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Dual-channel Filters
Series SR640
1290-D Reamwood Avenue, Sunnyvale, California 94089
e-mail: info@thinkSRS.com • www.thinkSRS.com
Copyright © 1989, 1999 by SRS, Inc. All Rights Reserved.
Notice: The information described in this manual pertains to the SR640 dual channel low-pass filter,
the SR645 dual channel high-pass filter, and the SR650 low-pass high-pass filter. The manual refers
to all three filter models as "SR640". Any differences between the three models are explicitly noted.
Revision 2.6
(12/99)
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TABLE OF CONTENTS
CONDENSED INFORMATION
Safety and Preparation for Use 3
Specifications 5
INSTRUMENT OVERVIEW
Front Panel Operation 7
Power 7
Frequency Display 7
Source Select 7
Coupling and Invert 8
Input and Output Gain 8
Filter In / Out 8
Clear / Local 8
Store and Recall 8
Error, Activity, and Remote 8
GUIDE TO PROGRAMMING
Communicating with the SR640 9
Configuration Switches 9
Command Syntax 9
RS-232 Echo 9
COMMAND LIST
Filter Commands 11
Status and Interface Commands 12
PROGRAMMING EXAMPLES
Basic 13
FORTRAN 13
C 14
INSTRUMENT DESCRIPTION
Circuit Description 16
PART LIST 18
Front Panel Board Parts List 18
Digital Board Parts List 20
Filter Board Parts List 22
Chassis Assembly Parts List 34
Miscellaneous Parts List 35
SCHEMATIC DIAGRAMS 36
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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.
Verify that the correct line fuse is installed before
connecting the line cord. For 100V/120V, use a 1
Amp fuse and for 220V/240V, use a 1/2 Amp fuse.
LINE CORD
LINE VOLTAGE SELECTION
The SR640 operates from 100, 120, 220 or 240
Volts nominal AC power source with a line fre-
quency 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 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
The SR640 series uses 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.
GROUNDING
The SR640's BNC connector shields are NOT
connected to the unit's chassis ground. This is
done in order to provide the user with maximum
flexibility in connecting the instrument's grounds
for optimum noise-reduction. However, the user
should be aware that improper connection of the
SR640 to a hazardous voltage source could result
in hazardous voltages present at the exposed
BNC shield terminals. Please see the guide to op-
erations for additional information concerning
grounding.
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SR640 SPECIFICATIONS
GENERAL INFORMATION
The SR640 consists of two independent low-pass filter channels. Each filter is preceded by a low-noise preamplifier with selectable gain of 0 to 60 dB. The filters are an 8-pole, 6-zero elliptical design with 0.1 dB maximum passband ripple and 115 dB/octave attenuation slope. A variable gain output amplifier with gains of 0dB,
10dB, and 20dB follows each filter. The filter may be bypassed to allow each channel to be used simply as an
amplifier.
The filters are controlled by a CMOS microprocessor which also handles the RS-232 and IEEE-488 interfaces. In addition, nonvolatile storage is provided for up to 9 complete instrument settings defined by the user.
No direct electrical connection exists between the microprocessor section of the SR640 and the filter channels. Filter and gain control is accomplished strictly through optoisolated interfaces. In addition, each channel's ground is isolated from the digital section's ground as well as from the other filter section. Each chan-
nel's isolated ground is made available to the user on a rear-panel BNC connector to provide maximum
flexibility in connecting the SR640.
