Filter Module 1A5 Detailed Functional Operation

TM 11-5820-873-34

In the first case, the positive going signal is formed into

strobe pulse when it is triggered by the pulse from the

a 100 to 200 millisecond positive differentiated pulse.

count filled detector ((c) above)

The positive differentiated pulse turns switch 1A4A6Q2

(3) Phase Detector. The 100 kHz reference

on Switch 1A4A6Q2 turns switch 1A4Q6Q1 on,

signal from spectrum generator 1A4A1 (a above) is

generating a blanking pulse. In the second case, the

applied to divide-by-four counter 1A4A4U11. The 25

negative going signal is formed into a 100 to 200

kHz output from 1A4A4U11 is applied to phase detector

millisecond negative differentiated pulse The negative

1A4A4U12. The phase detector compares the nominal

differentiated pulse turns switch 1A4A6Q1 on,

25 kHz output frequency of the preset counter ((2)

generating a blanking pulse In either case, the blanking

above) with the 25 kHz input from 1A4A4U11 and

pulse is applied to spectrum generator 1A4A1 (a above),

develops a DC output voltage that is proportional to the

and disables the 3rd local oscillator signal.

two signals. The output from the phase detector is

coupled to VCO 1A4A5 (e below).

2-10. Filter Module 1A5

Detailed

Functional

e. VCO 1A4A5 (Fig. FO-3 and Fig. FO-11).

(1) Band Oscillators.

The selected 10 MHz

Operation (Fig. FO-4)

FREQUENCY switch enables one of the three band

oscillators through one of the three transistor switches.

a. Filtering. The filter module provides low pass

The band oscillators are Colpitts oscillators similar to

filtering in transmit mode and band pass in receive

voltage controlled oscillator 1A4A2 (b(1) above). Band

mode. The filters have been divided among three

0 oscillator 1A4A5Q7 operates over a frequency range

different boards. Board 1A5A1 has the low pass filters

of 91.25 to 101.2499 MHz Band 1 oscillator 1A4A5Q7

for the odd number channels. Board 1A5A2 has the low

operates over a frequency range of 101.25 to 111.2499

pass filters for the even number channels. Board

MHz. And, Band 2 oscillator 1A4A5Q8 operates over a

1A5A3 has high pass filters. Only the low pass filters

frequency range of 111.25 to 121.2499 MHz. Coarse

are used in the transmit mode. The high pass filters are

frequency tuning is provided by the action of the course

used, along with the low pass filters, for band pass

frequency voltage from the selected 10 MHz

filtering in the receiver mode. Correct filtering for each

FREQUENCY switch. Fine frequency tuning is provided

band is accomplished automatically when the front

by the DC voltage from VHF divider 1A4A4 (d(3)

panel frequency control switches are set. Inactive

above). The output from the enabled band oscillator is

filters, for any given band, are switched to ground

coupled through amplifier 1A4A5Q11, 1A4A5Q12 to

through a 10 ohm resistor. This discharges the filter and

translator 1A4A3 (c above) and through amplifier

prevents unwanted interaction with active filters. Table

1A4A5Q9, 1A4A5Q10 and the 91 to 122 MHz bandpass

2-2 lists the filter band numbers and their corresponding

filter to receiver/exciter 1A3 (para 2-4).

frequency ranges.

(2) Binary Coded Decimal Converter. The BCD

converter consists of two transistor switches, 1A4A5Q1

Table 2-2. Band Numbers and Frequency Ranges.

and 1A4A5Q2, that convert the ground signal from the

selected 10 MHz FREQUENCY switch to tens

complement binary coded decimal. The output from the

BCD converter is applied to the 10 MHz preset gate

(d(2) above) in VHF divider 1A4A4.

f. Local Oscillator Blanker (Fig. FO-23). The local

oscillator blanker protects the power amplifier from

transients caused by a frequency change while

transmitting. When the selected 1 MHz FREQUENCY

switch setting is changed, the synthesizer may change

frequency by several MHz while the switch is

momentarily open. If this occurs while transmitting, it

b. Filter Selection. Correct filter selection for the

would result in a large transient that could damage RF

band in use is setup through wafer switches driven by a

power amplifier 1A7A1.

DC motor. The motor (B1) is mounted on the Motor

When the setting of the selected 1 MHz

Control Board 1A5A4. Motor control is accomplished

FREQUENCY switch is changed, the "preset 20 1 MHz"

with an "open seeking" circuit comprised of relay

frequency control line will change from either:

1A5A4K1 and wafer switch 1A5A4S1. When changing

logical "0" (approximately 0.2 volts) to

the 1 MHz or 10 MHz switch of the selected frequency

logical "1" (approximately 3 volts) state

changes the band, relay 1A5A4K1 is energized thru

1A5A4S1. The now closed contacts, 5 and 9, of

logical "1" to logical "0" state

1A5A4K1 supply + 12 VDC to motor 1A5A4B1. The

motor simultaneously rotates the wafer switches on

2-14