

DISCRIMINATORS

Counters A, B, and T have independent
discriminators when counting the signal inputs.
All discriminator levels may be set to a fixed level
or scanned. A rear panel INHIBIT input (TTL
active high) can inhibit the discriminators to stop
the count.

Referenced to the signal inputs:
Discriminator range: -300 mV to +300 mV.
Discriminator slope: Rising or Falling
Resolution: 0.2 mV
Input offset voltage: 1 mV
Minimum pulse input: 10 mV
Pulse pair resolution: 5 ns.
DISC outputs: NIM levels into 50 Ohms

TRIGGER INPUT

Impedance: 10 KOhms
Threshold: ±2.000 Vdc in 1 mV steps
Slope: Rising or Falling

Protection: 15 Vdc, 100 V for 1 µs

GATE GENERATORS

There are two independent gates, AGATE and
BGATE, which enable the A and B counters. Both
gates are triggered by the TRIGGER input. The
gates may be fixed in time or scanned. The GATE
outputs show the positions of the gates with
respect to the discriminator outpu ts.

Insertion delay: 25 ns
Maximum delay: 999.2 ms
Minimum gate width: 5 ns
Maximum gate width: 999.2 ms or CW
Resolution: 0.1%, 1 ns minimum
Accuracy: 2 ns +1%
Jitter: 200 ps rms +100 ppm
Maximum trigger rate:1 MHz
GATE view outputs: NIM levels into 50 Ohms
GATE view error: < 2 ns

COUNTING MODES

A and B for a period determined by T preSET and
T input.
A ± B for a period determined by T preSET and T
input.
A for a period determined by B preSET and B
input.

SCAN AND DWELL

The number of count PERIODS or data points in a
scan may be set from 1 to 2000. The duration of
one count period is determined by the preset
condition.

The time between consecutive count periods is the
DWELL TIME and can be set from 2 ms to 60 s.
The DWELL output will be TTL high during the
DWELL time. This output can be used to trigger
external devices.

At the end of a scan (of 1 to 2000 count periods)
counting may be programmed to STOP or START
the scan over again.

The START key begins the first count period of
the programmed scan.

5

SR400 Specifications

The STOP key terminates the current count period
and pauses the scan. If scanning, gates and disc
levels are held at their last value. The STOP key
pressed while in a paused condition will reset the
scan and all scanned parameters will return to their
start values. The START key pressed while paused
resumes the scan by starting the next count period.

The DWELL TIME may also be set to
EXTERNAL. In this mode, count periods begin
with the START key or EXTERNAL START
INPUT (TTL rising edge). Count periods
terminate with the preset condition, the STOP key,
or the EXTERNAL STOP INPUT (TTL rising
edge). A STOP key while not counting resets the
scan. This allows completely variable dwell times
and/or counting periods.

All count data is internally buffered for one scan.
Data may be read over the computer interfaces
during or after a scan.

DISPLAY MODE

CONTINUOUS: Displays current counter value
HOLD: Displays final count value

D/A OUTPUT

The front panel D/A Output is proportional to A,
B, A-B, or A+B depending upon the counting
mode selected. The D/A output is updated at the
end of each count period.

There are two rear panel D/A outputs, PORT 1 and
PORT 2. These outputs may be set or scanned
from the front panel or via the computer interface.

Full scale: ± 10 Vdc
Resolution: 12 bits (5 mV)
Current rating: 10 mA
Output Impedance: < 1 Ohm
Accuracy: 0.1% + 5 mV

COMPUTER INTERFACE

IEEE-488 (GPIB) and RS-232 (up to 19.2 kbaud).
Full instrument control and data transmission.

GENERAL

Dimensions: 16" x 13" x 3.5"
Weight: 10 lbs
Power: 35 Watts from 100, 120,

220, or 240 Vac

Warranty: One year parts and labor

on materials and
workmanship

6

Menu Display List

MENU DISPLAY LIST

MODE

COUNT=A,B FOR T PRESET
A=INPUT 1
B=INPUT 2 B SET=1E3
T=10MHZ T SET=1E0 s
N PERIODS=2000 [at 1234]
AT N=STOP DWELL=1E0s
D/A=A
D/A RANGE=LOG
DISPLAY=CONTINUOUS

LEVEL

TRIG SLOPE=RISE
TRIG LVL=+2.000 V
A DISC SLOPE=FALL
A DISC=SCAN ∆=-1.0 mV
A DISC LVL=-10.0 mV
B DISC SLOPE=FALL
B DISC=SCAN ∆=-1.0 mV
B DISC LVL=-10.0 mV
T DISC SLOPE=FALL
T DISC=SCAN ∆=-1.0 mV
T DISC LVL=-10.0 mV
PORT1=SCAN ∆=+0.100 V
PORT1 LVL=+0.500 V
PORT2=SCAN ∆=+0.100 V
PORT2 LVL=+0.500 V

A GATE

AGATE=SCAN ∆=1.000 µs
A DELAY=1.000 µs
A WIDTH=1.000 µs

SETUP

LCD CONTRAST=12
STORE=3 EXECUTE=>
RECALL=3 EXECUTE=>

B GATE

BGATE=SCAN ∆=1.000 µs
B DELAY=1.000 µs
B WIDTH=1.000 µs

COM

GPIB ADDR=23
RS-232 BAUD=9600
RS-232 BITS=8
RS-232 PARITY=NONE
RS-232 WAIT=6
RS-232 ECHO=OFF
DATA=

7

Menu Display List

8

Abridged Command List

ABRIDGED COMMAND LIST

The parameter i is 0,1,or 2 to select counter A,B, or T

MODE

CM j Set COUNT to mode j; A,B(0), A-B(1), A+B(2) for preset T,or A FOR B preset(3).
CI i,j Set counter i to input j; 10 MHz(0), INP 1(1), INP 2(2), TRIG(3).
CP i, n Set counter i preSET to 1 <= n <= 9E11.
NP m Set Number of PERIODS in a scan to 1 <= m <= 2000.
NN Read current count period number or scan position.
NE j Set end of scan mode to mode j; START(1) or STOP (0).
DT x Set DWELL time to 2E-3 <= x <= 6E1 s or EXTERNAL(0).
AS j Set D/A to source j; A(0), B(1), A-B(2), A+B(3).
AM j Set front panel D/A to RANGE j; LOG(0) or LINEAR (1-7).
SD j Set DISPLAY to mode j; CONTINUOUS(0) or HOLD(1).