FILTER SPECIFICATIONS
Frequency Range 1Hz to 100 kHz with 3-digit resolution
Type 8-pole, 6-zero elliptic
Rolloff 115 dB/octave
Passband Ripple 0.1 dB pk-pk, typical
Stopband Attenuation 80 dB, typical
INPUT SPECIFICATIONS
Impedance 1 MΩ // 15 pF
Configuration Single ended (A or B) or Differential (A-B)
Common Mode Rejection >90 dB at 100 Hz
Coupling AC or DC
Input Noise 10 nV/√Hz at 1 kHz with 60 dB input gain
Gain 0,10,20,30,40,50,60 dB ±0.2 dB
Maximum Input Signal 10 Volts pk-pk
OUTPUT SPECIFICATIONS
Impedance <1 Ω
Full Scale Output Signal 10 Volts pk-pk into >300 Ω
DC Offset Adjustable to 0 Volts DC
Gain 0,10,20 dB ±0.2 dB
Harmonic Distortion No greater than 80 dB below full scale at 100 Hz signal
Spurious Components No greater than 80 dB below full scale with input source <50 Ω
Crosstalk Between Channels No greater than 80 dB below full scale with input source <50 Ω
Phase Match Between Channels ±2°, DC to cutoff frequency
GENERAL
Interfaces IEEE-488 and RS-232 standard. All instrument functions can be
controlled and read through the interface.
Stored Settings 9 complete 2 channel instrument configurations may be stored in
nonvolatile memory.
Power 100/120/220/240 VAC, 50/60 Hz
Dimensions 15.7"W X 3.0"H X 14.0"L
Warranty One year parts and labor on materials and workmanship
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I N S TRU ME N T O VERVIEW
FRONT PA NEL SUMMARY
FRONT PA NEL OPERATION
The front panel has been designed to be nearly
self-explanatory. The effect of each keypress is
usually reflected by a change in the state of a
nearby LED.T he fron t panel c ontrols for e ach filte r
channel are identical. The following discussion of
the front panel contr ols applies to both channel 1
and channel 2.
POW E R BUT T O N
The SR640 is tur ned on by depressi ng the POWER button. When t he unit is turned on, t he model
number, se rial number and fir mware versi on num ber will appear briefly on the LED displays. When the unit is powered off, all instrument settings are stored in non-volatile memory.
Upon power up, the SR640 c hecks the consi stency of the stored settings. If an error appears on
power up, the instrument is returned to its default
state. The default state for each filter channel is:
Cutoff Frequency: 5 kHz
Coupling: DC
Source: A
Invert: Off
InputGain: 0 dB
Output Gain: 0 dB
Filter: In
FREQUENCY DISPLAY
The three digit LED displays indi cate the cutoff fre quency of each fil ter. The adjacent unit LEDs in-
dicate wheth er the d isplayed frequency is in Hz o r
kHz. The cutoff frequency for either channel may
be changed using the frequency increment and
decrement buttons to the right of the display. When one of these b uttons is pressed, the
displayed frequency will begin scrolling up or
down. Note, however, that the actual filter cutoff
frequency does N OT change unti l the ENTER bu tton is pressed. The ENTER LED will light to indicate that the d isplayed frequency d oes not match
the actual filter cutoff frequency. When the ENTER button is pressed, the ENTER LED will turn
off and the filter frequency will change to match
the displayed frequency. If the freq uency display
has been chan ged but it is not desired to chan ge
the actual filter cutoff frequency, pressing the CLR/
LOCAL button will reset the display to the actual
filter frequency.
SOURCE SELECT
There ar e two input BNC connectors prov ided for
each filter channel. When source LED 'A' is on,
the input signal is measured between the center
conductor of the 'A' BNC connector and its
shield. When the 'B' LED is on, the signal is
measured betwee n the center conductor of t he 'B'
BNC connector and its shield. When the 'A-B' LED
is on, the signal i s taken between the center con ductor of th e 'A' co nnecto r and th e center conductor of the 'B' connector. In all cases, the shields
of the 'A' and 'B' BNC connec tors ar e connected to
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Instrument Overview
each other and to the filter ground. The filter
ground is NOT connected to the unit's chassis
ground.
Filter ground for each channel is available at a
rear panel BNC connector. With this connector,
the user can connect filter ground to chassis
ground or to any other desired point.
COUPLING AND INVERT
The COUPLING button selects either AC or DC input coupling for both the 'A' and 'B' input connectors. The AC coupling has a nominal -3dB point
of 0.1Hz. Selecting INVERT on the front panel
simply inverts the sign of the output signal with respect to the input.
