Tait TEL0035, TEL0033 Cct Description Part List Manual
M850-00
T856/857 General Information
C1.1
1 T856/857 General Information
This section provides a brief description of the T856 transmitter and T857 exciter, along
with detailed specifications and a list of types available.
The following topics are covered in this section.
Section Title Page
1.1 Introduction 1.7
1.2
1.2.1
1.2.2
1.2.3
1.2.4
1.2.4.1
1.2.4.2
1.2.4.3
1.2.5
1.2.6
1.2.6.1
1.2.6.2
1.2.6.3
1.3 Product Codes 1.13
1.4 T856 Standard Product Range 1.14
1.5 T857 Standard Product Range 1.15
Specifications
Introduction
General
RF Section
Audio Processor
Inputs
Modulation Characteristics
CTCSS
Microcontroller
Test Standards
European Telecommunication Standard
DTI CEPT Recommendation T/R-24-01
Telecommunications Industry Association
1.8
1.8
1.8
1.9
1.10
1.10
1.11
1.11
1.11
1.12
1.12
1.12
1.12
Figure Title Page
1.1 T856 Main Circuit Block Identification 1.4
1.2 T856 Front Panel Controls 1.4
1.3 T857 Main Circuit Block Identification 1.6
1.4 T857 Front Panel Controls 1.6
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C1.3
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replace A4 pages C1.3/C1.4 with A3 pages C1.3/C1.4
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C1.5
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1.1 Introduction
T856/857 General Information
C1.7
The T856 is a synthesised, microprocessor controlled FM base station transmitter
designed for single or multichannel operation in the 400 to 520MHz frequency range
with a standard power output of 25W. The RF section of the transmitter comprises a frequency synthesiser which provides 100mW of frequency modulated RF drive to a two
stage, wide band output driver followed by a 25W power amplifier. A thermal shutdown feature is provided in the T856 in case operating temperatures exceed acceptable
levels.
The T857 is a synthesised, microprocessor controlled FM base station exciter designed
for single or multichannel operation in the 400 to 520MHz frequency range
standard power output of only 1W, the exciter is designed for use with the T858 50W or
T859 100W power amplifiers. The RF section of the exciter comprises a frequency synthesiser which provides 100mW of frequency modulated RF drive to a two stage, wide
band output amplifier.
A wide selection of audio characteristics may be obtained from the audio processor.
Optional circuit blocks are an audio compressor and a pre-emphasis stage. They can be
bypassed or linked to one or both audio inputs, and then back into the remaining audio
circuitry in almost any combination. All audio processor options are link selectable.
The synthesiser frequency is programmed via the serial communications port. Eight
channel select lines are accessible via an optional D-range connector (D-range 2 T800-03-0000) at the rear of the set.
1
. With a
1
All components except those of the VCO are mounted on a single PCB. This is secured
to a die-cast chassis which is divided into compartments to individually shield each section of circuitry. Access to both sides of the main circuit board is obtained by removing
each of the chassis lids. There is provision within the chassis to mount small option
PCBs.
The front panel controls include line sensitivity, microphone socket and carrier switch.
This switch turns on the carrier (unmodulated) as an aid to servicing.
The T856 and T857 are both 60mm wide and each occupies a single space in a Tait rack
frame, which has the ability to accommodate up to seven standard modules.
1. Although capable of operating over the 400-520MHz frequency range, the T856 and
T857 have an 8MHz switching range (see Section 1.2.3 and Section 3.1).
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T856/857 General Information
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1.2 Specifications
1.2.1 Introduction
The performance figures given are minimum figures, unless otherwise indicated, for
equipment tuned with the maximum switching range and operating at standard room
temperature (+22°C to +28°C) and standard test voltage (13.8V DC).
Where applicable, the test methods used to obtain the following performance figures are
those described in the EIA and ETS specifications. However, there are several parameters for which performance according to the CEPT specification is given. Refer to Section 1.2.6 for details of test standards.
Details of test methods and the conditions which apply for Type Approval testing in all
countries can be obtained from Tait Electronics Ltd.
The terms "wide bandwidth", "mid bandwidth" and "narrow bandwidth" used in this
and following sections are defined in the following table.