LEVELS

TS j Set TRIG to SLOPE j; RISE(0) or FALL(1).
TL v Set TRIG LVL to -2.000 <= v <= 2.000 V.
DS i,j Set DISC i to SLOPE j; RISE(0) or FALL(1).
DM i,j Set DISC i to mode j; FIXED(0) or SCAN(1).
DY i,v Set DISC i scan step to -0.0200 <= v <= 0.0200 V.
DL i,v Set DISC i LVL to -0.3000 <= v <= 0.3000 V.
DZ i Read current DISC i LVL (during scan).
PM k, j Set PORT k (1 or 2) to mode j; FIXED(0) or SCAN(1).
PY k,v Set PORT k (1 or 2) scan step to -0.500 <= v <= 0.500 V.
PL k,v Set PORT k (1 or 2) LVL to -10.000 <= v <= 10.000 V.
PZ k Read current PORT k (1 or 2) LVL (during scan).

GATES

GM i,j Set GATE i to mode j; CW(0), FIXED(1), or SCAN(2).
GY i,t Set GATE i DELAY scan step to 0 <= t <= 99.92E-3 s.
GD i,t Set GATE i DELAY to 0 <= t <= 999.2E-3 s.
GZ i Read current GATE i DELAY position (during a scan).
GW i,t Set GATE i WIDTH to 0.005E-6 <= t <= 999.2E-3 s.

FRONT PANEL

CS Count start, same as START key.
CH Count pause, same as STOP key while counting.
CR Count reset, same as STOP key pressed twice.
CK j Simulate key press j.
RR Rotate knob right (CW or UP) 1 step.
RL Rotate knob left (CCW or DOWN) 1 step.
SC Read cursor position. Left(0), right(1), or inactive(2).
MI j Set front panel to mode j; local(0), remote(1), locked-out(2). RS-232 only.
MS string Display string on menu line.
MS Return menu line to normal display.
MD j,k Display line k of menu j.
MM Read menu number of display (j).
ML Read menu line of display (k).

9

Abridged Command List

STORE/RECALL

ST m Store instrument settings to location m (1 to 9).
RC m Recall instrument settings from location m (1 to 9).
RC 0 Recall default settings.

INTERFACE

CL Clear instrument.
SV m Set GPIB SRQ mask to 0 <= m <= 255.
SS Read status byte.
SS j Read bit j(0-7) of status byte.
SI Read secondary status byte.
SI j Read bit j (0-2) of secondary status byte.
SW m Set RS-232 character wait interval to m*3.33 ms 0 <= m <= 25. RS-232 only.
SE j,k,l,m Set RS-232 terminator sequence to j,k,l,m (ASCII codes). RS-232 only.
SE Clear RS-232 terminator sequence to defaults. RS-232 only.

DATA

QA Read last count in counter A.
QB Read last count in counter B.
QA m Read from scan buffer point m(1-2000) for counter A.
QB m Read from scan buffer point m(1-2000) for counter B.

EA Send entire counter A buffer.
EB Send entire counter B buffer.
ET Send entire counter A and B buffer.

FA Start scan and send N PERIODS data points from counter A.
FB Start scan and send N PERIODS data points from counter B.
FT Start scan and send N PERIODS data points from both counters.

XA Read current contents of counter A.
XB Read current contents of counter B.

STATUS BYTE

Bit Description

0 Parameter changed
1 Data ready
2 Scan finished
3 Counter overflow
4 Rate error
5 Recall error
6 Service request
7 Command error

SECONDARY STATUS BYTE

Bit Description

0 Triggered
1 Inhibited
2 Counting
3-7 Unused

10

Instrument Overview

INSTRUMENT OVERVIEW

SR400 OVERVIEW

The SR400 architecture is diagrammed above.
There are three fast counters identified as counters
A, B, and T. All three counters operate at rates up
to 200 MHz. The input to each counter is selected
from a number of sources, including the two
analog signal inputs, the crystal timebase, and the
external trigger input. Counter T is presettable and
determines the measurement period. For pulsed
experiments, counters A and B may also be
synchronized to external events via the two
independent gate generators. The gate generator
provides gates from 5 ns to 1 s in duration with a
delay from external trigger ranging from 25 ns
to 1 s.

SIGNAL INPUTS AND DISCRIMINATORS

There are two independent analog signal inputs
labelled INPUT 1 and INPUT 2. They are

internally terminated into 50 Ω. The inputs can

accept signals of either polarity up to ±300 mV
and are protected to ±5V dc. Each input is
followed by a dc to 300 MHz amplifier. This
allows detection of pulses as s mall as 10 mV. If
greater sensitivity is required, the SR445A remote
amplifier is available. The SR445A can provide
gains from 5 to 625 at bandwidths up to 350 MHz.

There are three discriminators, one for each
counter. Each discriminator has selectable slope
and a threshold programmable from -300 mV to
+300 mV in 0.2 mV steps (referenced to the
inputs). Pulse-pair resolution is 5 ns and pulses of
either polarity may be detected. Each threshold

11

Instrument Overview

may be programmed to scan in either direction
with selectable step size.

COUNTING

Each counter's input may be selected from a
number of sources. All multiplexing of counter
inputs is done internally to provide accurate timing
and ease of operation. Input cables should rarely
need to be swapped or disconnected to change
measurement modes.

The actual inputs to the counters can be viewed as
NIM level pulses from the DISC outputs on the
front panel. The DISC pulses are negative going
from 0 to -0.7 V. The falling edge is the active or
counted edge regardless of the discriminator slope
setting. The DISC outputs are very useful when
adjusting discriminator thresholds or gate timing.