INPUT AND OUTPUT GAIN
The input and output gain indicators display the
amount of gain present before and after the filter. Changing the gain is accomplished with the
up and down buttons to the right of the respective
indicators. The gain can be changed in 10 dB increments up to a maximum of 60 dB of input gain
(prefilter) and 20 dB of output gain (postfilter).
FILTER IN/OUT
Selecting filter IN on the front panel puts the filter
in the signal path. Selecting filter OUT leaves the
input and output amplifiers in the signal path but
bypasses the filter. The -3dB point for the input
and output amplifiers with the filter bypassed is approximately 450 kHz.
turn the unit to local operation if the unit has been
placed in the LOCAL LOCKOUT mode.
STORE AND RECALL
The SR640 can store and recall up to 9 complete
instrument setups. To store the current instrument
configuration, press the STORE button. The setup number (1-9) will appear in the channel 2 frequency display. To change the setup number, use
the channel 2 frequency up and down buttons. When the setup number is correct, press
the channel 2 ENTER button to store the setup. Recalling previously stored setups is done in
the same manner. Recalling setup '0' places the
instrument in its default configuration. Pressing
the CLR/LOCAL button while storing or recalling a
setup aborts the operation and returns the display
to its normal mode. If an attempt is made to recall a setup which has not been previously stored,
the display will indicate an error. In this case, the
CLR/LOCAL button should be pressed to resume
normal operation.
ERROR, ACTIVITY, and REMOTE LEDs
The ACT LED lights to indicate any activity over
the interface, such as the receipt of a command.
The ERROR LED will light when a bad command
is received over the computer interface. The
REMOTE LED lights to indicate the SR640 is either in the REMOTE or LOCAL LOCKOUT mode
of operation. In either of these modes, the front
panel buttons are not operational. Only in the
REMOTE mode can front panel operation be regained by pressing the CLR/LOCAL button.
OFFSET ADJUST
The front panel offset adjust allows the user to dial
in approximately ±150 mV of input offset voltage. Note that this is a true input offset and the
actual output offset voltage will depend on the selected gain.
CLR/LOCAL
The CLR/LOCAL button has two functions. If the
displayed frequency for either filter channel has
been changed, pressing the CLR/LOCAL button
resets the display to the actual filter frequency. If
the SR640 has been placed in the REMOTE mode
by a command from the computer interface, pressing the CLR/LOCAL button will return the unit to
the LOCAL mode and enable front panel control. Note that the CLR/LOCAL button will not re-
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GUIDE TO PROGRAMMING
The SR640 dual channel low-pass filter is remotely programmable via both RS-232 and GPIB interfaces. All front panel features (except offset adjustment) may be controlled and read via the
computer interfaces.
Communicating with the SR640
Command Syntax
Communication with the SR640 is accomplished
by sending and receiving strings of ASCII characters. Characters sent to the SR640 may be sent
in either UPPER or lower case.
Before using either the RS-232 or GPIB interface,
the appropriate configuration switches must be
set. The configuration switch is located on the
rear panel and label 'SW1'. Bit 8 of the switch selects GPIB or RS-232 operation. If bit 8 is 'on',
GPIB operation is selected, if it is 'off', the RS-232
interface is enabled. The other bits of the switch
take their meaning depending upon whether RS232 or GPIB operation has been selected.
When a bit is 'on', it is in the DOWN position.
GPIB:
Bit Explanation
8 Always on for GPIB
7 not used
6 not used
5 high order bit of GPIB address
4 GPIB address (on = 1 off = 0)
3 GPIB address
2 GPIB address
1 low order bit of GPIB address
RS-232:
Bit Explanation
8 Always off for RS-232
7 on = echo off = no echo (normally
off)
6 on = 8 bit word off =7 bit word
5 on = even parity off = odd parity
4 on = parity enabled off = disabled
3 baud rate select
2 baud rate select
1 baud rate select
Bit 3 2 1 Baud Rate
on on on 19200
on on off 9600
on off on 7200
on off off 4800
off on on 3600
off on off 2400
off off on 2000
off off off 1800
Note that the configuration switch is only read on
power-up. If a setting needs to be changed, the
unit must be turned off and turned on again for the
change to be recognized.
