4.
Description of Frequency Determining and Stabilizing Circuitry
4-1. Introduction
The Frequencies for transmitter and receiver first local frequencies are all derived from a signal
10.695MHz cry
Phase lcoked loop.
stal and second local frequencies are derived from 10.240 MHz crystal by means of a
The first local oscillator frequencies are 16.270MHz(CH1) to 16.710MHz(CH40). The second local
frequency is fixed at 10.240MHz to generate second IF 455KHz.
Transmit, the VCO of the PLL operates 13.4825MHz(CH1) to 13.7025MHz(40CH). The VCO frequency
goes to the doublers circuit DW2, LT1, LT2 which doubles the frequency to generate 26.965MHz(CH1)
to 27.405MHz(40CH).
QV1
VCO
QW1
Doublers
13.4825MHz(CH1)
~
13.7025MHz(CH40)
26.965MHz(CH1)
~
27.405MHz(CH40)
To transmitter
The VCO operating frequency for the receiver is 16.27MHz(CH1) to 16.710MHz(CH40) as the
first local oscillator, injected through the buffer amplifier Q302 into the first FET balanced mixer QR2,3.
4-2.
Basic Synthesis Scheme
The crystal frequency(10.24MHz) is divided by 4069times to make 2.5KHz which is fed to one
side of the phase detector. The VCO output is divided by a programmable divider, and fed to other side of
the phase detector Pin 22, 23 of IC6.
The feedback loop is closed by passing the phase detector output through an active low pass filter and using
the output to control the VCO frequency through varicap diode DV1,2.
Under locked conditions, both of phase detector input signal must be identical at 2.5KHz.
The VCO frequency is then given by ;
Receiver
: Fvco / N = 0.0025 [MHz] OR Fvco = 0.0025 X N [MHz]
Since “N” is an integer, the VCO frequency can be stepped up with 2.5 KHz increments.
By suitable choice of “N” the desired output frequency can be obtained.
Channel 1
Channel 40
N Fvco N
Fvco
Transmit 5393
3254
13.4825
16.2700
5481
3342
13.7025
16.7100
Receiver
See the [Table 1]for other Channels
The VCO frequency goes to the doublers circuit, with doubles the incoming signals.
VCO Output Frequency
Doublers Output Frequency
Содержание TCB-550
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