When the START key is pressed, a START
command is received from the computer interface,
or the EXTERNAL START input is pulsed,
counter T is enabled to count. The COUNT
PERIOD begins with the first pulse from T
counter's input after START. The COUNT
PERIOD enables counters A and B to count their
respective inputs. If the A GATE and B GATE are
CW or continuous, all pulses that occur during the
COUNT PERIOD are counted. If the gates are
enabled, only the pulses that occur during those
gates which fall within the COUNT PERIOD are
counted.

Counter T is presettable from 1 to 9E11. When
counter T reaches the preset count, the COUNT
PERIOD is terminated and counting is halted on
all counters. When the input to counter T is the 10
MHz crystal timebase, the COUNT PERIOD is a
fixed time interval from 100 ns to 25 hours. When
the input is INPUT 2, the COUNT PERIOD is
externally determined and can be as short as 5 ns
and indefinitely long. This mode is used for source
compensation or reciprocal measurements. If the
input is TRIGGER, the COUNT PERIOD is a
fixed number of gates (each trigger provides one
gate). This results in a constant gate aperture time
during each COUNT PERIOD. N+1 Triggers are
required for N gates per period. If the input is B
PRESET, then counter T counts the B DISC

output gated by B GATE. This is for gated ratio or
reciprocal counting.

The SR400 may be programmed to cycle through
1 to 2000 COUNT PERIODS in a single scan. At
the end of the programmed scan, the counters may
be stopped or the scan may be restarted. If the end
of scan mode is STOP, then after the last COUNT
PERIOD in the scan, the counters are halted
indefinitely. A counter RESET is required to
rearm the counters and a START command starts
another scan. If the Number of PERIODS in the
scan is 1, then each START command results in
one COUNT PERIOD. If N PERIODS is greater
than 1, then consecutive COUNT PERIODS are
separated by the DWELL time. The DWELL time
may be set from 2 ms to 60 s. During the DWELL
time, counting is disabled and data may be
transferred or external parameters scanned. The
DWELL OUTPUT is a TTL output which is high
during the DWELL time. At the end of the
DWELL time, the next COUNT PERIOD begins
and the scan continues until N PERIODS have
been measured. If the end of scan mode is
START, one DWELL time separates the last
COUNT PERIOD of the completed scan and the
first COUNT PERIOD of the next scan.

Another DWELL mode is EXTERNAL. In this
mode, the EXTERNAL START input enables
counter T. The COUNT PERIOD begins with the
first pulse to counter T after START. Counting
then proceeds normally until the preset is reached.
At the end of the preset COUNT PERIOD, the
DWELL time is indefinite and the next COUNT
PERIOD starts with t he n ext EXTE RNAL ST ART
pulse. In this way, preset COUNT PERIODS may
be synchronized to external events. If externally
determined COUNT PERIODS are desired, then
the preset COUNT P ERIOD can be set to a value
much larger than actually required and the
EXTERNAL STOP input used to terminate the
COUNT PERIOD. The COUNT PERIOD then
starts with the first pulse to counter T after
EXTERNAL START and ends with EXTERNAL
STOP. The next EXTERNAL START be gins the
next COUNT PERIOD in the scan. When the scan
is finished and the scan end mode is STOP, a
RESET from the front panel or computer interface
is required to reset the scan. If the scan end mode

12

Instrument Overview

is START, the next EXTERNAL START begins
the first COUNT PERIOD of the next scan.

SCANNING

In all scan modes, a number of parameters may be
scanned. These parameters are the three
discriminator thresholds (Pulse Height Analysis),
the two gate delays (Boxcar mode), and the two
D/A output ports (X-axis of recorder, scope, or
analog control of other apparatus).

After each COUNT PERIOD, each scanning
parameter is adjusted by one step. All changes are
made during the DWELL time so that all values
are stable during the COUNT PERIODS. The scan
limits are determined by the start position, the step
size, and the number of PERIODS in a scan. The
range of a scan is N PERIODS times the step size.
When the counters are reset, all scanned
parameters return to their start positions.

GATING

The gate generators may be disabled by setting the
gate modes to CW. In this mode, the gates are
continuously open and counting is enabled solely
by the COUNT PERIOD. This is appropriate for
CW sources or long counting intervals. If the
signal is pulsed or so weak that synchronous (lock­in) measurement is required, then the gate
generators are required. The gates prevent pulses
that occur outside the time interval of interest from
being counted and improve signal to noise ratio.
When the gates are scanned, the time behavior of
the signal may be recovered.

The gates are triggered by the TRIGGER input.
The TRIGGER threshold is adjustable from -2V to
+2V and slope may be either RISE or FALL. The
discriminated trigger pulse may be viewed through
the T DISC output when the input to counter T is
set to TRIG. This can be helpful when setting the
trigger threshold.

The minimum insertion delay from trigger to gate
is 25 ns. The additional delay may be programmed
from 0 ns to 999.2 ms with a resolution of 1 part in
1000 or 1 ns, whichever is greater. The gate width
is adjustable from 5 ns to 999.2 ms with the same
resolution. The two gates are output as NIM pulses

from the GATE outputs. The gates are open when
the outputs are low. The gate outputs have a
timing accuracy with respect to the discriminator
outputs of 2 ns. This allows accurate timing of fast
gates relative to the signal.

The two gates have independent modes, delays,
and widths. This allows for a variety of
measurements. For example:

1) "Boxcar" mode. The gates are used to enable
the counters only when the signal is present. If the
gates are scanned, then the time profile of the
signal is recovered. Background subtraction can be
included by using one gate for the signal and an
equal width gate on the background, either before
or after the signal. The signal gate may be scanned
while the background gate held fixed. Gated
source compensation is also possible since
counters A and B may count different sources at
the same time or the same source at different
times. In the compensation mode, the COUNT
PERIOD is determined by the gated output of the
B discriminator and the COUNT MODE is A FOR
B PRESET.

