Ametek 5000 Instrument Manual page 76

the solenoid L1 winding and lighting LED CR11. Sample gas flows through the cell. Every 30 seconds,
+5 volts is applied to transistor Q5, turning it on, grounding the control side of solenoid winding L2
and LED CR10. The solenoid opens reference gas flow through the cell, and the LED lights.
In normal operation, +5 volts is on TB1-8, biasing transistor switch Q4 on, turning Q3 off, de-energizing
solenoids L3 and L4, allowing sample gas to flow. Pressing the MOIST GEN key applies 0 volt to the
base of Q4, turning it off. This turns Q3 on, grounding the control sides of L3 and L4, opening moisture
generator gas flow.
Cell Output
There are two crystal oscillator (Y — reference and Y — measure) circuits with amplifier/inverters U1
1 2
and U2 and rc filters. Except for their frequencies, the two oscillators function alike.
U2A-1/-2 and U2F-13/-12 provide positive feedback to sustain the Y (Reference) and Y (Measure)
1 2
nominal oscillations at 8.982500 MHz and 8.982000 MHz, respectively.
U2B/C and U2E/D amplify the outputs.
Assuming a dry Y frequency of 8.982000 MHz, (L2 energized) and a Y frequency of 8.982500 MHz, low
2 1
pass filter C9 develops a difference frequency of 500 Hz (∆F ). This frequency is amplified to produce
2
a square-wave voltage of 0 to 12 at 500 Hz applied to the controller processor.
During the 30-second wet cycle, moisture is sorbed in the crystal cell which adds to its weight. The
additional weight decreases the frequency to 8.981000 MHz (assumed), which produces a wet differ-
ence frequency (∆F ) of 1500 Hz (8.982500 – 8.981000 = 0.0015 MHz). This frequency is amplified and
1
applied to the microprocessor as a square-wave voltage of 0 to 12 at 1500 Hz.
6-10
5000 Multi Point Moisture Analyzer