Tektronix 2715 Strobe Frequency Generator (VCO Module)

Theory of Operation
2715 Spectrum Analyzer Service Manual 3-25
When the FM coil driver is in use (instrument not phase locked) there aretwo
potential signal sources summed into thedriver circuit. One that is always
present is induced by DAC U850A and subsequent circuits.It is steered into
U821 by switch U757A, and serves to finely set thecenter frequency. The other
is selected only in moderate spans. It i s the sawtooth sweep waveform used to
sweep the 1st LO over spans rangingfrom 50 kHz/div through 5 MHz/div.
Functionally, this module is a voltage controlled frequencysource. It has an
output frequency rangefrom 25.73 MHz through 26.05 MHz in response to a
tuning voltage range of approximately4 V to 12.5 V. There is also a switch to
select which of two signals is countedby the microprocessor, and there are
means to turn off theoutput.
The strobe frequency comes froma VC O that rangesfrom 102.9 MHz through
104.2 MHz and whose output frequency is dividedby four to yield the desired
strobe frequency.Its frequency is controlled by phase locking it (with a 100 MHz
offset) to 1/4 the frequencyof a free running VCO (the LFVCO) that operates
over a frequencyrange of 11.68 MHz through 16.84 MHz. It is this LFVCO that
is actually tuned by the incoming tuning voltagereferred to earlier, and this
phase locked loop within the VCO module is called the inner loop. The strobe
signal is generated in this mannerin order to control close in phase noise and to
provide a countable signal (the LFVCO output) whose frequency is not
condensed into a very narrowrange that would require long times to count to
adequate resolution.
The LFVCO consists of Q426, T430, varactorsCR532 and CR540, and
associated components. Output (from the collector of Q422) is made available
for external counting throughselector switch U240. The LFVCO output
frequency is also divided by fourthrough U410, and the result is the reference
frequency for theinner loop. U512 and U510 serve as a phase/frequency detector
for the inner loop. The differentialoutput of the phase detector is passed through
error amplifier U435 and becomesthe tuning voltage for the HFVCO.
The HFVCO consists of Q522, L610, varactorCR511, trim capacitor C611, and
associated parts. Output is taken froma tap on L610, then split through a pair of
isolation amplifiers (Q400 and Q410 plus Q316). Output fromQ400 is divided
by four in U300, providing the useful strobe frequencyoutput of the module.
HFVCO output from Q316 is applied to one input of mixer U320, whose other
input is supplied with 100 MHz energy fromthe Reference Oscillator. The useful
output of the mixer is the differencefrequency between the HFVCO (for
example, 103 MHz) and the 100 MHz reference, or 3 MH z. This signal is
amplified (by Q310, Q312, and so forth.) and appliedto the remaining input of
the phase/frequency detector,thus closing the inner loop.
U240 serves two functions; both controlledby the logic signals CNTSELA and
CNTSELB. First, it allows turning the HFVCO on or off by means of Q425.
When the oscillator is turnedoff, Q101 is turned on at the same time to shut
Strobe Frequency
Generator (VCO Module)