2) "Lock-in" or "Chopped" mode. In this case, a
cw signal is chopped on and off. This can be
accomplished with a light chopper such as the
SR540. The light chopper reference output is used
to trigger the gates. The A GATE is positioned
during the "open" cycle and counts signal plus
noise. The B GATE is positioned during the
"closed" cycle and only counts the noise. The
counter outputs are subtracted to give only the
signal. The widths of both gates should be equal
and less than half of the chopper period. This
prevents frequency jitter in the chopper from
affecting the overlap of the gates with the open
and close cycles of the chopper.

When counter T is preset and its input is set to
TRIG, then each COUNT PERIOD consists of a
constant number of gates. N+I triggers are needed
for N gates per preiod. This is the usual mode of
gated operation. However, a number of other
modes are available. For example, if the input is
set to 10 MHz, each COUNT PERIOD consists of
the number of gates which occur during the
COUNT PERIOD time interval. If the input is a
signal input, then counting is both gated and

13

Instrument Overview

source compensated. If the DWELL mode is
EXTERNAL, an EXTERNAL START p ulse s tart s
counting and an EXTERNAL STOP pulse stops
counting. The counters are enabled only during
those gates which occurred between the two
EXTERNAL pulses.

COUNT MODES

The length of a COUNT PERIOD is the time it
takes counter T to reach its preset count. If N
the preset count in counter T, and R

and Rt are the

a

is

t

mean pulse rates of the inputs to counters A and T,
then the COUNT PERIOD is

T = N

/ Rt

t

The number of counts in counter A will be

A = R

T = Ra Nt / Rt

a

Rate:

If R

= 10 MHz, then the COUNT PERIOD is a

t

constant amount of time and

A = R

( Nt / Rt ) ~ Ra

a

and counter A measures the mean rate of its input.

Gated:

If R

is the gate trigger rate and counter A is

t

gated, then the COUNT PER IOD is N
"effective" time the counter is enabled is N
where W

is the width of A gate and

a

but the

t/Rt

tWa

A = R

( Nt Wa ) ~ Ra

a

In this case, counter A measures the mean rate
during the A gate.

Source Compensated:

If Rt is INPUT 2, then the COUNT PERIOD is
determined by the pulse rate of INPUT 2 and the T
discriminator. Count A will be

A = N

Ra / Rt ~ Ra / Rt

t

and is proportional to the ratio of the rates of
counters A and T. If INPUT 2 monitors source

intensity, then the COUNT PERIOD will adjust
for fluctuations in the source. Counter A measures
the ratio of its mean input rate and the mean rate
of INPUT 2.

Reciprocal:

If R

is INPUT 2, then the COUNT PERIOD is

t

N

. If Ra is 10 MHz, then counter A measures

t/Rt

A = R

Nt / Rt = ( Ra Nt ) / Rt ~ 1 / Rt

a

and is proportional to the reciprocal of the count
rate of INPUT 2. Not e that ea ch COUNT PE RIOD
is determined by N

counts of INPUT 2, thus the

t

measurement time is determined by the signal
strength. Measurements of both strong and weak
signals are made to the same accuracy for constant
signal to noise ra tio .

Counter B:

Counter B may be used in all of the above modes
except Reciprocal. Since counter B is completely
independent of counter A, 2 sources of counts may
be measured simultaneously. These sources may
be the same signal or 2 different signals; the
sources may be discriminated at separate
thresholds; they may be non-gated or gated; if
gated, the 2 gates may be independently set.
Almost any measurement can be made.

In addition, counter B has a preset mode called A
FOR B PRESET. In this mode, the input to
counter B is sent to counter T. The COUNT
PERIOD is now determined by the B
discriminator and the B gate. If R

is INPUT 1,

a

then counter A is source compensated by the gated
input of counter B. This allows source
compensation of gated signals and sources. If R

a

is
10 MHz, then counter A measures the reciprocal
of the rate of counter B's gated input.

OUTPUTS

The front panel can display counts up to 10

9

- 1.
Counters A and B can be displayed as separate
counters or combined as A-B or A+B. When A
FOR B PRESET is the count mode, then only A
data is displayed.

14

Instrument Overview

The front panel D/A output provides an analog
output proportional to A, B, A-B, or A+B
depending on the counting mode. The scale may
be logarithmic (1V/decade) or linear to any 3
digits.

If a chart recorder is used, the D/A output should
drive the Y axis. The X axis can be recorder
driven (strip chart mode) or the PORT1 or PORT2
outputs can be used to drive the X axis (scans).
This latter method allows accurate determination
of the X value of each point. If the DWELL output
is used as the pen lift, the points will be
unconnected.

If count rates are high and count periods short,
then scans may be displayed on an X-Y scope. The
D/A output is the Y drive and PORT1 or PORT2
is the X drive. The DWELL output should be used
as the blanking pulse.

15

Instrument Overview

16

Measurement Examples

MEASUREMENT EXAMP L ES

Example 1: Internally Triggered Gate Mode

This example, described under the heading “QUICK TEST” on page 59 of this manual, is easy to perform and
will quickly acquaint the user with the basic operation of the SR400.

Example 2: Externally Triggered Gate Mode

In this example, the SR400 is programmed to make 10 counting measurements of a 10 kHz pulse train. For
each measurement, a gate will be delayed 500 ms from an external trigger pulse and will open of 500 ms
duration. Following the gate, there will be a 6 second dwell period* during which the counter will be
disarmed. After the dwell period, the counter will reset to 0 counts, re-arm and repeat the measurement. After
10 measurement cycles, the SR400 will stop counting and disarm. You will need a 10 kHz signal source and a

0.5 Hz square wave generator for this example.

*It is important to note that the dwell period begins at the first external trigger pulse arriving after the
gate is closed. Thus, even though there is only one gate opened for each measurement cycle, a minimum
of two external triggers are needed: one to trigger the gate and another to begin the dwell period.

The following timing diagram illustrates this example.

17

Measurement Examples

1. Turn the SR400 off. Hold down the STOP (RESET) button while turning the SR400 on. Wait three seconds
and release the button.