A command to the SR640 consists of 4 ASCII
characters followed by an optional question mark
'?' character, followed by a list of arguments separated by commas, followed by a line-feed (<lf>)
character. If spaces are included in the command string, they are ignored by the SR640. The
SR640 interprets the GPIB EOI message sent with
the last character of a string as equivalent to a <lf>
terminator. Examples of commands are:
FREQ1,23600.<lf> set the channel 1 filter
frequency to 23.6 kHz.
FREQ?2<lf> read the channel 2 filter
frequency
*CLS<lf> clear the status byte
Multiple commands may be sent on a single line.
The commands must be separated by semicolon
characters. An example of a multiple command
would be:
FREQ1,23.6E3;FREQ2,100<lf>
The presence of a question mark character in a
command indicates that the command is a request
to the SR640 to send back a value. Values returned by the SR640 are terminated with a carriage return <cr> and a linefeed <lf>. If the GPIB
interface is used, the <lf> is accompanied by the
EOI message.
RS-232 Echo and No Echo Operation
In order to allow the SR640 to be operated from a
terminal, an echo feature has been included which
causes the unit to echo back characters received
over the RS-232 port. This feature is enabled by
setting bit 7 of the configuration switch on. When
using the unit with a computer, this feature should
be disabled to prevent spurious characters from
being sent to the computer. The echo feature is
not operational when the unit is being used with
the GPIB interface.
TIps on Using National Instruments
GPIB cards
To successfully interface the SR640 to a PC via
the GPIB, the instrument, interface card, and inter-
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Guide to Programming
face driver must all be configured properly. To configure theSR640, the GPIB address must be properly set using the DIP switch on the back panel.
The SR640 is shipped with the GPIB address set
to 23.
Make sure that you follow all the instructions for installing the GPIB card. The National Instruments
card cannot simply be unpacked and put into your
computer. To configure the card you must first set
jumpers and switches to set interrupt levels and I/
O addresses. You must then run the program IBCONF to configure the DOS resident GPIB driver.
Please refer to the National Instruments manuals
for complete details.
Once all the hardware and software has been configured properly, try using the IBIC program supplied with the National Instruments card to send
commands and receive data from the SR640. If
you cannot talk to the SR640 with the IBIC pro-
gram, there is no point in trying to write a program.
Although the National Instruments language con-
tains many commands, to avoid confusion it is
best to start with a few simple ones. Use IBWRT
and IBRD to write and read from the SR640. After
you are familiar with the simple commands you
can explore other, more complex, programming
commands.
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COMMAND LIST
The command list of the SR640 is detailed below. Commands that may be queried have a
question mark ? in parentheses (?) after the four
character command mnemonic. Commands that
may ONLY be queried have a ? after the mnemonic. Parameters shown in brackets {} should be
omitted if the command is used as a query. Do
not send () or {} as part of the command. The variable i used in many of the commands indicates
which filter channel the command will operate on.
This variable can only be 1 (for channel 1) or 2 (for
channel 2). The variable n is an integer variable,
and must be sent as an integer, i.e. no decimal
points or exponents. The variable x is a real
number and may be expressed in fixed or floating
point format, with or without a decimal point.
FILTER COMMANDS
ACDC(?) i{,n}
The ACDC command sets the input coupling for
channel i to AC (n=0) or DC (n=1).
FLTR(?) i{,n}
If n=1, the FLTR command puts the channel i filter
into the signal path. If n=0, the filter is bypassed.
