HP 203A - Variable Phase Channel RF; Amplifier; Reference Phase Channel RF; Variable Frequency Oscillator

Section IV
Paragraphs 4-20 to 4-30
the variable phase channel. Refer to the schematic
diagram (figure 5-12) for circuit details.
4-20. VARIABLE PHASE CHANNEL RF
AMPLIFIER,
4-21. The signal from the variable phase shifter
is amplifiedby A2Q6 then applied to the base of А297.
A2Q7 and A2Q8 act as anover-driven amplifier which
amplifies and clips the signal applied to the base of
A2Q7; this operation produces a square wave of cur-
rent at the collector of А298. The zero crossing of
the square wave of current coincides with the zero
crossing of the sine wave Signal applied to the base of
A2Q7 so that the phase of the applied signal is pre-
Served. А tuned network, formed by A2C24, A2C25,
А2С29, A2C30, A2L7, and A2T2 filters the 555 Кс
Square-current waveform toa nearly pure sine wave.
4-22. The output of the RF amplifier circuit, which
is taken across A2C30, is maintained at a constant
amplitude by the level controlling circuit. If the out-
put should increase, the voltage at A2C27 increases,
resulting ina voltage increase at the base of A2Q9.
This increase is applied to the bases of A2Q7 and
A2Q8 which then conduct less average current. When
A2Q8 conducts less,the signal at its collector de-
decreases and the output voltage decreases opposing
the original change. The result is that the amplitude
of the output remains nearly constant despite vari-
ations in the amplitude of the input signal. The out-
put signal is then applied to АЗТ4 in the modulator
assembly A3 (figure 5-16).
4-23. REFERENCE PHASE CHANNEL RF
AMPLIFIER.
4-24. The signal present at the emitter of A2Q2 is
applied to the reference phase channel RF amplifier
section, A2Q3 through A2Q5. This stage operates
FROM
VFO
BP FILTER
(5.4-5,6МС)
C9,Clo,
&
TO MIXER IN 2ND
THROUGH 6TH
DECADE MODULES | * THE INSTRUMENT CONTAINS
SIX DECADE MODULES.
ALL ARE IDENTICAL AND
INTERCHANGEABLE.
Model 203A
the same as the variable phase channel RF amplifier
described in paragraph 4-20. The Output signal is
then applied to A3T3 in the modulator assembly A3 го
(figure 5-16).
4-25. VARIABLE FREQUENCY
OSCILLATOR (АТО).
4-26. The variable frequency oscillator assembly
A10 generates asignal thatis variable from 495 kc to
550
ke
by
rotation
of
the
front
panel
FREQUENCY
dial.
The FREQUENCY dial is calibrated so that with the
dial set at 5 the VFO is oscillating at 550 ke and with
the dialset at 60 the VFO is oscillatingat 495 кс. Тһе
output Signal from the variable frequency oscillatoris
applied to the І К position of the MULTIPLIER (ire-
quency range) Switch andalsoto the 1st decade module.
Refer to the schematic diagram, figure 5-14, for
circuit details.
4-27. DECADE MODULES (А11-А16).
4-28. The six decade module assemblies A11 thru
A16 each consist of a mixer, a bandpass filter, anda
10:1 divider. These decades produce a band of high
frequency signals that are mixed inthe modulators (A3)
with the 555 kc fixed frequency signal from the RF
amplifiers to produce a signal in the 0.005 cps to 60
ke range (refer to paragraph 1-5 for options). Refer
to the schematic diagram, figure 5-14and figure 4-1
for circuit details.
4-29. DECADE MODULE (A11).
4-30. The 4. 995 Mc signalfrom the crystal oscillator
is applied to the emitter of A11Q1 isolation stage and
subsequently appears across the priímaryof А11Т1. The
Signal from AliTlisappliedto asuppressed carrier,
balanced modulator. The 495 kc VFO signal (assume
TO ІК POSITION OF
MULTIPLIER SWITCH SIB
TO 100 POSITION OF
MULTIPLIER SWITCH 518
TO MIXER IN
2ND DECADE
MODULE (Al2)
203A-M- ІВА
Figure 4-1. Circuit Details
4-2
01760-3