2. Press the MODE key. Using the four arrow keys and the front panel knob, select the following parameters.
Unless otherwise indicated, leave all other settings at their default values:

T = TRIG TSET = 1E0
N PERIODS = 10 [AT 0]
AT N = STOP DWELL = 6E0 S

3. Press the A GATE key and select the following parameters:

A GATE = FIXED
A DELAY = 500 mS
A width = 500 mS

4. Press the LEVEL key and select the following parameters:

A DISC SLOPE = RISE
A DISC LVL = +150 mV

5. Using a function generator or a pulse generator, apply a 0.5 Hz, 5 V (TTL) signal to the TRIGGER INPUT
on the SR400.

6. Using a second signal source, apply a 10 kHz square wave (or pulse train) with an amplitude of
approximately 500 mV to the SIGNAL INPUT 1 on the SR400.

7. To view the COUNT PERIODS, press the MODE key and use the up/down arrow keys to display “N
PERIODS = 10 [at 0]”

Arm the counter by pressing the START button. (Alternatively, a TTL pulse applied to the START BNC on
the front panel will arm the counter.) Each measurement cycle should result in approximately 5,000 counts.

As an extension of the above example, the SR400 can be programmed to accumulate counts over numerous
gates. This is often done to improve the signal to noise ratio of low light level experiments. To accumulate
counts, set TSET (MODE menu) to the desired number of gates. For example, if you wish to accumulate over
five gates, set TSET – 5E0. In this case, six external triggers are required for each measurement cycle: five to
trigger the five gates and one to start the DWELL PERIOD. Any additional triggers received during the
DWELL PERIOD are ignored.

18

Measurement Examples

Example 3: Externally Triggered Gate Mode

This example is a variation of Example 2, but requires no additional trigger pulses to begin the DWELL
PERIOD. In this example, the SR400 is programmed to make 10 counting measurements of a 10 kHz pulse
train. The counting gates, each 500 ms wide, are delayed 300 ms from external triggers arriving at a 1 Hz rate.
In this case, the SR400 requires only one trigger per COUNT PERIOD, however the trigger configuration is a
bit trickier than the previous example.

The following timing diagram illustrates this example.

To avoid the triggered dwell period, the T counter is logically ANDed with the A gate. This is accomplished
by letting the T counter count the SR400’s internal 10 MHz oscillator signal for a period extending our
beyond the A gate, but not exceeding the trigger repetition period (1 second in this case). Mathematically, this
is expressed:

A DELAY + A WIDTH < TSET < (1/Trigger Rate)

Furthermore, DWELL must be set to EXTERNAL in the MODE menu. This enables the SR400 to reset and
begin each COUNT PERIOD upon receiving a TTL pulse at the START BNC input.

19

Measurement Examples

1. Turn the SR400 off. Hold down the STOP (RESET) button while turning the SR400 on. Wait three seconds
and release the button.

2. Press the MODE key. Using the four arrow keys and the front panel knob, select the following parameters.
Unless otherwise indicated, leave all other settings at their default values:

T = 10 MHz TSET = 9E -1 s
N PERIODS = 10 [AT 0]
AT N = STOP DWELL = EXTERNAL (Turn the knob counterclockwise until “DWELL = EXTERNAL” is
displayed.)

3. Press the A GATE key and select the following parameters:

A GATE = FIXED
A DELAY = 300 mS
A width = 500 mS

4. Press the LEVEL key and select the following parameters:

A DISC SLOPE = RISE
A DISC LVL = +150 mV

5. Using a function generator or a pulse generator, apply a 1 Hz, 5 V (TTL) signal to the EXTERNAL
START input on the SR400. Use a BNC ‘T; connector at the EXTERNAL START input and extend a cable
from the EXTERNAL START input to the TRIG input.

6. Using a second signal source, apply a 10 kHz square wave (or pulse train) with an amplitude of
approximately 500 mV to the SIGNAL INPUT 1 on the SR400.

7. To view the COUNT PERIODS, press the MODE key and use the up/down arrow keys to display “N
PERIODS = 10 [at 0]”

8. Arm the SR400 by pressing the STOP (RESET) button.

Each counting period should result in approximately 5,000 counts. After 10 counting measurements the
SR400 will disarm. To re-arm, press the STOP (RESET) button.

20

Guide to Operation

GUIDE TO OPERATION

FRONT PANEL SUMMARY

POWER BUTTON

The SR400 is turned on by depressing the
POWER button. All instrument settings are stored
in nonvolatile memory and are retained when the
power is turned off. The model, firmware version,
and serial numbers of the unit will be displayed
briefly when the power is turned on. The recall
status is also dipslayed. If recall is OK, then the
instrument returned to the settings in effect when
the power was last turned off. If recall is ERR,
then the stored settings were lost and the default
settings are used. If recall is DEFAULT, then the
instrument is resetting to default conditions. This
is done if the STOP (RESET) key is held down
when the power is turned on. The default setup is
listed in the next chapter.

LIQUID CRYSTAL DISPLAY

The 2 line by 24 character LCD is the user
interface for data display and front panel
programming operations. The top line always
shows count data. The lower line is the menu line
and displays one line of setup information. The
LCD is backlit by an electroluminescent panel for
convenient operation in low light level conditions.
When in the SETUP menu, the contrast of the
LCD may be adjusted for optimum viewing.

MENU KEYS

All operating parameters of the SR400 are
grouped into six function menus. The six menu

keys select the group of functions to be
programmed. The four arrow keys select
individual parameters within each menu. The
selected parameter is displayed on the menu line
of the display.

The MODE menu selects the counting mode,
counter inputs, counter presets, scan mode and
length, dwell time, analog output, and display
mode. The LEVEL menu adjusts the
discriminator, trigger, and D/A levels. The A
GATE and B GATE menus program the gate
generators. The SETUP menu provides STORE
and RECALL of 9 different instrument setups and
adjustment of the LCD contrast. The COM menu
sets up the computer interface parameters and
displays received data. Detailed descriptions of the
menus are provided at the end of this section.

SPIN KNOB

All parameters are altered using the spin knob.
Whenever the menu line has an active or flashing
cursor, the displayed parameter can be adjusted
with the knob.