FREQ(?) i{,x}
The FREQ command sets the filter cutoff frequency for channel i to x. Allowed values are 1.0 ≤ x ≤
99,900 (Hz).
nary number. The binary number is coded as follows:
bit meaning
3 (msb) channel 2 output overload
2 channel 2 input overload
1 channel 1 output overload
0 (lsb) channel 1 input overload
PREG(?) i{,n}
The PREG command sets the input gain for channel i to the value specified by n. Allowed values
of n are:
n Input Gain
0 0 db
1 10 db
2 20 db
3 30 db
4 40 db
5 50 db
6 60 db
PSTG(?) i{,n}
The PSTG command sets the output gain for
channel i to the value specified by n. Allowed values of n are:
n Output Gain
0 0 db
1 10 db
2 20 db
INVT(?) i{,n}
The INVT command inverts the output signal with
respect to the input (n=1), or leaves it uninverted
(n=0).
LOCL n
The LOCL command sets the local/remote mode
of the instrument. Allowed values are:
n mode
0 local
1 remote
2 remote with local lockout
OVLD?
The OVLD? command queries the overload status
of the instrument. The result is returned as a
decimal number (0-15) which represents a 4 bit bi-
SRCE(?) i{,n}
The SRCE command selects the input source for
channel i. The permitted values for n are:
n Input Source
0 A
1 A-B
2 B
STATUS AND INTERFACE COMMANDS
*IDN?
The *IDN? command returns the SR640's identification string. The string format is:
Stanford Research Systems,SR640,S/N0001,v
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Command List
1.01<cr><lf>
*RST
The *RST command resets the instrument. It is
equivalent to a RCL 0 command (recall default setup).
*STB? {n}
The *STB? command queries the value of the
SR640's status byte. The byte is returned as a
decimal number (0-255). The *STB? n command
reads the value of bit n. The range of n is 0-7. The
value returned is either 0 or 1. The various bits in
the status byte have the following meanings:
bit meaning
0 READY. Indicates unit has completed all
pending commands.
1 OUTPUT FULL. Indicates the unit's output
queue is full.
2 EXECUTION ERROR. Indicates an illegal
value was encountered while executing a
command.
3 SYNTAX ERROR. Indicates the SR640
could not interpret received characters as a
command.
4 OUTPUT NOT EMPTY. Indicates the SR640
has characters waiting to be sent.
*SRE(?) {n}
The *SRE command sets the service request enable byte to the value specified by n. Allowed
values are 0 through 255. When a bit is set in the
status byte, the byte is logically 'anded' with the
service request enable byte. If the corresponding
bit in the service request enable byte is set, the
SR640 issues a SRQ (service request) to the
GPIB controller. This command may only be used
with the GPIB interface.
*RCL n
The *RCL n command recalls the nth stored instrument settings. Allowed values for n are 0 through
9.
*SAV n
The *SAV command saves the current instrument
configuration as stored setting n. Values for n are
1 through 9.
*PSC (?) n
The *PSC command sets the power-on status
clear bit. If n=1, the service request enable byte
is cleared upon power up. If n=0, the status byte
and service request enable byte maintain their values when the unit is powered down. This allows
the generation of a service request upon power
up.
WAIT(?) n
5 INPUT FULL. Input queue full. The user
should send commands more slowly.
6 SRQ. Indicates the unit has requested ser-
vice from the GPIB controller.
7 POWER ON. Indicates the SR640 has been
powered on.
Each of the bits is set when the associated condition first becomes true. The status byte is only
cleared after the *STB? or *CLS command.
*CLS
The *CLS command clears the SR640's internal
status byte. This status byte is the byte returned
by the SR640 when it is serial polled via the GPIB
interface. It may also be queried using the *STB?
command.
The WAIT command sets the RS-232 transmission delay between characters. This is sometimes useful when interfacing with slower computers and terminals. The wait interval is set to about
2ms * n. Allowable values for n are 0 < n <255.
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PROGRAMMING EXAMPLES
The following are examples of programs which communicate with the SR640 in a variety of
languages over both RS232 and GPIB. In each case, the program does the same thing: Set the
channel 1 pregain to 10 db, the Channel 2 postgain to 20 db, and read the value of the channel
1 cutoff frequency. Once the basics of writing to and reading from the SR640 are mastered,
the task of programming is reduced to simply finding the appropriate command from the
command list.