Channel Spacing
Wide Bandwidth 25kHz ±5.0kHz 15.0kHz
Mid Bandwidth 20kHz ±4.0kHz 12.0kHz
Narrow Bandwidth 12.5kHz ±2.5kHz 7.5kHz
Modulation
100% Deviation
Receiver
IF Bandwidth
1.2.2 General
Number Of Channels .. 128 (standard)
Supply Voltage:
Operating Voltage .. 10.8 to 16V DC
Standard Test Voltage .. 13.8V DC
Polarity .. negative earth only
Polarity Protection .. diode
Supply Current:
Transmit - T856 .. 5.5A (typical)
- T857 .. 750mA
Standby - T856 .. 165mA (typical)
- T857 .. 150mA (typical)
1
Operating Temperature Range .. -20°C to +60°C
1. Additional channels may be factory programmed. Contact your nearest Tait Dealer or
Customer Service Organisation.
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Dimensions:
Height .. 183mm
Width .. 60mm
Length .. 320mm
Weight .. 2.1kg
Time-Out Timer (optional) .. 0 to 5 minutes adjustable in 10 second
Tail Timer .. 0 to 5 seconds adjustable in 100ms
Transmit Key Time .. <30ms
Transmit Lockout Timer .. 0 to 1 minute adjustable in 10 second
T856/857 General Information
steps
steps
steps
C1.9
1.2.3 RF Section
Frequency Range .. 400-520MHz (refer to Section 1.4 and
Section 1.5)
Modulation Type .. FM
Frequency Increment .. 5 or 6.25kHz
Switching Range .. 8MHz (i.e. ±4MHz from the centre
frequency)
Load Impedance .. 50 ohms
Frequency Stability .. ±1ppm, -20°C to +60°C
(see also Section 1.4 and Section 1.5)
Adjacent Channel Power (full deviation):
Wide Bandwidth (WB) .. -75dBc
(±25kHz/15kHz B/W)
Mid Bandwidth (MB) .. -70dBc
(±20kHz/12kHz B/W)
Narrow Bandwidth (NB) .. -65dBc
(±12.5kHz/7.5kHz B/W)
(Transmitter switching must comply with ETS 300 113)
Transmitter Side Band Noise:
(no modulation, 15kHz bandwidth)
At ±25kHz .. -95dBc
At ±1MHz .. -105dBc
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Intermodulation .. -40dBc with interfering signal of
T856 Mismatch Capability:
Radiated Spurious Emissions:
Conducted Spurious Emissions: (T856 Only)
T856/857 General Information
-30dBc
.. -70dBc with 25dB isolation
& interfering signal of -30dBc
(PA with output isolator)
Ruggedness .. refer to your nearest Tait Dealer or
Customer Service Organisation
Stability .. 3:1 VSWR (all phase angles)
Transmit .. -36dBm to 1GHz
-30dBm 1GHz to 4GHz
Standby .. -57dBm to 1GHz
-47dBm 1GHz to 4GHz
Transmit .. -36dBm to 1GHz
-30dBm 1GHz to 4GHz
Standby .. -57dBm to 1GHz
-47dBm 1GHz to 4GHz
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Power Output:
T856 - Rated Power .. 25W (see Duty Cycle)
- Range Of Adjustment .. 5-25W
T857 .. 1W ±300mW
Duty Cycle (T856 Only) .. 100% @ 25W at +25°C
.. 25% @ 25W at +60°C
.. 100% @ 10W at +40°C
1.2.4 Audio Processor
1.2.4.1 Inputs
Inputs Available .. line, microphone and CTCSS
Line Input:
Impedance .. 600 ohms (balanced)
Sensitivity (60% modulation @ 1kHz)-
With Compressor .. -50dBm
Without Compressor .. -30dBm
Microphone Input:
Impedance .. 600 ohms
Sensitivity (60% modulation @ 1kHz)-
With Compressor .. -70dBm
Without Compressor .. -50dBm
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1.2.4.2 Modulation Characteristics
Frequency Response .. flat or pre-emphasised (optional)
(below limiting)
Line And Microphone Inputs:
Pre-emphasised Response-
Bandwidth .. 300Hz to 3kHz (WB & MB)
Below Limiting .. within +1, -3dB of a 6dB/octave
Flat Response .. within +1, -2dB of output at 1kHz
Above Limiting Response .. within +1, -2dB of a flat response
Distortion .. 2%
Hum And Noise:
T856/857 General Information
.. 300Hz to 2.55kHz (NB)
pre-emphasis characteristic
(ref. 1kHz)
C1.11
Wide Bandwidth .. -55dB (300Hz to 3kHz [EIA]) typical
Mid Bandwidth .. -54dB (CEPT)
Narrow Bandwidth .. -50dB (CEPT)
Compressor (optional):
Attack Time .. 10ms
Decay Time .. 800ms
Range .. 50dB
1.2.4.3 CTCSS
Standard Tones .. all 37 EIA group A, B and C tones
plus 13 commonly used tones
Frequency Error .. 0.08% max.
