AIOAI
AIOAI Phase Lock Loop 1 (PLLI) Voltage-Controlled Oscillator (VCO), Circuit Description
AlOAl contains the loop amplifier and voltage-controlled oscillator (VCO) for the PLLl phase lock loop. The PLLl 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 PLLl VCO drives a counter which divides the frequency by 10. The counter output goes through a switch and a filter to the 20/30 output. The PLLl VCO is turned off in spans greater than n xl00 kHz but less than or equal to n x 5 MHz to prevent spurious responses.
Loop Amplifier @
The loop amplifier consists of a low-noise differential pair input stage, Q9 and &lo, and an operational amplifier U6A. 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 kHz low-pass filter which has two notches tuned to reject the 50 kHz subharmonics created by fractional-n division (5 MHz i 100).
U6B compares the dc varactor tune voltage to about $17.5 V. If the tune voltage goes higher than this, U6B switches and pulls it down to about +5 V. Hysteresis in the switching point of U6B provided by R24 keeps U6B from switching back to the original state until the tune voltage reaches about +5 V.
Q2 keeps the tune voltage from going below about +3 V. This is to ensure that the PLLl VCO always oscillates and the varactors do not become forward biased in spans where PLLl is active.
Gain Switch @
U5 latches the four most significant bits of the programming of the PLLl 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 kHz is achieved.
200 to 300 MHz VCO @ and Output Amplifier @l
The PLLl VCO consists of Qll 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 kHz but less than or equal to n x 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 AlOA3 PLLl IF and the Divide by 10 0. In spans of n x 100 kHz, the VCO is tuned as low as 199.000200 MHz.