START, STOP, AND RESET

The two count keys, START and STOP, are used
to start, pause, and reset the counters.

A scan can be started only if the counters are in
the reset condition; all scanned parameters are at
their st art va lu es a nd the c ount er s ar e res et t o zero .

21

Guide to Operation

In general, pressing the STOP key twice in
succession puts the counters into the reset
condition.

START begins the first COUNT PERIOD of the
programmed scan. If the DWELL time is
programmed, then the scan will continue
automatically. A STOP will pause the scan; the
current COUNT PERIOD is terminated and
ignored. A START while paused will resume the
scan. A STOP while paused resets the scan. If the
DWELL time is EXTERNAL, then a STOP during
the COUNT PERIOD terminates the COUNT
PERIOD and advances the scan. START begins
the next COUNT PERIOD i n the scan. A START
during a COUNT PERIOD or programmed
DWELL period is ignored.

A STOP while in a dwell period or pause resets
the scan; all scanned parameters return to their
start values and the counters are reset. In general,
2 consecutive presses of the STOP key will reset
the counters.

The SCAN led is on when a scan is in progress.
The SCAN led is off when the scan is paused or
the counters are reset.

INSTRUMENT STATUS

Six LED's indicate the instrument status. REM is
on when the front panel is locked out, ACT flashes
when there is activity on the computer interfaces,
and ERR flashes when there is a computer
interface error. COUNT is on during each COUNT
PERIOD, OVRN indicates that a counter has
exceeded 10

9

- 1 counts during a COUNT
PERIOD, and RATE flashes whenever a gate
delay or width exceeds the trigger period minus
1 µs. The RATE led also flashes if the trigger rate
exceeds 1 MHz, even if the gates are in the CW
mode.

COUNTER LED'S

The source LED's in the counter sections indicate
what each counter's input is. The counter inputs
are output through the DISC outputs. The SET
LED's indicate which counter, B or T, is the preset
counter. The B SET LED is on when the mode is

A FOR B PRESET, the T SET LED is on
otherwise.

INPUTS

The rising or falling edge of the TRIGGER input
triggers the gate generators. The input impedance
is 10 KΩ and the minimum pulse width is 10 ns.
The trigger level is adjustable from -2V to +2V
with either slope. The minimum pulse amplitude is

0.1 V.

The SIGNAL INPUTS, 1 and 2, are 50 Ω analog

inputs. Photomultiplier or preamplifier outputs are
connected to these inputs. These inputs are linear
between -300 mV to +300 mV and protected to
± 5 Vdc.

EXTERNAL START is a TTL input with the
rising edge active. The minimum pulse width is 20
ns. When the DWELL time is EXTERNAL, the
COUNT PERIOD begins with the rising edge of
START and ends with the preset condition or the
rising edge of STOP. When the DWELL time is
programmed, EXT START begins the scan.

EXTERNAL STOP is a TTL input with the rising
edge active. The minimum pulse width is 20 ns.
When the DWELL time is EXTERNAL, EXT
STOP ends each count period. Reset must be
accomplished via the STOP key or computer
interface. When the DWELL time is programmed,
EXT STOP resets the scan.

OUTPUTS

The DISC outputs are 50 Ω, NIM level, outputs

showing the actual inputs to the three counters.
These outputs are active low (-.7 V) and counting
occurs on the falling edge. The outputs are low
when the signal inputs exceed their threshold
levels with the correct slope.

Cables attached to the DISC outputs should be
kept far from the cables which carry the SIGNAL
inputs. The 0.7 V DISC pulses can be picked up
by the SIGNAL cables and discriminated. This
causes the discriminators to oscillate. This is not a
problem if the threshold is greater than 10 mV. If
smaller thresholds are required, careful
arrangement of the cables is necessary, or the

22

Guide to Operation

DISC output should just be terminated at the front
panel.

The GATE outputs are 50 Ω, NIM level, outputs

showing the actual gates to counters A and B. The
GATE outputs are low (-.7 V) during the gate. The
timing accuracy between the GATE outputs and
DISC outputs is 2 ns.

The D/A analog output provides a -10V to +10V
output proportional to the count data with linear or

logarithmic scaling. Output impedance is < 1 Ω

and the maximum current output is 10 mA.

The DWELL output is a TTL level output which is
high whenever the counters are paused. The rising
edge may be used to trigger changes in external
parameters or data acquisition.

23

Guide to Operation

REAR PANEL SUMMARY

POWER ENTRY MODULE

The power entry module is used to fuse the line,
select the line voltage, and block high frequency
noise from entering or exiting the instrument.
Refer to the first page of this manual for
instructions on selecting the correct line voltage
and fuse.

IEEE-488 STD PORT

The 24 pin IEEE-488 connector allows a computer
to control the SR400 via the IEEE-488 or GPIB
instrument bus. The address of the instrument is
set from the front panel using the COM menu.

RS-232 CONNECTOR

The RS-232 interface connector is configured as a
DCE. The baud rate, parity, character length, and
echo mode are programmed from the front panel
using the COM menu.

INPUT

INHIBIT is a TTL input which disables the A,B,
and Tdiscriminators when it is high. The minimum
pulse width is 10 ns.

OUTPUT

The PORT1 and PORT2 D/A analog outputs
provide -10 V to +10V, user programmable
outputs. The outputs may be scanned to drive the
X axis of a recorder or scope during scans. Output

impedance is < 1 Ω and the maximum current

output is 10 mA.

24

Guide to Operation - Menus

FRONT PANEL MENUS

The lower line of the LCD display is the menu
line. Menu lines are grouped into six menus. To
select a new group or menu, press a new menu
key. The menu line that will be displayed is the
most recently active menu line of the new menu.

Each menu contains several lines. Each line may
consist of 1 or 2 displayed parameters. The
DOWN and UP keys change which menu line
within a menu will be displayed. The DOWN key
displays the next menu line and the UP key, the
previous line. Pressing either key many times in
succession will scroll through all of the menu lines
within a menu.