(from EIA tones)
Generated Tone Distortion .. 1.2% max.
Generated Tone Flatness .. flat across 67 to 250.3Hz to within 1dB
Modulation Level .. adjustable
Modulated Distortion .. <5%
1.2.5 Microcontroller
Auxiliary Ports:
Open Drain Type .. capable of sinking 2.25mA via 2k2Ω
max. .. 5V
V
ds
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T856/857 General Information
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1.2.6 Test Standards
Where applicable, this equipment is tested in accordance with the following standards.
1.2.6.1 European Telecommunication Standard
ETS 300 086 January 1991
Radio equipment and systems; land mobile service; technical characteristics and test
conditions for radio equipment with an internal or external RF connector intended primarily for analogue speech.
ETS 300 113 March 1996
Radio equipment and systems; land mobile service; technical characteristics and test
conditions for radio equipment intended for the transmission of data (and speech) and
having an antenna connector.
ETS 300 219 October 1993
Radio equipment and systems; land mobile service; technical characteristics and test
conditions for radio equipment transmitting signals to initiate a specific response in the
receiver.
ETS 300 279 February 1996
Radio equipment and systems; electromagnetic compatibility (EMC) standard for private land mobile radio (PMR) and ancillary equipment (speech and/or non-speech).
1.2.6.2 DTI CEPT Recommendation T/R-24-01
Annex I: 1988
Technical characteristics and test conditions for radio equipment in the land mobile
service intended primarily for analogue speech.
Annex II: 1988
Technical characteristics of radio equipment in the land mobile service with regard to
quality and stability of transmission.
1.2.6.3 Telecommunications Industry Association
ANSI/TIA/EIA-603-1992
Land mobile FM or PM communications equipment measurement and performance
standards.
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T856/857 General Information
C1.13
1.3 Product Codes
The three groups of digits in the T850 Series II product code provide information about
the model, type and options fitted, according to the conventions described below.
The following explanation of T850 Series II product codes is not intended to suggest that
any combination of features is necessarily available in any one product. Consult your
nearest Tait Dealer or Customer Service Organisation for more information regarding
the availability of specific models, types and options.
Model
The Model group indicates the basic function of the product, as follows:
T85X
-XX-XXXX T855 receiver
T856 25W transmitter
T857 exciter
T858 50W power amplifier
T859 100W power amplifier
Type
The Type group uses two digits to indicate the basic RF configuration of the product.
The first digit in the Type group designates the frequency range:
T85X-X
The second digit in the Type group indicates the channel spacing:
T85X-XX
X-XXXX ’1’ for 400-440MHz
’2’ for 440-480MHz
’3’ for 480-520MHz
-XXXX ’0’ for wide bandwidth (25kHz)
’3’ for mid bandwidth (20kHz)
’5’ for narrow bandwidth (12.5kHz)
Options
T85X-XX-XXXX The Options group uses four digits and/or letters to indicate
any options that may be fitted to the product. This includes
standard options and special options for specific customers.
’0000’ indicates a standard Tait product with no options fitted.
The large number of options precludes listing them here.
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T856/857 General Information
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1.4 T856 Standard Product Range
The following table lists the range of standard T856 types (i.e. no options fitted) available at the time this manual was published. Consult your nearest Tait Dealer or Customer Service Organisation for more information.
Frequency Range (MHz) 400-440
Deviation (kHz) 2.5 2.5 4 5
TCXO ±1ppm -20°C to +60°C
Transmitter Type: T856- 16-0000
Frequency Range (MHz) 440-480
Deviation (kHz) 2.5 2.5 4 5
TCXO ±1ppm -20°C to +60°C
Transmitter Type: T856- 26-0000a25-0000 23-0000 20-0000
Frequency Range (MHz) 480-512 480-520
Deviation 2.5 2.5 5
TXCO ±1ppm -20° to +60°C
Transmitter Type: T856- 36-0000a35-0000 30-0000
a. United States market only.
