R
OUTINE
S
ERVICING
748250-L Rosemount Analytical October 1999
6-9
Model 880A Non-Dispersive Infrared Analyzer
6.8 ELECTRONIC CIRCUITRY
6.8.1 O
SCILLATOR
C
IRCUIT
B
OARD AND
A
SSOCIATED
E
LEMENTS OF
A
MPLITUDE
-M
ODULATION
C
IRCUIT
In the Oscillator Circuit (Drawing 623995) the 10 MHz carrier wave is generated by a
crystal-controlled radio-frequency oscillator using crystal Y1 and transistors Q1 and
Q2.
The modulation circuit is driven by the detector, the sensing element of the analyzer.
Mechanical functioning of the detector is explained in Section 4.4. Considered
electronically, the detector is a two-plate variable capacitor. The modulator is coupled
inductively, through one winding of inductance T1, to the oscillator. Amplitude of the 10
MHz carrier thus varies with the 5 Hz modulation signal.
6.8.2 F
UNCTIONING OF
M
ODULATION
S
YSTEM IN
TUNE M
ODE
Preparatory to oscillator tuning, access Oscillator Tune (OT=XX) in the Diagnostic
Display (Figure 3-7). In this mode the display indicates the rms value of the
halfwave-rectified carrier. The tank circuit is now adjusted in the following two-step
sequence:
Tuning:
Initially, the OSC TUNE adjustment is set somewhat counterclockwise from
its correct setting. Then, it is rotated clockwise to move the slug into the core, thus
increasing inductance and decreasing resonant fre-quency. The adjustment is set for
maximum obtainable reading. At this setting, tank-circuit resonant frequency is the
same as oscillator frequency (i.e., nominal 10 MHz). See Resonance Curve Number 1,
Figure 5-2B.
Detuning:
By counterclockwise rotation of the OSC TUNE adjustment, the slug is
partially withdrawn from the core, thus de-creasing inductance and increasing
resonant frequency. The adjustment is set so reading decreases to between 75 % and
80 % of the maximum obtainable value noted in Step 1, above. See Resonance Curve
Number 2, Figure 5-2B. This curve has the same shape as that obtained in Step 1,
immediately preceding, but is displaced to the right.
6.8.3 F
UNCTIONING OF
M
ODULATION
S
YSTEM IN
O
PERATING
M
ODE
Overall sensitivity of the analyzer system may now be checked by placing SPAN gas
in the sample beam to simulate absorption of sample-beam energy and thus provide
the maximum obtainable 5 Hz detector-output signal.
During that portion of the chopping cycle, while the chopper is not blocking the sample
and reference beams, the diaphragm distends away from the metal button, thus
decreasing detector capacitance and shifting the tank-circuit resonance curve to the
right. At the moment the diaphragm reaches maximum distention, the curve reaches
the position of Curve 3, Figure 5-3B.
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