4028 HF RECEIVER

TM 11-5820-917-13

4028 HF RECEIVER - Unit 2

4-64.

4-65. The HF receiver unit consists of three principal assemblies: the sweep

synthesizer, receiver, and power supply. The receive antenna (figure 4-1) is

selected by the antenna switch and applied to one of eight preselector bandpass

filters. The filtered RF is then amplified, filtered and translated to baseband

and audio signals. The receiver has bandwidths of 500 Hz and 5 kHz. The

baseband signal is sent to the spectrum analyzer in the 6025 unit. Additionally,

the receiver translates the same baseband information to an audio band starting

at 700 Hz which is applied to a speaker for operator monitoring. The receiver

frequency tuning is determined by the sweep synthesizer. Ranging from 42.2

to 70.2 MHz, the local oscillator is 40.2 MHz higher than the tuned frequency.

The first LO is filtered by a bandpass filter before going to the receiver. The

synthesizer receives a 5 MHz reference and the start, stop, reset, sweep rate, and

frequency-limit programming from the 6025 unit.

4-66. SWEEP SYNTHESIZER (2A1) (P/N 5030-1001 Only) (figures 4-9, FO-23,

and FO-24). The sweep synthesizer consists of two circuit card assemblies: the

microphage synthesizer, operating between 42 and 70 MHz, and the synthesizer

converter, which offsets the microphage output by 40 MHz to produce the 2-30

MHz sweep. In addition, the synthesizer module has two fixed frequency outputs,

5 and 40 MHz, used in the receiver, and a 2.1- 3.5 MHz count output used by the

frequency counter logic. Logic to control the frequency programming of the syn-

thesizer RF sweep is contained on the converter assembly. This logic digitally

increments the synthesizer frequency program every 20 microseconds to produce

a linear frequency sweep. External control inputs to the synthesizer converter

logic provide means to start, stop, reset, and blank the sweep and to select sweep

limits and rates. Transmit blanking of the 2-30 MHz output is implemented by dis-

abling the 40 MHz mixer conversion of the 42-70 MHz signal. All synthesis opera-

tions are based on an externally supplied 5 MHz standard.

4-67. The synthesizer module may be used without modification in either the

receiver or transmitter. For TCS-4B applications the 2-30 MHz output is used

directly to drive the 5018 RF amplifier. For RCS-4B applications, the basic RF

sweep is offset 200 kHz higher to produce a 42.2-70.2 MHz receiver 1st L.O.

from the microphage synthesizer and a 2.2-30.2 MHz receiver calibration signal

from the synthesizer converter. Selection of transmit or receive frequency for-

mats is provided by a digital programming line in the unit wiring harness connect-

ing to the sweep synthesizer assembly.

4-68. The sweep synthesizer uses a single digital phase-lock loop (PLL) design

employing a fractional phase computation technique that provides a phase-con-

tinuous (coherent) output sweep with 2 Hz frequency resolution. The fractional

phase computation technique is a hybrid approach that combines the operation of

a conventional phase coherent, high frequency, low resolution, PLL synthesizer

and a digitally controlled, low frequency, high resolution, direct phase computa-

tion waveform generator. The result is a PLL synthesizer capable of locking

properly with a continuously changing programmed phase error within the loop.

The programmed phase error capability of this hybrid loop extends the frequency

resolution of the basic PLL by almost five decades.

4-28