Caution:
T856-1X-XXXX Product Types
••••
a
15-0000 13-0000 10-0000
••••
•••
The 406 to 406.1MHz frequency range is reserved worldwide for use by
Distress Beacons.
Do not program transmitters to operate in this fre-
quency range.
You can identify the transmitter type by checking the product code printed on a label on
the rear of the chassis (Figure 1.1 in Part A shows typical labels). You can further verify
the transmitter type by checking the placement of an SMD resistor in the table that is
screen printed onto the PCB (refer to Section 6.1 for more details).
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T856/857 General Information
C1.15
1.5 T857 Standard Product Range
The following table lists the range of standard T857 types (i.e. no options fitted and
cyclic keying disabled) available at the time this manual was published. Consult your
nearest Tait Dealer or Customer Service Organisation for more information.
Frequency Range (MHz) 400-440
Deviation (kHz) 2.5 2.5 4 5
TCXO ±1ppm -20°C to +60°C
Exciter Type: T857- 16-0000
Frequency Range (MHz) 440-480
Deviation (kHz) 2.5 2.5 4 5
TCXO ±1ppm -20°C to +60°C
Exciter Type: T857- 26-0000
Frequency Range (MHz) 480-512 480-520
Deviation 2.5 2.5 5
TXCO ±1ppm -20° to +60°C
Exciter Type: T857- 36-0000
a. United States market only.
Caution:
T857-1X-XXXX Product Types
••••
a
15-0000 13-0000 10-0000
••••
a
25-0000 23-0000 20-0000
•••
a
35-0000 30-0000
The 406 to 406.1MHz frequency range is reserved worldwide for use by
Distress Beacons.
Do not program exciters to operate in this frequency
range.
You can identify the exciter type by checking the product code printed on a label on the
rear of the chassis (Figure 1.1 in Part A shows typical labels). You can further verify the
exciter type by checking the placement of an SMD resistor in the table that is screen
printed onto the PCB (refer to Section 6.1 for more details).
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T856/857 Circuit Operation
C2.1
2 T856/857 Circuit Operation
This section provides a basic description of the circuit operation of the T856 transmitter
and T857 exciter.
Note:
Refer to Section 6 where the parts lists, grid reference index and diagrams will provide
detailed information on identifying and locating components and test points on the
main PCB. The parts list and diagrams for the VCO PCB are in Part E.
The following topics are covered in this section.
Unless otherwise specified, the term "PGM800Win" used in this and following sections refers to version 2.00 and later of the software.
Section Title Page
2.1 Introduction 2.3
2.2 Microcontroller 2.4
2.3
2.3.1
2.4
2.4.1
2.5
Synthesised Local Oscillator
Two Point Modulation
VCO
VCO Supply
Audio Processor
2.5
2.6
2.7
2.7
2.8
2.5.1
2.5.2
2.5.3
2.5.4
2.5.5
2.6 Power Supply & Regulator Circuits 2.10
2.7 Transmit Timers 2.11
2.8 T856 Drive Amplifier & PA 2.12
2.9 T857 Exciter Drive Amplifier 2.13
General
Audio Inputs
Keying Inputs
Compressor (Automatic Level Control (ALC))
Outputs To Modulators
2.8
2.8
2.9
2.9
2.9
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C2.2
T856/857 Circuit Operation
Figure Title Page
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2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
T856 High Level Block Diagram
T857 High Level Block Diagram
T856/857 Microcontroller Block Diagram
T856/857 Synthesiser Block Diagram
T856/857 Two Point Modulation
T856/857 Audio Processor Block Diagram
T856/857 Power Supply & Regulators Block Diagram
T856/857 Transmit Timers
2.3
2.3
2.4
2.5
2.6
2.8
2.10
2.11
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T856/857 Circuit Operation
C2.3
2.1 Introduction
The individual circuit blocks which make up the T856 and T857 are:
• synthesiser
•VCO
• audio processor
• drive amplifier
• power amplifier (T856 only)
• voltage regulators.
Each of these circuit blocks is set in its own shielded compartment, formed as an integral part of the main chassis.
The configuration of the circuit blocks may be seen on a functional level in Figure 2.1
and Figure 2.2. Refer to the circuit diagrams in Section 6.2 (T856) or 6.3 (T857) for more
detail.
Reference
Oscillator
12.8MHz
Microphone
Line
Tx Reg
Reference
Oscillator
12.8MHz
Microphone
Transformer
Line
Transformer
Power
Control
Drive
Amp.
