SEA 222 Operator's Manual page 56

5.5.8
5.5.9
modulation from the counter train or mixer. Typical near field noise
sidebands are 60 dB or better below normal output on all channels
in
the frequency range of the equipment.
THE VCXO AND ASSOCIATE BUFFERS
The VCXO (voltage controlled crystal oscillator) serves as the second
conversion oscillator for the SEA 222 and operates at approximately
38.6 MHz.
The crystal oscillator is a transistor overtone type, with an in-
ductor in series with the crystal. Coupling varactor CR23 to the
series inductor allows the crystal oscillator to be voltage tuned over
the required frequency range. Q16 amplifies the VCXO signal to the
level required by the second mixer, A2. A sample of the VCXO signal is
also sent to the VCXO down converter, Ul7.
THEORY OF OPERATION: SECONDARY (VCXO) LOOP
The VCXO signal is down converted in Ul7, a HCMOS exclusive OR gate.
In this application the exclusive OR gate operates as a double balanced
harmonic mixer. An input from the VCXO at 38.6 MHz is mixed with a
sample of the 6400.0 KHz master clock signal. The mixer output is
taken at approximately 200 KHz and is the difference signal between the
38.6 MHz VCXO signal and the sixth harmonic of the master clock signal
at 38.4 MHz.
The 200 KHz difference signal is buffered and filtered by the com-
ponents associated with L16 and Q17, further buffered by another
exclusive OR gate (operating as a non-inverting buffer) and then sent
to the signal frequency input of synthesizer chip, Ul6.·
In Ul6, an appropriate di vi de-by-N number is
1
oaded into the di vi de-
by-N counter by the control computer. This will result in the 200 KHz
down converted VCXO signal being further reduced in frequency to
approximately 100 Hz.
The down converted, down counted VCXO s i gna
1
is now app 1 i ed to the
tri-state phase detector in U16 and compared in phase with the 100 Hz
signal derived from the master clock. (See ~ 5.5.3.) The resultant DC
error signal is passed through a second order R/C loop filter and
applied to the VCXO tuning varactor, CR23.
I