4
SYNTHESIZER
The primary function of the synthesizer is to provide transmitter injection signal, and local asciIIatar signal
to the receiver. This design allows a single crystal oscillator to provide stable frequency generation without
the need for tuning crystals for each channel. By dividing
the reference oscillator
signal by an amount
respective of the selected channel, the synthesizer can provide a range of frequencies.
Programming
of the synthesizer is accomplished
via the diode matrix board, located at the back of the unit.
Tlıese diodes determine the proper divide ratio for the synthesizer. (Refer to diode programming
instruc-
tions in chapter 4.)
The design used in the SP-605HffM
is a premix PLL configuration
which allows a lower frequencyoutput
from the VGü. This is desired to allow accurate division by the digital divider circuitry.
Voltage
Controlled
Oscillator
(VCO)
-
The VGü consists of oscillator TR11 O and varactor diode
0106. The effective capacitance of 0106 is controlled by the OC tuning voltage. As the tuning valiage is
changed,
the oscillator
frequency
of the VGü follows. The output of the VGü is applied to the doubler
circuits of TR101 and TR103. TR101 feeds the receiver, and TR103 provides signal into the transmitter.
The VGü output is alsa applied to cascaded buffer stages TR111 and TR112 which feed the mixer circuit
for the phased-Iocked-Ioop
(PLL).
Pre-Mix
Local
Oscillator
-
In order to reduce the VGü to a frequency suitable for division by the low
speed logic, the VGü output is mixed with the output of the oscillator/doubler
TR115. In the receive mode,
the VGü output must be offset from the transmit frequency by half of the first IF frequency. This, and a
semi-duplex
split is accornplished
by using a different crystal frequency at the pre-mix oscillator.
Mixer
and
Low
Pass Filter
-
The mixer circuit of TR113 mixes the VGü frequency with the output of
the pre-mix asciIIatar TR115. The desired resultant, which is the difference frequency (or lower frequency
component) is passed by the low pass filter consisting of GH1
OB,
G177, and
G17B.
Pre-Mix
Local
Oscillator
-
In order to reduce the VGü to a frequency suitable for division by the low .
speed logic, the VGü output is mixed with the output of the oscillator/-':~'lbler
TR115. In the receive mode,
the VGü output must be offset from the transmit frequency by half of the first IF frequency. This, and a
serni-duplex split is accomplished
by using a different crystal frequency at the pre-mix oscillator.
Oivider
Buffer
-
The output of the low pass filter is buffered and amplified by TR114 to the level reqired
by programmable
divider IG101.
Programmable
Oivider
-
Oivision of the buffered pre-mix output is accomplished
by programmable
divider IG101. The division ratio is set by the programming
diodes on the diode matrix board. This division
process determines the output of the VGü.
._
Phase
Comparator,
Reference
Oscillator/Oivider"":-
By comparing the divided VGü frequency to
the reference oscillator, the synthesizer can determine if the VGü is on the proper operating frequency, as
determined by the programming
diodes. If a difference is detected, the OG tuning voltage will be changed
to drive the VGü to the correct outpul.
The DG tuning voltage is derived from the fillered ou~put ofphase
comparalar
IC1 03. üutput
from IG103
represents
the difference
between the divided output of. programmable
divider IC1 01, and the 2.5 kHz
output from the reference oscillator/di~ider
IC1 02,. The 2.5 kHz refere0ce
signal is derived from the
10.240 Mhz reference oscillator
and a fixed division ratio in the. referance divider of 4096. Any detected
difference will alter the OC tuning voltage to bring the VCü output to the correct frequency.