Ramp
Gen.
Synthesiser
Audio
Processor
+22dBm
Lock
Detect
&
Gate
PIN
Switch
Time
Delay
Time
Delay
Figure 2.1 T856 High Level Block Diagram
PIN
Switch
Time
Delay
Synthesiser
Audio
Processor
+22dBm
Lock
Detect
&
Gate
Exciter
Output
(+30dBm to
+33dBm)
Drive
Amp.
Ramp
Gen.
PA
Time
Delay
Switching
Control
Directional
Coupler
Low Pass
Filter
Low Pass
Filter
Exciter
Output
(+28dBm to
+31dBm)
PA Control Signal
(via exciter RF coax)
Transmitter
Output
(25W)
VCO
Tx Reg
Time
Delay
VCO
Figure 2.2 T857 High Level Block Diagram
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T856/857 Circuit Operation
2.2 Microcontroller
(Refer to the microcontroller circuit diagram (sheet 8) in Section 6.2 or 6.3.)
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Watchdog
Reset
Microcontroller Cavity
Channel
Auxiliary
Output
EEPROM
Select
Port
Dual Digital
Potentiometer
Port
Synthesiser
12.8MHz
Clock
External
Serial
Port
CTCSS Tone
5V Digital
Regulator
5V Reset
Microcontroller
Converter
CTCSS
Encoder
Figure 2.3 T856/857 Microcontroller Block Diagram
Overall system control of the T856/857 is accomplished by the use of a member of the
80C51 family of microcontrollers (IC810). It runs from internal ROM and RAM, thus
leaving all four ports free for input/output functions.
Non-volatile data storage is achieved by serial communication with a 16kBit EEPROM
(IC820). This serial bus is also used by the microcontroller to program the synthesiser
(IC740) and deviation control EPOTS (IC220).
The main tasks of the microcontroller are as follows:
• program the synthesiser and EPOT;
• interface with the PGM800Win programming software at 9600 baud via the
serial communication lines on D-range 1 (PL100) & D-range 2;
• monitor channel change inputs from D-range 2;
• generate timing waveforms for CTCSS encoding;
• coordinate and implement timing control of the exciter/transmitter.
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m
T856/857 Circuit Operation
C2.5
2.3 Synthesised Local Oscillator
(Refer to the synthesiser circuit diagram (sheet 7) in Section 6.2 or 6.3 and the VCO circuit diagram in Part E.)
12.8MHz
Reference
Oscillator
Ref
Mod
Serial
Bus
Clk
Data
En
Fixed
Divider
/64
Phase
Modulator
FREQUENCY SYNTHESISER IC
Reference
Divider
f
ref
Controller
/R
Phase
Detector
Programmable
Divider
/N
Charge
Pump
Loop
Filter
Prescaler
64/65
Modulation
f
in
VCO
Σ
VCO Buffer
Divider Buffer
VCO PCB
Output
Buffer
Figure 2.4 T856/857 Synthesiser Block Diagram
The synthesiser (IC740) employs a phase-locked loop (PLL) to lock a voltage controlled
oscillator (VCO) to a given reference frequency. The synthesiser receives the divider
information from the control microprocessor via a 3 wire serial bus (clock, data, enable).
When the data has been latched in, the synthesiser processes the incoming signals from
the VCO buffer (f
) and the phase modulator (f
in
ref
).
A reference oscillator at 12.8MHz (=IC700) is buffered (IC710 pins 3 & 4) and divided
down to 200kHz (IC730). This 200kHz square wave is then summed with the modulating audio and passed to an integrator (IC720 pins 9 & 8, Q710, Q720). This produces a
ramping waveform which is centred around a DC level determined by the incoming
audio. IC720 pins 5 & 6 perform as a comparator, ultimately producing a phase-modulated 200kHz square wave. This is followed by another phase shifting stage (IC720 pins
3 & 4, Q730, Q740), before being divided down to 6.25kHz or 5kHz within the synthesiser IC (IC740).
+22dB
L.O.
A buffered output of the VCO (Q795) is divided with a prescaler and programmable
divider which is incorporated into the synthesiser chip (IC740). This signal is compared
with the phase modulated reference signal at the phase detector (also part of the synthesiser chip). The phase detector outputs drive a balanced charge pump circuit (Q760,
Q770, Q775, Q780, Q785) and active loop filter (IC750 pins 5, 6 & 7, Q790) which produces a DC voltage between 0V and 20V to tune the VCO. This VCO control line is further filtered to attenuate noise and other spurious signals. Note that the VCO frequency
increases with increasing control voltage.
