Theory Of Operation - Marti Electronics SRPT-30 User Manual

THEORY OF OPERATION

Refer to Block Diagram Drawing No. 702-124, 702-117, and appropriate Schematic Diagrams.
CIRCUIT BOARD DESCRIPTION
PRE-AMP/MIXER Board, 800-251A
Each of the four microphone inputs is fed to a low-noise differential op-amp (half of an NE-5532). Critical resistors
in the input circuits are low-noise, precision, temperature stable types to obtain maximum performance from the pre-
amps. Monolithic chip capacitors are used to filter RF voltages that may be present at the microphone inputs. The four
op-amp outputs are fed to gain pots then resistively mixed and routed to the COMPRESSOR BOARD.
COMPRESSOR Board, 800-166A (SRPT30/40A-xxx: uses 800-166A40A;
Several functions are performed on this board. Integrated Circuit IC-1 serves as a (a) pre-amp (not used on the
SRPT-30/40A), (b) pre-emphasis amplifier, (c) voltage-controlled attenuator, and (d) regulator /ripple rejection. Pre-
emphasized audio out of IC-1B is also fed to D2 - D3 which form an adjustable series peak-limiting circuit. This circuit
is adjusted to limit only audio peaks which get past the compressor. The limiter circuit feeds a low-pass filter (L1, C23,
and R46) which reduces the audio bandwidth to that specified for the operating channel of the transmitter. To this is
mixed the output of the tone encoder, IC-2A, which is a low-distortion Wien bridge oscillator. This composite signal is
then fed to the Modulation port (on P2) of the Transmitter Synthesizer Board, 800-375AT. This audio signal is also fed
to IC-2B which amplifies it to a level suitable for a 600 ohm headphone monitor. IC-2C is a DC amplifier the input of
which is connected to the AGC (automatic gain control) circuit and the output of which drives the audio compression
meter.
14.5 VOLT STEP-UP REGULATOR Board, 800-392A
The 800-392A is a DC/DC converter that powers the synthesizer, audio, and mixer boards. It allows the user to
apply an external input voltage (through the 12-15 VDC accessory input pins) as low as 7 VDC and still maintain 14.5
volts to the synthesizer and 10.0 volts to the audio and mixer boards. Therefore, the synthesizer will stay locked during
most power surges and excessive voltage drops. The 800-392A also suppresses any external (or internal) voltage noise
such as an alternator from a running vehicle. As a result, the SRPT-30/40A still maintains a high signal-to-noise.
TRANSMITTER SYNTHESIZER Board, 800-375AT
The fundamental purpose of this board is to accomplish two things: (1) Generate the final output frequency and (2)
FM modulate the mixed audio. The circuitry to achieve this consists of a Phase-Locked Loop (PLL), which includes a
Frequency Synthesizer IC, Voltage-Controlled Oscillator, a pre-scaler, a reference frequency oscillator, and a low-
frequency loop filter. The Frequency Synthesizer IC is a programmable device for setting internal counters for allowing
the reference frequency oscillator to be a perfect multiple of the final output frequency. The reference frequency
oscillator is a 12.8 MHz TCXO. The low-frequency loop filter is a one-Hertz active type. The 64/128 pre-scaler is used
to help aid in the multiplying.
The audio output from the 800-166 COMPRESSOR board is fed into the TRANSMITTER SYNTHESIZER's VCO
which FM modulates the signal at the final output frequency. This modulated RF output signal is then sent to the 800-
388A Two-Stage RF PA board for final amplification.
Included on the TRANSMITTER SYNTHESIZER board is a high-speed microcontroller. This controller decodes
and acts on commands sent from the Front Panel Control & Meter board (800-378A). These commands include new
frequency change (direct or channel select), control switch settings, calibration, etc. The controller also monitors and
regulates forward power, monitors VSWR and PA temperature, performs auto foldback of power due to high VSWR
and then recovers when VSWR lowers, and performs auto shutdown of power due to very high temp. It has internal
EEPROM for storing important information such as frequency channels and historical info. The controller also detects
synthesizer lock and unlock as well as enabling a fastlock feature for far frequency changes.
SRPT30/40A-450T: uses 800-166A40ATSL)
8/7/2003
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