HP 5305 B, Operating And Service Manual

Page: 18 / 48

9E-4-20. Assume, however, the channel B frequency
increases. This causes the negative pulses at U22(1) to
arrive a little earlier than before, arriving ahead of the
edges at pin 3. When an edge arrives at pin 1, the UP
output goes active low and stays low until the edge at pin 3
arrives. The UP output, then, begins generating active low
pulses. They are inverted into positive pulses that forward
bias CRB Current flows through CRB and RBI into the
integrator amplifier U23, causing the tuning voltage to
integrate downward, this increases the PLO's output and
increases the feedback frequency at U22 pin 3. This
process continues until the signals at U22 pins 1 and 3 are
again matched in frequency and phase. The phase detector
outputs are inactive high, and the loop is at reset.
9E-4-21. THE VCO. The phase detector's corrections are
integrated and saved by the loop amplifier, an integrating
amplifier (refer to A1 schematic). This makes a tuning
voltage that sets the frequency of the VCO (voltage-
controlled oscillator). The oscillator is a sawtooth
generator driving a Schmitt trigger. The voltage-controlled
current source (Q16) charges capacitor C42 in a linear
ramp until the ramp reaches about +2.4 volts. This goes
through an emitter follower (Q17) and appears at U24C(9)
at + 1.8 volts. This is the upper trigger point on the
Schmitt gate. The output of U24C snaps low and
discharges C42 through CR21 to about +0.6 volts. This is
below the negative threshold, so the Schmitt output snaps
high and the sawtooth starts again. The negative pulse
train at U24C(8) is used as the PLO output, and it is this
frequency that is counted by the decade counter. The
waveforms would appear as follows in Figure 9E-4-2.
Figure 9E-4-2. VCO Waveforms at about 2 MHz
9E-4-22. When the VCO is running at much less than 10
MHz, it becomes very difficult to see the narrow pulses at
U24C(8) on a scope. It is easier to see the sawtooth or a
squarewave at half the VCO frequency at U21(9).
Model 5305B
Theory of Operation
It is important to remember that the frequency/tuning
voltage (U23 pin 6) is a negative relationship. That is, the
lower the tuning voltage, the higher the frequency.
9E-4-23. SEARCH INDICATOR CIRCUIT. The VCO
runs continuously whether the loop is locked (normal
operation) or not. In the high resolution range, there is
always a signal from the VCO to the 5305B counting
circuitry, whether or not there is a signal at the Channel B
input. As a result, the counter is always armed and tries to
count the VCO frequency (typically 25 kHz) even without
an input. This is prevented by the search indicator circuit,
U16A, U18C, U24A, Q19.
9E-4-24. When the detector's inputs are in phase, the U22
output pins (2 and 13) are always TTL high. If the loop is
not locked, one or the other will be low much of the time.
Whenever either is low, U24B(6) is high, signaling an
error in the loop. These error pulses are averaged by R77,
R78, and C36. If the errors are large enough, the LOST
line from comparator U16A will snap low in about 50
msec. In the high resolution range, LOST passes through
gate U18C and becomes STOP (active high). STOP lights
the SEARCH lamp (S annunciator) through Q14 and gates
off the 10 MHz clock at U10A. This prevents the counter
from continuing with more measurements or display
updates and saves the last valid reading in the display.
LOST enables gates U18 Band D, which pass the phase
correction signals through resistor R76 (5.1K Ω ). R76 is in
parallel with R81 (82K Ω , so R81 is effectively reduced
from 82K ohm to about 5K ohm. This greatly increases
loop bandwidth and allows for rapid searches and short
acquisition times.
9E-4-25. When an in-range input signal is restored to
Channel B, the loop will lock. The phase corrections stop,
U24B(6) (the error signal) stays low, and C36 discharges
through R77 (100K ohm). As the average error signal
drops t1 volt, the lower threshold, U16A(1) snaps
high, and STOP goes low. U24A(3) inverts this negative
edge into a positive edge and Q19 is switched on for about
0.1 seconds. This clears all the counters and the time base,
so a new measurement is begun, making the first reading
correct. Meanwhile the SEARCH light is switched off,
and the loop bandwidth is reduced to a low value to allow
proper stability for accurate measurements.
9E-4-26. NON-LINEAR VCO CHARACTERISTIC.
When no signal is available at the PLO input, the VCO is
tuned to its lowest frequency (about 25 kHz). If a high
frequency (e.g., 10 kHz) is applied, the VCO must be
slewed all the way to 10,000 kHz to achieve lock. This
would take a loop that is stable at 50 Hz almost two
minutes to achieve lock. Therefore, gates U18 Band D,
were added, and the VCO characteristic has a bend in it.
See Figure 9E-4-3.
9E-4-3