If the synthesiser loop loses lock, a pulsed signal appears at LD (pin 2) of IC740. This
signal is filtered and buffered by IC750 pins 1, 2 & 3, producing the Lock-Detect signal
used to shut off the power supply to the drive amplifier. IC750 pin 1 is at 20V when the
synthesiser is out of lock.
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T856/857 Circuit Operation
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2.3.1 Two Point Modulation
Frequency modulation occurs by modulating both the VCO input and the synthesiser
reference input. This process is called two point modulation and ensures a flat modulation response from 67Hz to 3kHz (2.55kHz for narrow bandwidth).
The PLL has a fast response time, allowing a Tx key-up time of <30ms. Because of this
fast response time the PLL sees lower modulation frequencies superimposed on the
VCO as an error and corrects for it, resulting in no modulation on the carrier. At modulation frequencies greater than 300Hz the loop cannot correct fast enough and modulation is seen on the carrier. The response of the loop to VCO modulation is shown by f
in Figure 2.5 below.
To achieve low frequency modulation, the reference oscillator is also modulated so that
the phase detector of IC740 detects no frequency error under modulation. Thus, the
synthesiser loop will not attempt to correct for modulation and the audio frequency
response of the transmitter remains unaffected. The response of the loop to reference
frequency modulation is shown by f
in Figure 2.5.
1
2
Figure 2.5 T856/857 Two Point Modulation
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T856/857 Circuit Operation
C2.7
2.4 VCO
(Refer to the VCO circuit diagram in Part E.)
The VCO transistor (Q1) operates in a common emitter configuration, with an LC tank
circuit coupled between its gate and drain to provide the feedback necessary for oscillation. The VCO control voltage from the loop filter (IC750) is applied to the varicaps
(D1-D2) to facilitate tuning within an 8MHz band of frequencies. A trimcap (C6) is used
for coarse tuning of the VCO. The output from the oscillator circuit drives a cascode
amplifier stage (Q2, Q3) which supplies +10dBm (typically) to a further stage of amplification, Q5. This is the final amplifier on the VCO PCB, and delivers +20dBm (typically)
to the exciter drive amplifier.
A low level "sniff" is taken from the input to Q5 and used to drive the divider buffer for
the synthesiser (IC740).
The VCO operates at the actual output frequency of the exciter, i.e. there are no multiplier stages. It is modulated by superimposing the audio signal onto the control voltage
and by phase modulating the reference signal.
2.4.1 VCO Supply
The VCO is supplied from two switched +9V supplies under the control of the Tx-Reg.
supply.
The VCO and buffer amplifier are supplied from one +9V switched supply by Q540 via
the C multiplier (Q550, C530 on the T856 and C550 on the T857).
The output amplifier is supplied from the other +9V supply by Q520, Q530, and Q510.
A delay circuit holds the VCO on for a short time after the Tx-Reg. supply has been
switched off. This is to allow the RF power circuits (both exciter and PA) to ramp down
in the correct manner before the VCO is switched off.
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T856/857 Circuit Operation
2.5 Audio Process or
(Refer to the audio processor circuit diagram (sheet 2) in Section 6.2 or 6.3.)
Carrier
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Opto-Key
Tx Key
PTT
Microphone
Input
Line Input
+
_
Constant Current Sink
Opto-coupler
Microphone
Pre-amp.
Audio 1 Audio 2
Link
34
5
PL205
Compressor
5
N
3
M
12
L
Pre-
emphasis
B
6
C
mic.
Inputs
line
Multiplexer
Output
Inhibit
Output
6
4
Link
PL210
Figure 2.6 T856/857 Audio Processor Block Diagram
Tx Enable
Link
3
4
H
5
6
I
7
8
J
PL215
Link
34
E
2
1
D
PL220
Buffer
Output
To VCO
Limiter
Set Deviation
Digital Pot.
Σ
Low Pass
Filter
Ref. Mod. Adjust
Digital Pot.
Integrator
Ref.
Mod.
CTCSS
2.5.1 General
The audio processor comprises several link selectable circuit blocks which may be configured in a variety of combinations to suit individual requirements. The pre-emphasis
network and compressor may be linked individually or cascaded between either or both
audio inputs and the limiter.