If the menu line contains two parameters, the
LEFT and RIGHT keys select between the two.
On some lines, the right hand parameter is active
only for certain values of other parameters. On
these lines, the right hand parameter may be
inactive.

The spin knob is used to modify the active
parameter. A flashing cursor precedes the value
which the knob will alter.

The following section details each menu line. The
lines are listed in the order in which the DOWN
key scrolls through each menu. For parameters
with limited choices, all selections are shown. For
numerical parameters, example values are shown.

MODE MENU

COUNT= A,B FOR T PRESET
A-B FOR T PRESET
A+B FOR T PRESET
A FOR B PRESET

This line selects the data display and the preset
counter. The upper line of the LCD is the count
display. There are four displays available; A and B
separately, A-B, A+B, or only A, depending upon
the COUNT selection. All count data is right
justified with leading zeroes off. The preset
counter is either T or B. The appropriate SET LED
will be on indicating which counter is preset. Note
that the choice of inputs to the counters determines

whether the count data measures rate, ratio, or
reciprocal.

Changing this line causes the counters to reset.

A= INPUT 1
10 MHz

The input to counter A can be either the internal
10 MHz timebase or the signal at INPUT 1
discriminated by the A discriminator. The selected
counter input can be viewed at the A DISC output.
The 10 MHz source is used for reciprocal
measurements or for quick testing of the counter
operation. The appropriate A counter source LED
is also lit.

Changing this line during a COUNT PERIOD may
result in a bad data point.

B= INPUT 2 BSET= ****
INPUT 1 1E0
9E11

The input to counter B can be either the signal at
INPUT 1 or INPUT 2 discriminated by the B
discriminator. The selected counter input can be
viewed at the B DISC output. The appropriate B
counter source LED is lit. Changing the counter
input during a COUNT PERIOD may result in a
bad data point.

If counter T is preset, the B SET parameter is
inactive (indicated by the **** on the display). If
counter B is preset (ratio or reciprocal
measurement), B SET is the B preset count from
1E0 to 9E11. Changing B SET during a scan
causes the counters to pause; any COUNT
PERIOD in progress is terminated and ignored. A
START begins the next COUNT PERIOD with
the new preset value.

T= 10 MHz TSET= 1E0 s
INPUT 2 1E7
TRIG 1E3
****

25

Guide to Operation - Menus

The input to counter T can be the internal 10 MHz
timebase (rate measurement), the signal at INPUT
2 discriminated by T discriminator (ratio or
reciprocal measurement), or the TRIGGER input
discriminated by the trigger discriminator
(constant gate aperture). The selected counter
input can be viewed at the T DISC output. The
appropriate T counter source LED is lit.

If counter B is preset, this line is inactive
(indicated by the **** on the display). If counter T
is preset, both parameters on this line are active. T
SET is the preset value of counter T from 1E0 to
9E11. If the T input is 10 MHz, then T SET is
displayed as real time = T SET cycles of 10 MHz.
ChangingT SET during a scan causes the counters
to pause; any COUNT PERIOD in progress is
terminated and ignored. A START begins the next
COUNT PERIOD with the new preset value.

N PERIODS=2000 [at 1234]

This line programs the number of COUNT
PERIODS in one scan from 1 to 2000. N
PERIODS determines the length of the internal
data buffer and the range of any scanned
parameters. The quantity in brackets is the number
of completed COUNT PERIODS in the current
scan.

Changing N PERIODS during a scan changes the
length of the current scan. Reducing N PERIODS
past the current scan position will cause an end of
scan condition.

AT N= STOP DWELL= 1E0 s
START 2E-3 s
EXTERNAL

This line sets the end of scan mode and the
DWELL time. When a scan reaches N PERIODS,
the counters may be paused or restarted. AT
N=STOP causes an end of scan pause. All scanned
parameters are held at their final values. A counter
reset is required before another scan can begin. If
N PERIODS is 1, then each START results in one
COUNT PERIOD. If N PERIODS is gre ater than
1, then consecutive COUNT PERIODS are
separated by the DWELL time. During the
DWELL time, counting is disabled and scanned

parameters updated. The DWELL OUTPUT is
TTL high during the DWELL time.

If AT N=START, the counters will wait one
DWELL time at the end of each scan, during
which all scanned parameters return to their start
positions, and then another scan begins. This is
used for continuous data taking with the
measurement rate determined by the COUNT
PERIOD plus the DWELL TIME.

The DWELL time may be programmed from 2E-3
s to 6E1 s (1 minute) or set to EXTERNAL. The
internal DWELL time is based on the 10 MHz
timebase and may have 200 ns of indeterminacy.
Changing the DWELL time during a scan causes
the counters to pause; any COUNT PERIOD in
progress is terminated and ignored.

Setting the DWELL time below 2E-3 s results in
an EXTERNAL dwell. In this mode, the START
key, EXT START, or remote start command starts
the COUNT PERIOD. The COUNT PERIOD las t s
until the preset condition is reached. The following
DWELL time is indefinite; the next COUNT
PERIOD begins with another START. A STOP
key, EXT STOP, or remote stop command during
the COUNT PERIOD terminates the COUNT
PERIOD. Thus by programming the preset
COUNT PERIOD much longer than required, the
COUNT PERIOD may be determin ed by START
and STOP. Scanning proceeds as normal.

NOTE: If AT N=STOP, the counters will pause at
the end of a scan. A reset (STOP key, EXT STOP,
or remote reset command) is required to rearm the
counters before another scan can begin. In general,
to reset the counters at any time, the STOP ke y is
pressed twice in succession.

D/A= A
B
A+B
A-B

The D/A output is proportional to the count data
on the top line of the LCD. When the COUNT is
A-B, A+B, or A only, the D/A output can only
follow the data display. In these modes, the cursor
is inactive and the knob does nothing. If the

26

Guide to Operation - Menus

COUNT is A,B FOR T PRESET, then the D/A
output can follow either A or B.

