Circuit Description - Realistic HTX-100 Service Manual

CIRCUIT DESCRIPTION
1. PLL circuit
The PLL circuit used in this model consists of two 10kHz and 5kHz reference loops, and in the 5kHz loop
1 kHz step is obtained by a 1 /1 0 divider. Moreover, by controlling frequency of the VCXO (oscillator) with
a microcomputer and a D/ A converter, 1 00 Hz step is obtained.
Theory of operation will be given below.
0305 is the reference oscillator (1 0.24 MHz) and supplies the output to two PLL ICs, IC302 and IC305.
The 0305 is also used as the RIT circuit which varies the frequency by
±
1.5 kHz with the varicap (D303)
voltage varied. The reference oscillator signal applied to IC302 and IC305 is counted down to 1/2048 and
1 /1 024 by the respective divider and then fed to phase comparators. 0302 is a VCO for the 1 05MHz band
and the output enters IC302 in passing through 0301 buffer amplifier. IC302 counts down the input and
applies the output to the phase comparator.
The phase comparator output passes a loop filter consisting of 0303 and 0304 and controls 0302 (VCO)
frequency.
fROM
CPU
r
!
"'
6
ori
0301 buffer output is applied to IC303, counted down to 1/1 0, and then applied to IC304 mixer.
0309 is a 22M Hz oscillator which uses a VXO (Variable Crystal Oscillator) circuit to obtain a 1OOHz step
operation. The microprocessor develops 6 bit data and the data are converted into a DC voltage by the
IC307, D/ A converter circuit, and the DC voltage is used as a control voltage for the VXO, thus developing
the 1OOHz step. The oscillating frequency is 22.0000- 22.0009MHz (1OOHz step). The signal is injected
into the IC306 mixer. 0307 output is also injected to the IC306 and the mixed output passes through a
LPF and injected to IC304 mixer.
The IC304 mixer output passes through the LPF and enters a divider inside IC305 and applied to the
phase comparator with the reference oscillator output.
The phase comparator output passes through IC307 loop filter and controls 0306 (VCO) frequency.
The VCO output passes through 0308 buffer amplifier and BPF (L318, L319) and enters the RX/TX mixer
as the local output.
Operations shown above can be given as an equation shown below:
F1
=
(N2/M2)Fr+fo+f1
=
Fr(N2/M2+1 /10 N1 /M1 + fo/fr)
where, F1
=
Local frequency
Fr
=
Reference oscillator frequency 10.24 MHz
M1
=
2048
M2
=
1024
N2 =10kHz step 500- 669 (step 1)
N1
=
1 kHz step 23390- 23408 (step 2) CW
23393- 23411 (step 2) SSB
to=
0.1 kHz step 22.0000 MHz- 22.00009 MHz
When the 500kHz key is pushed, the microprocessor computes N1, N2, and fo required to step up by
500kHz from a current frequency and transfers the data to the PLL circuit.
- 8 -