resultant feedback signal from a frequency to voltage con-
verter and ramp generator. Any differential between the
two signals is a voltage output that controls the bias on
frequency-determining capacitance diode D314. D314 is part
of the swept oscillator circuit which shifts the swept oscil-
lator frequency by an amount proportional to the signal
output from the comparator.
Q240 is the constant current source for the sweep com-
parator. About 3.4 mA of current through the comparator
is set by the bias of Q240. Output DC level at the collector
of Q230 is set by the IF Center Freq Range adjustments in
the collector circuit of Q260.
Sweep Oscillator. The oscillator frequency is primarily
a function of the L (L314) and C (diode D314 in series with
blocking capacitor C314) in the collector circuit of Q310.
Capacitance of diode D314 is varied by the signal from
comparator Q230-Q220. An increase in back bias decreases
the capacitance of the diode and increases the frequency of
the swept oscillator. Capacitance change of the diode is
not proportional to the voltage ramp, but high gain in the
discriminator feedback loop to the comparator reduce this
non-linearity. At maximum dispersion, the oscillator sweeps
from 225 MHz to 325 MHz.
Output signal from the oscillator is tapped across the
partial winding of L314 and capacitively coupled to trans-
formers T330 and T331. The transformers step the voltage
up about 2:1 and converts the single-ended signal to a bal-
anced push-pull drive signal for the output amplifier Q340
and Q350.
Fig. 3-6 is a simplified drawing of the transformer circuit.
The oscillator is the signal source or generator which supplies
the signal voltage (e). The input windings of T330 and T331
are connected in series; therefore, the voltage across each
winding equals e/2 (assuming an ideal transformer). The
polarity of the signal at a particular instant of time is shown
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.
The generator, or source, is in series with the output wind-
ing for T331, therefore, the voltage ot the output with refer-
ence to point A equals 3e/2. This voltage adds to the volt-
age output of T330 to provide a total output signal of 4e/2
or 2e.
If the reference point is changed to the common side of
the input windings of T330 and T331 (shown os a phantom
ground on the simplified drawing) the impedance looking
into the output terminals of the transformers is balanced,
Fig. 3-5. Block diagram of sweeper circuits.
3-6
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