charge of diodes D1134 and D1139. When the charge has
The back bias across D1116 is controlled by INT REF FREQ
dissipated, the diodes generate a fast recovery step. This
control R1106. This change in back bias increases or de-
recovery step is differentiated and coupled through trans-
creases the diode capacity and shifts the resonant frequency
mission line transformer T1140, T1150 and T1160 to the phase
of crystal Y1104. The pulling range on the crystal frequency
detector as a series of equal amplitude positive and negative
by the INT REF FREQ control is about 1 kHz. This is sufficient
strobe pulses.
to maintain phase lock condition through frequency gaps
that occur above 1 GHz when the oscillator shifts phase lock
The phase detector (Fig. 3-4) consists of a two diode
mode,
gate and a low pass filter network. The diode gate is turned
on by the combined application of the local oscillator signal
When the local oscillator shifts to a different lock mode,
and the very narrow strobe pulses, During the on time, the
the fundamental frequency of the oscillator shifts 1 MHz.
phase detector samples the amplitude and phase of the local
This produces frequency gaps in the upper scales which will
oscillator signal and develops a voltage at the output of
shift the signal off screen with dispersions of 100 kHz/div
the filter (C, and the junction of R1, R2) that approximately
or less. The INT REF FREQ control shifts the reference
equals the instantaneous value of the local oscillator signal.
oscillator frequency about 0.1% (1 kHz). This shifts the local
oscillator by the same percentage, so the frequency gaps
The sample of the local oscillator signal has a finite width
between lock modes are filled. If the observed signal should
determined by the duration of the strobe pulse. The phase
shift off screen, it can be returned on screen or slid along
detector operates on either the positive or negative slope
the display by the INT REF FREQ control.
of the local oscillator signal, depending on the total dif-
ference between the detector output voltage and the phase
The pulse generator consists of tunnel diode D1124, driving
of the local oscillator signal. The strobe pulse width, there-
amplifier Q1120. The quiescent current of tunnel diode
fore, must not exceed one-half period of the highest local
D1124 is approximately 2.5 mA. The positive-going portion
oscillator input frequency, which is 4.2 GHz. This period is
of the input reference signal switches the tunnel diode to its
0.21 ns.
high state and a fast rise positive pulse is generated. The
pulse is amplified and differentiated by Q1120 and the short
If the input local oscillator frequency is not a harmonic
RC time constant in the emitter circuit.
of the reference frequency, the output of the phase detector
is approximately zero. H o w e v e r , a s t h e l o c a l o s c i l l a t o r
The output pulse of Q1120 is transformer coupled through
frequency approaches a harmonic of the reference frequency,
T1128 to Q1121. The positive portion of the coupled pulse
an AC or beat frequency signal is developed at the detector
is of sufficient amplitude to trigger Q1121 into avalanche,
output. This is amplified and applied through the LOCK
and the resulting collector current sweeps out the stored
3-4