D/A RANGE= LOG
##765####

The D/A output scale can be either logarithmic or
linear. If the output is logarithmic, the scale is 1V
per decade with 1 count being 0V, 0 counts being ­1V, and negative counts being -2V. If the scale is
linear, the output is proportional to 3 digits of the
count. These digits are shown while the ignored
digits are represented by #'s.

The D/A output is updated at the end of each
COUNT PERIOD. If the D/A output is driving a
chart recorder, the end of scan mode should be
STOP or the pen reset will mar the graph. If the
DWELL output is connected to the recorder pen
lift, data will be plotted as unconnected points,
otherwise, the discrete points will be connected.

DISPLAY= CONTINUOUS
HOLD

The count data on the upper line of the LCD can
be updated in two ways. CONTINUOUS mode
updates the display 20 times a second during the
COUNT PERIOD. Data is held during the
DWELL time at the final value reached during the
previous COUNT PERIOD. HOLD updates the
display only at the end of each COUNT PERIOD.

CONTINUOUS is useful when COUNT
PERIODS are long, HOLD is better when
COUNT PERIODS are short.

LEVEL MENU

TRIG SLOPE= RISE
FALL

The gate generator may be triggered on either the
rising or falling edge of the TRIGGER input.

TRIG LVL=+2.000 V

The gate TRIGGER threshold level may be
adjusted from -2.000 V to +2.000V. The

discriminated trigger pulses may be viewed
through the T DISC output when counter T is
preset and its input is TRIG. This can be helpful in
selecting a trigger level. The minimum pulse
amplitude required to trigger is 100 mV.

A DISC SLOPE= RISE
FALL

When A counter's input is INPUT 1, A
discriminator may be triggered by rising or falling
edges of the INPUT 1 signal. If the signal pulses
are negative going, then the slope should be
FALL. If the signal pulses are positive, then the
slope should be RISE. In either case, the A DISC
output is always active low; i.e., a discriminated
rising edge of the signal will be output as a
negative going NIM pulse at the A DISC output.
The falling edges of the A DISC output are
counted by counter A.

A DISC= FIXED
SCAN ∆= -1.0 mV

The threshold level of A discriminator may be
FIXED or SCANned. If the level is fixed, the right
hand parameter is not displayed. If the level is
scanned, then the right hand parameter is the step
size. At the end of each count period in a scan, the
level is incremented by one step size. The step size
can be varied from -20.0 mV to + 20.0 mV. The
range of a scan is N PERIODS times the step size.
The starting level is set in the next menu line.

The scan mode may be changed during a scan. If it
is changed to SCAN, the level begins scanning
from the start value on the next count period. If it
is changed to FIXED, the level returns to the start
level immediately.

The step size may also be changed during a scan.
The new step size will be in effect at the end of the
next count period. The scan is not reset or paused.

A DISC LVL = -10.0 mV

: -11.0 mV

The threshold level of A discriminator may be
adjusted from -300 mV to + 300 mV. This level is

27

Guide to Operation - Menus

the fixed threshold when A DISC is FIXED and
the starting level when A DISC is SCANned.

When the A DISC mode is SCAN and a scan is in
progress, this line displays the current position of
the A DISC level. The cursor is inactive and
replaced by a ':'. The spin knob cannot adjust the
threshold while it is scanning. When the mode is
SCAN, the starting level may only be adjusted
when the counters are in reset.

B DISC SLOPE= RISE
FALL

B discriminator may be triggered by rising or
falling edges of the INPUT 1 or INPUT 2 signal.
If the signal pulses are negative going, then the
slope should be FALL. If the signal pulses are
positive, then the slope should be RISE. In either
case, the B DISC output is always active low; i.e.,
a discriminated rising edge of the signal will be
output as a negative going NIM pulse at the B
DISC output. The falling edges of the B DISC
output are counted by counter B.

B DISC= FIXED

SCAN ∆= -1.0 mV

The threshold level of B discriminator may be
FIXED or SCANned. If the level is FIXED, the
right hand parameter is not displayed. If the level
is scanned, then the right hand parameter is the
step size. At the end of each count period in a
scan, the level is incremented by one step size.
The step size can be varied from -20.0 mV to +

20.0 mV. The range of a scan is N PERIODS
times the step size. The starting level is set in the
next menu line.

The scan mode may be changed during a scan. If it
is changed to SCAN, the level begins scanning
from the start value on the next count period. If it
is changed to FIXED, the level returns to the start
level immediately.

The step size may also be changed during a scan.
The new step size will be in effect at the end of the
next count period. The scan is not reset or paused.

B DISC LVL = -10.0 mV
: -11.0 mV

The threshold level of B discriminator may be
adjusted from -300 mV to + 300 mV. This level is
the fixed threshold when B DISC is FIXED and
the starting level when B DISC is SCANned.

When the B DISC mode is SCAN and a scan is in
progress, this line displays the current position of
the B DISC level. The cursor is inactive and
replaced by a ':'. The spin knob cannot adjust the
threshold while it is scanning. When the mode is
SCAN, the starting level may only be adjusted
when the counters are in reset.

T DISC SLOPE= RISE
FALL

When T counter's input is INPUT 2, T
discriminator may be triggered by rising or falling
edges of the INPUT 2 signal. If the signal pulses
are negative going, then the slope should be
FALL. If the signal pulses are positive, then the
slope should be RISE. In either case, the T DISC
output is always active low; i.e., a discriminated
rising edge of the signal will be output as a
negative going NIM pulse at the T DISC output.
The falling edges of the T DISC output are
counted by counter T.

T DISC= FIXED
SCAN ∆= -1.0 mV

The threshold level of T discriminator may be
FIXED or SCANned. If the level is FIXED the
right ha nd parameter is not displayed. If the level
is scanned, then the right hand parameter is the
step size. At the end of each count period in a
scan, the level is incremented by one step size.
The step size can be varied from -20.0 mV to +

20.0 mV. The range of a scan is N PERIODS
times the step size. The starting level is set in the
next menu line.

The scan mode may be changed during a scan. If it
is changed to SCAN, the level begins scanning
from the start value on the next count period. If it
is changed to FIXED, the level returns to the start
level immediately.

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