Johnson DM3474, Руководство по обслуживанию

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SECTION 4
4-1
Part No. 001-3474-002
CIRCUIT DESCRIPTION
4.1 GENERAL
4.1.1 INTRODUCTION
The main subassemblies of this transceiver are the RF board, VCO board, and TCXO. A block diagram of the
transceiver is located in Figure 4-1.
The VCO board is enclosed by a metal shield and soldered directly to the RF board. The VCO is not
serviceable.
The 3474 is available with a reference oscillator stability of
±
1.5 PPM. The TCXO (Temperature
Compensated Crystal Oscillator) is soldered directly to the RF board.
4.1.2 SYNTHESIZER
The VCO (voltage-controlled oscillator) output signal is the receiver first injection frequency in the Receive
mode and the transmit frequency in the Transmit mode. The first injection frequency is 52.95 MHz above the
receive frequency. The frequency of this oscillator is controlled by a DC voltage produced by the phase detector in
synthesizer chip U801.
Channels are selected by programming counters in U801 to divide by a certain number. This programming is
performed over a serial bus formed by the Synth Clock, Synth Enable, and Synth Data pins of J201. This
programming is performed by user supplied hardware and software (see Section ).
The frequency stability of the synthesizer in both the receive and transmit modes is established by the
stability of the reference oscillator described in the preceding section. These oscillators are stable over a
temperature range of -30
°
to +60
°
C (-22
°
to +140
°
F).
4.1.3 RECEIVER
The receiver is a double-conversion type with intermediate frequencies of 52.95 MHz / 450 kHz. Two helical
bandpass filters reject the image, half IF, injection, and other unwanted frequencies. A four-pole crystal filter
enhances receiver selectivity