FILTER SET

TM 11-5820-918-13

potentiometers R2, R10, R18, R6, and R14 are factory set.

4-60. FILTER SET (figure FO-35) (S/N 400100 and before). The filter set comprises

eight sequentially- enabled, half-octave, low -pass filters which are used to attentuate

harmonics generated by the 5018 power amplifier. The eight half-octave filters cover

a 2-30 MHz transmit frequency range and are individually enabled by the filter decode

assembly as the sweep progresses. For example, a sweep beginning at 2 MHz enters

at J1 (figure FO-35) and passes into filter FL1 whose input (CR1) and output (CR2)

diode gates are enabled by forward biasing provided by the filter decode assembly.

The remaining seven pairs of diode gates are reverse biased by a - 270-volt potential

developed by the filter decode circuit. Note that C16 on schematic figure FO-35 is

only included in the last filter. As the sweep passes through 2.8 MHz, the CR1 and

CR2 gates are disabled by reverse biasing (- 270 volts at filter select input 1); thereby

removing FL1 from the RF circuit (figure FO-26). At the same time, FL2 is enabled

by switching its filter select input to +5 volts. Due to a permanent bias of -250 volts

at the junction of R1 and R2, the +5 volt reverse biases CR1 and CR2 by 255 volts.

This again causes a current flow such that CR3 and CR4 are forward biased. When

the selection cycle has passed through FL- 7, a direct connection (RG - 58) is enabled,

and only FL8 is in the RF path. This filter, which is at all times in the RF line, now

acts solely as the last half-octave filter for 23-30 MHz. During the blank period be-

tween sweeps, FL8 is the only pass filter selected until the sweep restarts.