FILTER DECODE

TM 11-5820-918-13

Filter

Frequency (MHz)

LPF1

2.0- 2.8

LPF2

2.8- 4.0

LPF3

4.0- 5.8

LPF4

5.8- 8.0

LPF5

8.0- 11.0

LPF6

11.0- 16.0

LPF7

16.0- 23.0

LPF8

23.0 - 30.0

LPF9

2.0- 30.0

As indicated in the table, LPF9 is in the RF path for all transmitted frequencies.

This filter further attenuates any harmonic frequencies above 30 MHz not removed

by the selected filter, LPF1- 8. When LPF8 is selected, the RF signal is fed directly

to the LPF9. LPF8 contains no filtering elements and routes unfiltered RF to LPF9

which removes unwanted harmonics above 30 MHz.

4-49. FILTER DECODE (figure FO-27). Sequential switching for the filter set is

provided by the filter decode circuit. This circuit determines the transmitted fre-

quency and outputs +5 and -250 volts to bias the appropriate filters on and off. The

filter decode circuit uses the 10- watt output from the drive portion of the power am-

plifier. The input, at J1, is divided by two (U2) and gated by U3. The internal 1

MHz reference oscillator (U24) is divided down to 250 Hz which is used to gate the

RF (divided by two) at U3 and to produce counter preset-load-latch timing pulses.

Circuits U12, U5, U6, U 13, and U19 count the gated RF. Following a gate period,

the BCD count is delivered through latches U7, U20, and U26 to the decoder circuit

which determines the particular half-octave filter to be enabled. The transistor cir-

cuits that follow the decoder are the filter drivers which provide the necessary high

power bias for the filter set pin diode switches. The +6 and -270 volt supplies are

used to power the filter drivers. When loaded into the filter set, the output levels

of the drivers are approximately +5 and -250 volts.

NOTE

The later serial numbered units (S/N 400101 and on) provide bias

voltages of +5 and -250 as described. The earlier serial numbered

units (S/N 400100 and before) used bias voltages of +5 and -270.

Other than these outputs, the circuits are the same.

4-50. DIPLEXER (figure FO-28). The diplexer is essentially a high-power, direc-

tional coupler matched on both sides to 50 ohms. The two transformers (T1 and T2)

are configured to couple -17 dB (2 watts) of the chirp transmit signal to the antenna

with a communications transmitter straight through loss of only 0.4 dB. The coupler

also provides the TCS-4B transmitter with 40 dB of isolation from a displexed com-

munications transmitter. Capacitor C1 is used to block DC continuity to ground in

the diplexer during communications transmitter tests for antenna sensing. The two

watts of transmitter signal are added to the communications signal, and the balance

(98 watts) is absorbed by the 50 ohm load resistor (attached to the rear panel). The

0.4 dB of loss on the communications transmitter line is also absorbed by the load re-

sister.

4-16