Refer to Section 3.5.1 for linking details.
2.5.2 Audio Inputs
Two audio inputs are available: one from a 600 ohm balanced (or unbalanced) line, and
the other from a local microphone. The microphone signal is passed first to a pre-amplifier (Q210) and ultimately to a multiplexer (IC240), but in between may pass through
the compressor (depending on the linking details). The line transformer is also connected to the multiplexer and is disabled by the microphone PTT switch.
A third input for external CTCSS tones is also provided.
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T856/857 Circuit Operation
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2.5.3 Keying Inputs
There are four ways to key the exciter:
• pulling the Tx-Key line low (pin 13 on D-range 1 [PL100]) at the rear of the set);
• pushing the "Carrier" button on the front panel - this will inhibit all audio;
• using the PTT button on the local microphone, disabling audio from the line;
• via the opto-key inputs (pins 11 and 12 on D-range 1 [PL100]) when electrical
isolation is required. This features a constant current sink (Q270) to ensure reliable activation of the opto-coupler (IC250) at low keying voltages.
2.5.4 Compressor (Automatic Level Control (ALC))
The input signal is fed via a current controlled attenuator (Q230, Q220) to a high gain
stage (IC230) from which the output signal is taken. This signal is passed to a comparator (IC230) which toggles whenever the audio signal exceeds a DC threshold determined by RV220. Thus, the comparator produces a square wave whose mark-space
ratio is determined by the amplitude of the audio signal. This square wave pumps up
the reservoir capacitor (C233) which controls the attenuator (Q230, Q220), thus completing the feedback loop.
The compression level is set by adjustment of the comparator threshold (RV220).
Note:
Although the high dynamic range of the compressor allows the use of very
low audio signal levels, such conditions will be accompanied by a degradation of the signal-to-noise ratio. Very low audio input levels should therefore be avoided where possible.
2.5.5 Outputs To Modulators
The output signal from the limiter (IC210, IC230) is summed with a CTCSS tone at a
summing amplifier (IC260). The signal is then low pass filtered (IC260) and split to supply the two modulators.
Since the VCO modulator is a true frequency modulator, its audio is simply buffered
(IC260). The reference modulator, however, is a phase modulator and its audio must
first be integrated (IC210).
It is vital that the audio levels to the modulators are accurately set,
other. Hence the inclusion of level adjustment in the reference modulator path. Once
set, adjustments to absolute deviation may be made only via the deviation digital pot.
(IC220 - adjustable via PGM800Win).
relative to each
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T856/857 Circuit Operation
2.6 Power Supply & Regulator Circuits
(Refer to the regulators circuit diagram (sheet 6) in Section 6.2 or 6.3.)
+9V
Buffer
Tx Enable
Micro-
controller
Power
Switch
Tx Reg.
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13.8V Nom.
From Rear
D-Range
Crowbar
Diode
13.8V
Nom.
5V
Reg
DC
Amp
5V 5V Dig9V 20V
Switching
PS
5V Dig
Reg
Watchdog
Timer
LVI
µP
µP
Reset
Figure 2.7 T856/857 Power Supply & Regulators Block Diagram
The T856 and T857 are designed to operate from a 10.8-16V DC supply (13.8V nominal).
A 5.3V regulator (IC630) runs directly from the 13.8V rail, driving much of the synthesiser circuitry. It is also used as the reference for a DC amplifier (IC640, Q630, Q620)
which provides a medium current capability 9V supply. The T856 has a regulator
(IC370) which produces 9V for use in the exciter and audio circuits.
A switching power supply (Q660, Q670) runs from the 9V supply and provides a low
current capability +20V supply. This is used to drive the synthesiser loop filter (IC750),
giving a VCO control voltage range of up to 20V.
Ultimate control of the transmitter is via the Tx-Reg. supply, switched from 9V by Q610.
This is enabled via the Tx-Enable signal from the audio processor, and microprocessor.
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2.7 Transmit Timers
The transmit tail timer, transmit timeout timer and transmit lockout timer can all be set
from PGM800Win. The fields for setting these are found on the system information
page. These three timers operate as follows (refer also to Figure 2.8):
Timer Function Adjustment
Transmit Tail Sets the tail time during which the
transmitter stays keyed after the external key source has been removed.
