If the programmable divider value of N is increased by 1 to N+1, then for phase lock to occur
Fo'/N+1 = Fr, where Fo' is the new VCO frequency. By simple transposition of this equation it
can be shown that Fo' = Fo+Fr. Therefore, by setting Fr to equal the channel spacing, it is
possible to select a particular channel by choosing the appropriate value for N, providing of
course that the required frequency is within the electronic tuning range of the VCO.
When the synthesizer is used as the transmitter exciter, modulation must also be applied. This
is achieved using the two-point modulation technique. The VCO is directly modulated but
because this signal will be seen as a phase error which the loop will attempt to correct, it is
necessary to apply modulation to the reference oscillator.
The modulation polarity for the VCO and reference oscillator must be the same, ie, a
frequency increase of the reference must occur with a frequency increase of the VCO. Test
point TP401 is provided to align the modulation balance. When R567 is adjusted correctly the
error waveform at the analog phase comparator output will be reduced to a minimum
amplitude.
3.2.3.3
THE PRM80 SYNTHESIZER
Refer to figure 3.7.
The phase locked loop synthesizer of the PRM80 consists of discrete transistor voltage-
controlled oscillators for both the transmitter and the receiver. Both VCOs have two buffer
amplifier stages. The output of each VCO is combined in a passive resistor combining network
and a buffer amplifier is provided ahead of the synthesizer frequency divider chain. The
frequency dividers consist, of a dual modulus (divide by 64 or 65) prescaler device (Plessey
SP8718), followed by the programmable divider IC (Plessey NJ8822). In this integrated circuit
are two programmable dividers, one for the division of the VCO frequency, and the other for
division of the reference oscillator. The NJ8822 also contains the phase comparator which is
provided in two forms, one as a frequency detector and the other as a sample and hold phase
detector. The NJ8822 provides the control logic to operate the dual modulus prescaler, and a
logic output to indicate the locked state.
The output of the phase comparators are coupled to the loop filter which is realized by half of
a low noise dual operational amplifier IC. The other half of the dual OP AMP is used as a
tracking circuit to provide an appropriate control voltage for the receiver front end.
The reference oscillator consists of a discrete transistor crystal oscillator, with a voltage
control input for modulation. For higher temperature stability, a Temperature Compensated
Crystal Oscillator option may be fitted.
3.2.3.4
PRESCALERS AND PROGRAMMABLE DIVIDERS
IC402 is the prescaler.
The purpose of the prescaler is to divide down the operating frequency of the VCO, into a
frequency range which is low enough for the programmable divider device which uses low
current CMOS technology.
Section 3 Page 13