Progr am Exampl e 1
IBM PC, BASI C, vi a RS232
In this example, the IBM PC's COM2: serial port is used to communicate with the SR640. Note
that a "straight through" RS232 cable (NOT a null modem cable) should be used to connect the
PC to the SR640. The SR640 should be set to operate at 9600 baud, 8 bits, no parity.
10'Example program for the SR640 filter using IBM BASIC over the COM2: port
20'
30'Open the COM2: port for 9600, 8 bits, no parity, ignore handshake
40 OPEN "COM2:9600,N,8,2,CS,DS,CD" AS #1
50'
60' Clear COM2 port
70 PRINT #1," "
80' Set channel 1 pregain and postgain
90 PRINT #1,"PREG1,1;PSTG2,2"
100 'Now read channel 1 cutoff frequency
110 PRINT #1, "FREQ?1"
120 INPUT #1, FREQ 'read answer
130 PRINT "frequency = ",FREQ
140 STOP
Progr am Exampl e 2
IBM PC, Mi crosof t FORTRAN, Nati onal I nstruments GPI B Card
Please refer to the "Guide to Programming" section for additional information on using the
National Instruments GPIB card. The GPIB.COM driver must be configured with IBCONF before
using the National Instruments cards.
c example program for the SR620 using Microsoft FORTRAN and National Instruments GPIB
card
c compile with /FPa /AL and link with MFIBL.OBJ (supplied by National Instruments)
$storage:2
c this line must be in the source file
common /ibglob/ibsta,iberr,ibcnt
integer sr640
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Programming Examples
character *80 string
real*4answer
c initialize gpib card
call ibinit(ibsta)
c get device id for sr640
sr640 = ibfind('sr640 ')
c set pregain and postgain
call ibwrt (sr640,'PREG1,1;PSTG2,2',15)
c now query stage 1 filter cutoff
call ibwrt (sr640,'FREQ?1',6)
call ibrd (sr640,string,30)
read (string,1000) answer
1000 format (bn,f14.0)
write (*,*) answer
stop
end
Progr am Exampl e 3
IBM PC, Mi crosof t C, Nati onal I nstruments GPI B card
/* Program to test SR640 communication using National Instruments GPIB card and
Microsoft C. Compile with /AL /FPa /c and link with the CIBL.OBJ module supplied
by National Instruments */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
/* function prototypes */
void main(void);
/* variables defined in CIBL module */
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extern int ibsta;
extern int iberr;
extern int ibcnt;
void main (void)
{
int sr640;
char string[30];
float answer;
sr640 = ibinit("SR640"); /* locate device */
if (sr640<0)
error();
ibwrt (sr640,"PREG1,1;PREG2,2",15); /* set gain */
ibwrt (sr640, "FREQ?1",6);
Programmi ng Exam-
/* query frequency */
ibrd (sr640,string,30);
/* and get answer */
sscanf(string,"%f",&answer);
printf("answer = %f",answer);
}
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Ci rcuit Descripti on
Q
s + ω
p
2
I NTRODUCTI ON
Schematics for the SR640 series dualchannel filters are shown on the following
pages. They include the following: Preamplifier, filter stage, output amplifier, optoisolated interface, microprocessor section, communications section, front panel
interface, power supplies, and front panel.
The preamplifer, filter stages (4/channel),
output amplifer, and optoisolated interface are located on the individual filter
boards (2/instrument). The microprocessor
and commuinication components are located on the digital board at the rear of the
instrument.
Pr eampl i fi er
The preamplifier is a low-noise fully differential amplifier with gains from 0 to
60 db. Relays K1 and K2 select the input
source and coupling, while K4 switches in
a X10 attenuator used at the three lowest
gain settings. The Dual FET U2, along with
low-noise op-amps U3 and U4 form the
feedback stabilized differential pair at the
heart of the premplifier. U6 and U8 provide additional amplification. The output
of the preamplifier is switched by U9 to
the first filter stage or directly to the
output amplifier if the filter has been bypassed.