Transmit Timeout Sets the maximum continuous trans-
mission time. Once the timer has
timed out, the transmitter must be
keyed again, unless prevented by the
transmit lockout timer.
Transmit Lockout Sets the period of time that must
elapse after a timeout before the transmitter can re-transmit. Once the timer
has timed out, the transmitter can be
keyed again.
Tx-Enable
On
Tx-Enable
Tx-Reg.
0-5 seconds in 100ms
steps
0-300 seconds in 10
second steps
0-60 seconds in 10
second steps
Tx-Reg.
Tail Time
On
Lockout Time
Timeout
Time
Figure 2.8 T856/857 Transmit Timers
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T856/857 Circuit Operation
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2.8 T856 Drive Amplifier & PA
(Refer to Figure 2.1 and the exciter and PA circuit diagrams (sheets 3 & 4) in Section 6.2.)
The output power of the PA is maintained at a constant level via a power control loop
applied to the two-stage, wide band exciter amplifier (Q350, Q355). The forward and
reverse RF power levels are sensed via a dual directional coupler and detector diodes
(D440, D420 in the PA cavity). The detected DC signals are buffered (IC330 pins 3 & 5)
and then summed with the ’power control’ level and fed to the control integrator (IC350
pin 6). The output control voltage is buffered by Q310 and Q315, and applied to the collector of the wide band exciter amplifier.
Note:
To reduce the spurious output level when the synthesiser is out-of-lock, the Tx-Reg. and
Lock-Detect signals are gated to inhibit the PA control circuit and to switch off the RF
signal at the input to the drive amplifier. This is achieved by a PIN switch attenuator
(D340, D380, D360).
Cyclic keying control is provided by additional circuitry consisting of several time
delay, ramp and gate stages:
This is to allow the RF power circuits (both exciter and PA) to ramp up and down in a
controlled manner so that minimal adjacent channel interference is generated during
the transition.
The output of the wide band amplifier is approximately 1W (+30dBm) for an input of
100mW (+20dBm) from the VCO, when the power control is set to maximum.
Forward and reflected power signals are summed so that, under high
VSWR, the power control will turn the output RF level down.
• Q325, IC350 power ramping
• Q355, Q325 Tx-Reg. and Lock-Detect
• Q335, Q340, Q345 delay and PIN switch drive.
gate
A temperature sensor (R481) is provided so that the RF output power can be reduced to
a preset level when a set temperature is exceeded. This is a protection circuit (IC350 pin
1, Q320) to prevent overheating, as the unit is
to Section 1.2.3 for duty cycle specifications).
#R517, #R518 and #R519 form an attenuator to provide good VCO/exciter isolation as
well as the correct exciter drive level.
The attenuator (#R395, #R396 and #R397) aids in producing the correct exciter drive
level to the PA over the three frequency ranges.
The RF output from the exciter is fed to the driver stage (Q410) and then to the final
(Q420). DC is fed to the final via a low pass filter with special low frequency decoupling. CV475 tunes the output matching across the entire band.
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not rated for continuous operation (refer
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2.9 T857 Exciter Drive Amplifier
(Refer to Figure 2.2 and the exciter circuit diagram (sheet 3) in Section 6.3.)
A two-stage, wide band amplifier (Q304, Q305) provides an output level of approximately 1W (+30dBm) for an input of 100mW (+20dBm) from the VCO. IC300 pin 1,
Q302, and Q303 provide a 9V regulated supply for the exciter.
To reduce the spurious output level when the synthesiser is out-of-lock, the Tx-Reg. and
Lock-Detect signals are gated to inhibit the exciter control circuit and to switch off the
RF signal at the input to the drive amplifier. This is achieved by a PIN switch attenuator
(D201, D202, D203).
Cyclic keying control is provided by additional circuitry consisting of several time
delay, ramp and gate stages:
• Q301, Q302, Q303, IC300 power ramping
• Q310, Q311 Tx-Reg. and Lock-Detect
• Q306, Q307, Q308, Q309 delay and PIN switch drive.
This is to allow the RF power circuits (both exciter and PA) to ramp up and down in a
controlled manner so that minimal adjacent channel interference is generated during
the transition.
R517, R518 and R519 form a 3dB attenuator to provide good VCO/drive amplifier isolation as well as the correct exciter drive level
Note:
The exciter provides a DC control signal to the PA via the RF coax. This is
injected via L314.
gate
Copyright TEL 21/06/99
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T856/857 Circuit Operation
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