HP 5305 B, Operating And Service Manual

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