FREQUENCY SYNTHESIZER MODULE A2.

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

power amplifier module A8 and to receive injection

to start a new cycle. For the complete cycle, the number

frequency to receiver rf module A3. Refer to figure 4-1.

of pulses counted by the variable divider is 2 [(41)(A) +

The p11 frequency synthesizer operates as follows: The

variable divider divides the output frequency (FO) by N

40 (Np - A)]. (In the example frequency of figure 4-1, the

and applies the resulting frequency (FV) to the phase

number of pulses counted is 2 [(41)(35)- 40(325-35)] or

detector. The phase detector compares FV in both

26.070.) The total number of pulses counted is the

frequency and phase to the reference frequency (FR).

division ratio (N) of the variable divider. The system

The frequency or phase difference between the two

equation for the phase- lock loop is, therefore, given by

signals produces an error signal which is filtered and

the following equation:

applied to the voltage-controlled oscillator (vco). The vco

changes frequency until FV equals FR. Phase lock

FO = (FR)(2) [41 (A) + 40 (Np - A)]

occurs when FV equals FR and their phase difference is

Where:

constant, The system equation is FO = FR x N.

4-62. The variable divider functions as a counter

FR = reference frequency

where one output pulse is generated for every N input

pulse. The variable divider consists of a divide-by-2 fixed

prescaler, a divide-by-Np ( Np) programmable counter, a

A = preset count set into

divide-by-A ( A) programmable counter, and a 2-

A programmable counter

modulus prescaler. The A programmable counter is

programmed (preset to a certain count) by the three least

Np = preset count set into

significant digits of the radio control while the -Np

Np programmable counter

programmable counter is programmed by the three most

significant digits of the radio control and the

transmit/receive key line. The 2-modulus prescaler is

4-64. When operating in the receive mode, the

controlled by the A programmable counter to divide by

transmit/receive key line causes the division ratio of the

either 40 or 41.

Np programmable counter to decrease by 30. This

causes a 30-MHz decrease in the fre- quency

4-63. In the 2-modulus prescaling technique, fre-

synthesizer output frequency. When mixed with the

quency division occurs as follows: At the beginning of a

received signal, the difference results in the 30-MHz if

cycle, the A programmable counter is preset to a count

frequency.

of A and the Np programmable counter to a count of

Np as determined by the radio control. (For the example

4-65. The condition for phase lock occurs when the

frequency given in figure 4-1, A is 35 and Np is 325.) The

phase difference between the variable divider output and

2-modulus prescaler divides the output of the fixed

frequency reference output becomes constant at a value

prescaler by 41 to produce one output pulse for every 41

necessary to produce a vco frequency that when divided

input pulses (82 cycles of FO). The output pulse causes

by N produces 12.5 kHz at the phase detector. The

the preset count of the -A programmable counter and

output frequency of the variable divider locks onto and

+Np program- mable counter to decrement by 1. (These

tracks the phase of the reference frequency. The phase

counters count down from the preset count to zero.) This

difference between the variable divider output and the

process continues until the A programmable counter is

reference frequency produces a correc- tion voltage at

decremented to zero. When the

A programmable

the input to the vco. The level of this correction voltage

counter reaches zero count, it feeds back a control signal

is such that it will pull the vco frequency in a direction that

to the 2-modulus prescaler to change its division ratio

will cause the variable divider output to phase-track the

from 41 to 40. To this point, the total number of pulses

reference frequency. Thus, for each setting of the radio

(cycles of FO) counted is 2 x 41 x A. Since the Np

control, a digital code presets the variable count to an

programmable counter decrements by the same number

integral number (N) representing a desired out put

of counts as the A programmable counter, the count

frequency. For each desired output frequency there is a

remaining in the -. Np programmable counter is Np A.

corresponding unique phase difference and tuning

As the cycle continues, the output of the 2-

voltage to the vco. At each tuned condition the variable

modulusprescaler continues to decrement the

divider output is phase locked to the reference frequency

programmable counter except now it only requires 40

and the two frequencies are equal to 12.5 kHz. The

input pulses to produce one output pulse. When the

output fre- quency (vco frequency) is therefore equal to

Np programmable counter is decremented to zero, it

the following:

produces one output pulse to the phase detector and at

the same time resets the Np programmable counter

vco frequency = N x 12.5 kHz

and A programmable counter to the preset count of Np

and A, respectively.

4-66. Refer to figure FO-5 for a block diagram of

frequency synthesizer module A2. The low-band,

4-11