HP (Hewlett-Packard) HP 85660B ユーザーズマニュアル

The output of the 300 MHz Power Amplifier, approximately 

 dbm, is the local oscillator

for the 

 Last Converter. Capacitor 

 reduces the sub-harmonic content of the output.

The output filter C52, L21, and C53, reduces higher order harmonics while maintaining a 
output impedance.

Amplifier U2 drives the Sampler step-recovery diode (function block E) with the 100
MHz signal at a level of approximately 

 

 Capacitors C8, 

 and L4 match the

forward-biased impedance of the diode to U2. Resistor R5 loads the output of U2 during the
diode’s reverse biased condition.

The output of the sampler is a dc voltage proportional to the phase difference between the
33rd harmonic of the 100 MHz reference and the 3.3 

 second local oscillator 

Second Converter). Integrator U3 supplies a tune voltage to the 3.3 

 second local

oscillator. This heterodyne loop phase locks the oscillator to the 100 MHz reference signal.

Potentiometer R38 adjusts the output balance of the sampler. Capacitor C46 is the integrator
capacitor for U3.

The output tune voltage from the Loop Integrator (function block F) becomes   25 V when
the 3.3 

 Heterodyne Loop is unlocked. The Aided Acquisition circuitry detects this

condition and retunes the 3.3 GHZ oscillator on the 

 Second Converter assembly within

the capture range of the phase-lock loop. The Aided Acquisition circuitry also controls the
Lock Indicator (function block J).

The Negative Rail Detector 

 is an inverting comparator. The threshold voltage at pin 5 is

set at -23.5 V. During unlock, the input of 

 becomes more negative than the threshold

voltage and the comparator output becomes positive. Resistor R21 provides positive feedback,
stabilizing the circuit. Resistors R25 and R26 form a voltage divider reducing the input to

 below the value of the power supplies. The positive output of 

 triggers One Shot

multivibrator 

 The negative output of 

 forward biases diodes CR1 and CR2. The

diodes become a constant current source for integrator capacitor C46 (function block F),
causing a positive direction search ramp of voltage at the output of U3. This search ramp
tunes the 3.3 

 second local oscillator through the capture range of the phase-lock loop.

If the phase-lock loop does not phase-lock on the search ramp, the tune voltage will remain
positive until the One Shot recovers, reverse biasing CR1 and CR2. The output of U3 then
becomes negative (unlock condition), causing the Aided Acquisition circuitry to repeat the
search cycle until phase-lock occurs. This results in a sawtooth tune waveform with a period
of approximately 6 ms.

The One Shot pulse width of approximately 1 ms is controlled by R24 and C38. This is the
time it takes for the voltage on pin 7 to decay to -25 V to less the -20 V. During this time,
the input to 

 has become -20 V due to the positive tune voltage ramp. In high band the

3.3 

 second local oscillator is switched 

 the PIN signal goes negative, which disables

the search ramp and unlock detector through CR3.

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