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

 contains the loop amplifier and voltage-controlled oscillator (VCO) for the 

 phase

lock loop. The 

 VCO tunes from 199.9 to 300 MHz (start frequencies of 200.000001

to 300.000000 MHz) for a range of about 4 V to 16 V tuning of the varactor. The 
VCO drives a counter which divides the frequency by 10. The counter output goes through a
switch and a filter to the 

 output. The 

 VCO is turned off in spans greater than

n xl00 

 but less than or equal to n 

5 MHz to prevent spurious responses.

The loop amplifier consists of a low-noise differential pair input stage, 

 and 

 and

an operational amplifier 

 Feedback ( ac is

)

p rovided by C34 and R29. The output

of the amplifier goes through a programmable ac voltage divider consisting of R31, R32,
R33, R34, R35 with R36 and C36. The signal is filtered by a 40 

 low-pass filter which

has two notches tuned to reject the 50 

 subharmonics created by fractional-n division

(5 MHz   100).

 compares the dc varactor tune voltage to about $17.5 V. If the tune voltage goes higher

than this, 

 switches and pulls it down to about 

 V. Hysteresis in the switching point

of 

 provided by R24 keeps 

 from switching back to the original state until the tune

voltage reaches about 

 V.

 keeps the tune voltage from going below about 

 V. This is to ensure that the 

VCO always oscillates and the varactors do not become forward biased in spans where 

is active.

U5 latches the four most significant bits of the programming of the 

 fractional-n divider.

These are level translated by U4 which drives four FET switches. By changing the voltage
divider in   in this manner, a constant loop bandwidth of 5 

 is achieved.

The 

 VCO consists of 

 which operates in the grounded-base mode. The resonator is

principally CR3, CR4, and L4. Feedback is accomplished with L5, R16, and C17. In spans
greater than n x 100 

 but less than or equal to n 

5 MHz, the VCO is turned off by

forward biasing the varactors. Q5 is a common-base buffer amplifier. The output of the VCO
is applied to Q4 where it is amplified and applied to 

 

 IF and the Divide by 10 

In spans of n 

100 

 the VCO is tuned as low as 199.000200 MHz.

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