Fig. 3-5. Block diagram of sweeper circuits.

resultant feedback signal from a frequency to voltage con-

Output signal from the oscillator is tapped across the

verter and ramp generator. Any differential between the

partial winding of L314 and capacitively coupled to trans-

two signals is a voltage output that controls the bias on

formers T330 and T331. The transformers step the voltage

frequency-determining capacitance diode D314. D314 is part

up about 2:1 and converts the single-ended signal to a bal-

of the swept oscillator circuit which shifts the swept oscil-

anced push-pull drive signal for the output amplifier Q340

lator frequency by an amount proportional to the signal

and Q350.

output from the comparator.

Fig. 3-6 is a simplified drawing of the transformer circuit.

The oscillator is the signal source or generator which supplies

Q240 is the constant current source for the sweep com-

the signal voltage (e). The input windings of T330 and T331

parator. About 3.4 mA of current through the comparator

are connected in series; therefore, the voltage across each

is set by the bias of Q240. Output DC level at the collector

winding equals e/2 (assuming an ideal transformer). The

of Q230 is set by the IF Center Freq Range adjustments in

polarity of the signal at a particular instant of time is shown

the collector circuit of Q260.

in Fig. 3-6. This voltage, across the input windings, produces

an equal voltage (e/2) across the output windings with the

polarity as indicated in Fig. 3-6.

S w e e p O s c i l l a t o r . The oscillator frequency is primarily

a function of the L (L314) and C (diode D314 in series with

The generator, or source, is in series with the output wind-

blocking capacitor C314) in the collector circuit of Q310.

ing for T331, therefore, the voltage ot the output with refer-

Capacitance of diode D314 is varied by the signal from

ence to point A equals 3e/2. This voltage adds to the volt-

comparator Q230-Q220. An increase in back bias decreases

age output of T330 to provide a total output signal of 4e/2

the capacitance of the diode and increases the frequency of

or 2e.

the swept oscillator. Capacitance change of the diode is

If the reference point is changed to the common side of

not proportional to the voltage ramp, but high gain in the

the input windings of T330 and T331 (shown os a phantom

discriminator feedback loop to the comparator reduce this

ground on the simplified drawing) the impedance looking

non-linearity. At maximum dispersion, the oscillator sweeps

into the output terminals of the transformers is balanced,

from 225 MHz to 325 MHz.

Fig. 3-5. Block diagram of sweeper circuits.

3-6