Fi l ter Stages
Each of the four filter stages is derived
from a 3 op-amp biquadratic filter with a
low pass transfer function of:
ω
2
z
p
s2+ ω
T (s ) =
s2+
Stage paramaters for the low pass filter
are:
Stg ω
1 .6347 .5493 2 .8060 .9507 2.0793
3 .9850 2.095 1.9653
4 1.076 7.375 2.6776
For the high pass filter, the parameters
are simply related by the lowpass to highpass transformation.
Component designations for the filter
stage will be refered to as UnXX where n
designates the stage number. Un14A, C,
and D form the actual filter section. The
tuning capacitors Cn20-Cn27 are switched
by relays Kn05 and Kn06 according the the
decade. Fine tuning within a decade is accomplished by the 10 bit parallel resistor
networks composed of Rn40 through Rn59.
Switching of the resistors is done with the
FET switches Qn00 through Qn19 and their
associated driver diodes.
The dual 8 bit DAC Un12 and its related
output op-amps along with Un13A and differential amp Un13D allow the pole frequency and Q of the circuit to be fine tuned
by adj usting the values sent to the dual
DAC. This is how the unit is calibrated.
The magnitude of the 'fine tuning' is set by
Rn60 and Rn61.
The zero frequency of the circuit is not
digitally adj ustable but is instead set by
resistors Rn71 and Rn72. Jumpers JPn05n07 allow the zero-forming network to be
removed from the circuit in stages that do
not have zeros. The single op-amp Un15
forms the output buffer for the stage.
p
Q ωz/ω
p
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Ci rcui t Descri p-
Output Ampl i fi er
Un16A and B select the filter output or bypassed output from the preamplifier. The
8-bit DAC U17 provides output gain adj ustment. The actual output amplifier is
U119 which has its gain selected by
switch U21. Output inversion is accomplished with U22 in conj unction with the
switches U16C and U16D. The optional
component U23 provides a high current
output buffer. Dual Comparator U20 takes
care of output overload detection.
Optoi sol ated I nter face
No direct electrical connection exists between the microprocessor section and the
filter boards. Control signals for the filter boards originate from the two 32 bit
latched serial input shift registers U25
and U26. These shift registers are in turn
controlled by the output of optoisolator
U24. Optoisolator U11 provides output
isolation for the overload signals which
are read by the processor section. Shift
register U26 is run between -15V and
ground so that its open collector outputs
may be pulled up to +15V. These ±15V signals control the FET switches which determine the filter stage frequencies.
nication is accomplished with the UART
U15 and GPIB chip U12.
Power Suppl i es
Regulators U101 provides 5V power to the
microprocessor and communications circuits, while regulators U102-105 provide
±15V power to the filter sections. Note
that the 5V power supply is the only one
referenced to the chassis ground. Note
also that each filter power supply is taken
from a separate transformer secondary for
maximum isolation.
Note: The SR645 and SR650 units contain
additional regulators (U27 and U28) on the
high-pass filter boards to provide ± 5V to
the VTC4706 op-amps.
Fr ont Panel
Shift Registers U201-U206 provide control
signals for the front panel LEDs. Switches
SW301- SW324 comprise the switch matrix
which is read by the processor via input
port U 301. LED digits DS101-DS106 are
controlled via output ports U18-U25.
Mi cr opr ocessor and Communi cati ons
The microprocessor U4 is a Z80, clocked
at 3.6864 MHz by U3. U6 is an 8K ROM,
while U7 provides 2K of battery backed up
RAM. U1 is a 64K ROM which contains the
calibration information for the filter
channels. The calibration ROM is mapped
as I/O, requiring flip-flop U5B to switch
between the normal I/O space and the calibration ROM. U9A gates the clock to the
processor enabling the processor clock to
be removed during quiescent operation.
The clock is restored by a control signal
from flip-flop U5A in response to a keyboard or communication interrupt. Commu-
17
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