SEA 222 Operator's Manual page 55

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5.5.6
5.5.7
In practice, the high frequency signal from the VCO is applied to
U12 which normally operates in the divide-by-16 mode until a control
signal from the divide-by-A counter in Ull forces it into the divide-
by-15 mode. The number of cycles in each prescaler mode, along with
the i nterna 1 di vi de-by-N counter, determines the actua 1 di vi de-by-N
modulus. The required modulus is calculated by the control computer
for each given frequency and then the proper A and N counter register
numbers are loaded into the synthesizer chip (Ull) by the control
computer.
HIGH FREQUENCY PHASE DETECTOR/LOOP FILTER
Integrated circuit Ull also contains the high frequency phase detector,
whose output is proportional to the phase difference between the clock
derived 6.4 KHz reference frequency and a similar input derived from
the dual modulus/divide-by-N counter described in
~
5.5.5. The error
voltage is filtered through a second order R/C filter and applied to
the varactor tuning elements in the VCO circuit.
THE VCO SYSTEM AND ASSOCIATED BUFFERS
In the SEA 222 the first conversion oscillator consists of a pair
of voltage controlled oscillators. These oscillators cover the fre-
quency range of 47 - 68 MHz (corresponding to an operating frequency
range of 2 - 23 MHz) and make use of a UHF low noise transistor in a
Colpitts oscillator circuit. Each oscillator covers approximately one
half the overall frequency range and the active oscillator is selected
by the control computer through the transistor switches Q4 and Q5.
The frequency determining inductors are of the high Q toroirlal type,
while each oscillator uses a matched pair of hyperabrupt junction
varactors as the tuning element.
The two oscillators are coupled to the emitter follower buffer, Q3,
through small capacitors and the followers' low impedance output is
applied to the high speed prescaler, U12, and the VCO buffer amplifier
system. The VCO buffer consists of transistors 012, 013 and Q14. Ql2
and Ql3 are connected as a direct coupled two transistor wide band
amplifier, while Ql4 is connected as a common emitter, transformer
coupled power buffer. The output from Ql4 is terminated with a 3 dB
pad and the +7 dBm output from the pad is applied to the LO input port
of signal mixer, Al. The purpose of the 3 dB pad is to provide a
constant 50 Ohm termination point for the m1 xer LO port and the buffer
transistor output.
The two VCO circuits are essentially identical except for tuning
range and are sufficiently buffered to prevent "pulling" or noise
I