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Theory of Operation
3-18
2715 Spectrum Analyzer Service Manual
The heater circuit maintains a constant 15 V potential across the heater element
regardless of power supply ripple. The circuit is shown in Figure 3--9.
22.1 kΩ
+5.2 V
-- 11 V
+5 V
−11 V
−11 V
Heater
6.91 kΩ
10 kΩ
10 kΩ
22.1 kΩ
10 kΩ
27.4 kΩ
2.5 Ω
−
+
−
+
1kΩ
Figure 3- 9: Crystal Heater and 15 V Regulation Circuit
Oscillator. The oscillator generates the 100 MHz reference frequency that is used
by the instrument firmware to enhance frequency accuracy. Figure 3--10 shows
the AC equivalent for the oscillator. At resonance, the tank inductor and
capacitor Lt and Ct exhibit an open circuit condition, and the LC equivalent of
the crystal (Lm and Cm) exhibit a short circuit condition. Thus, at resonance, the
open loop gain is calculated as:
A =
R
t
(r
e1
+ r
e2
+ r
s
)
This gain must be greater than unity to ensure oscillation.