SYNTHESIZER - TM-11-5820-917-130090

TM 11-5820-917-13

NOTE

The synthesizer module is used interchangeably in both transmit (TCS-4B)

and receive (RCS-4B) applications. A programming line in the instrument

wire harness determines whether the synthesizer operates in the transmit

or the receive mode.

The sweep synthesizer module has five RF outputs: (1) the 42-70 MHz first mixer

L.O. injection for the receiver; (2) the 40 MHz second L.O. receiver injection;

(3) a buffered 5 MHz from which the receiver third mixer L.O. injection is derived;

(4) the 2.1- 3.5 MHz count output which is used by the frequency counter in the

TCS-4B transmit logic, or RCS-4B receiver control logic to drive the front panel

LED frequency display; and (5) the 2-30 MHz transmit sweep output which drives

the TCS-4B RF power amplifier or the RCS-4B receiver calibrator circuits. The

one RF input to the sweep synthesizer is the 5 MHz frequency standard signal

from which all RF outputs are derived. The synthesizer digital inputs select

parameters such as: sweep rate and limits; sweep start, stop and reset; RF blank-

ing; slip; auto sync and RF output power level. The module consists of three cir-

cuit card assemblies: synthesizer 2A1A1, down converter 2A1A2, and sweep

programmer 2A1A3.

4-72. SYNTHESIZER (figures FO-25 and FO-26). The 5035-2001 synthesizer cir-

cuit (2A1A1) is a digitally programmed, phase-locked-loop synthesizer capable of

generating any frequency between 42 and 70 MHz to 1 Hz resolution. It consists

of a 42-70 MHz VCO, a programmable divider (divide-by-N), a phase detector and

loop amplifier, and control logic (phase register and timing generator). Figure

4-11 is a simplified diagram of the circuit. A detailed functional block diagram is

in figure FO-25.

a. The output frequency of the VCO (and the synthesizer) is determined by

electrically tuning the VCO with a control voltage from the loop amplifier. The loop

amplifier produces this control voltage by integrating (smoothing) the phase-error

signals generated by the phase detector. If there is no phase error, the output of

the phase detector is zero and the loop amplifier will hold the VCO at its existing

frequency. If there is a phase error the phase detector will drive the loop ampli-

fier to change the VCO frequency until the error is corrected. The synthesizer

uses the phase detector to compare the output of the divide-by-N counter with a

fixed 100 kHz reference signal. If the phase or frequencies of these two signals

do not match, the phase detector will drive the loop amplifier to adjust the VCO

frequency until the divide-by-N output exactly matches the 100 kHz reference,

thereby achieving phase lock. The VCO output frequency is always N times 100

kHz. There are N cycles of the VCO output for every one cycle of the 100 kHz

reference. If N is an integer number, the VCO frequency will be an exact multi-

ple of 100 kHz. However, if N is a number consisting of both integer and fraction-

al components, intermediate frequencies between 100 kHz points may be synthe-

sized. For example, to produce a 43.5 MHz output the divide-by-N counter must

divide by 435. If an output of 43.501 MHz is desired, the required divide ratio is

435.01. The divide-by-N counter, however , is a 3 decade counter only capable of

dividing by integer numbers between 400 and 700. To divide by 435.01 the phase

and divide by 436 for the remaining 1%. The resulting average divide number is

(99+ 435) + (1 + 436)= 435.01

100

4-32