Instruction manual

TM 11-6625-3017-14

SECTION 3

TECHNICAL DESCRIPTION

3.1 SYSTEM OPERATION

General operation of the TF 2300A Modulation

Meter can be explained with reference to the block

schematic, Fig. 3-1.

The r. f. input is heterodyned in the mixer with the

local oscillator output, producing an intermediate

frequency signal of 1.5 MHz. The output of the mixer is

fed to an i.f. Amplifier which has a linear

phase/frequency response to f. m.

From the i. f. amplifier, the signal can be passed

through the f. m. or the a. m. sections, depending on the

positioning of the Function switch.

F.M. sections

From the i. f. amplifier, f. m. signals, as illustrated in

Fig. 3-2 (a), are passed through three limiting stages to

eliminate all amplitude changes and produce a

rectangular waveform as shown in (b).

This limited i. f. waveform is passed to the pulse

counter discriminator whose operation is illustrated in (c),

(d) and (e). A pulse of fixed amplitude and width is

generated every time the clipped i. f. signal passes

through zero in the positive-going sense as shown in (b)

and (c). At any given repetition frequency, these pulses

have a constant mean amplitude, Vm, provided the pulse

amplitude and width are fixed; thus when the p. r. f.

varies due to f. m. of the input, the mean amplitude will

also vary directly as the modulation frequency. This is

illustrated in (c) and (d). In practice the limiter output is

fed to a Schmitt trigger circuit, the resultant constant

rise-time rectangular waveform being differentiated and

used to drive a pulse generator. The pulses from here

are later passed through a low-pass filter to remove all

but the modulation frequency components.

The l. f. signal (e) is then amplified in the 1st l. f.

amplifier, the gain of which can be standardized by the

SET CAL-F. M. front panel preset in conjunction with the

calibrator, and passed through the 200 kHz low-pass

filter and, if required, to restrict the bandwidth to the

audio range, through the 15 kHz low-pass filter.

Fig. 3-1. Block schematic diagram.

3-1