Pll Local Oscillator; Channel Selection Program; Transmitting Frequency Calculation - Realistic TRC-449 Service Manual

1) PLL LOCAL OSCILLATOR
F L, the frequency of the VCO, is controlled by
the PLL synthesizer system as follows. The output
frequency of the VCO (I C5) is appl ied to one of
the inputs of the In-loop Mixer FET4 and mixed
with the off-set frequency wh ich is generated by
the Off-set Oscillator TR34 or TR38 and multi-
plied by three by Tripier TR40.
The resultant difference frequencies are produced
at the output of F ET 4 as follows:
F
1
Output frequency of F ET 4
Fo Off-set frequency, approximately, 11.2858
MHz on AM or USB mode and 11.2842
MHz on LSB mode.
3 x Fo: 33.8575 MHz on AM or USB mode.
33.8525 MHz on LSB mode.
F
1
falls into the range of 910kHz to 1350 kHz in
increments of 10 kHz.
F
1
is applied to the Programmable Frequency
Divider in the PLL Ie, IC7, after being amplified
by TR30 and TR31, and then is divided by the
programmed-divide-ratio, N, by the Programmable
Divider.
The resultant frequency is applied to the Phase
Detector in IC7 and is compared (phase-detected)
with the reference frequency of 10 kHz applied to
another input of the Phase Detector.
The Phase Detector generates the control voltages
for VCO to minimize the difference between F
1
divided by N and the reference frequency.
Thus, under locked condition of the loop, the local
frequency FL is locked stable and is calculated by
the following formula:
FL
=
N
x
10 kHz
+
3
x
Fo
2) CHANNEL SELECTION PROGRAM
The divide ratio, N, of the Programmable Frequen-
cy Divider is determined by supplied voltages to
the program input terminals, Pin No. 13 through
Pin No. 22 of the I C7.
The program input voltages for Pin No. 19, No. 22
and No. 23 are fixed to "Iow level", and the
voltages for Pin No. 13 through No. 18, No. 20
and No. 21 are supplied from the channel selector
switch S408.
The function of the program input terminals is as
follows:
Pin No. : 13 14 15 16 17 18 19 20 21 22
Function : 1A 1B 1C 10 2A 2B 2C 20 3A 3B
Significant 1
2
4 8 10 20 40 80 100200
number
*
Program Input "h igh level" 19 and 20 are
Kept "Iow" (off).
The divide ratio N of the Programmable divider is
provided by the sum of the significant numbers
which are effective by supplying "high level"
input. For example, when the channel switch is
set to Channel No.1, the input of 1 A, 2A and 2D
is in "high level" and the others are in "Iow level".
Therefore, the divide ratio N is determined as
follows:
N
=
1
+
10
+
80
=
91
In the same manner, the divide ratio N for Channel
No. 1 through No. 40 is determined as shown in
Frequency Chart.
3) TRANSMITTING FREQUENCY CALCULA-
TION
For an example, when the unit is operated on
Channel 19, the transmitting frequency is calculat-
ed as shown in the following table for each mode.
AM mode
LSB mode USB mode
N 113 113
113
3 x Fo (MHz)
33.8575 33.8525 33.8575
N x 10 kHz (MHz)
1.13 1.13 1.13
FL (MHz)
34.9875
34.9825 34.9875
FIF (MHz)
7.8025 7.7975 7.8025-
F
t
(MHz)
27.185 27.185
27.185
- 11 -