Theory of Operation
3-18 2715 Spectrum Analyzer Service Manual
The heater circuit maintains a con stant 15 V potential across the heater elemen t
regardless of powersupply ripple. The circuit is shown in Figure 3--9.
22.1kΩ
+5.2V
-- 1 1 V
+5V
−11 V
−11 V
Heater
6.91kΩ
10kΩ
10kΩ
22.1kΩ
10kΩ
27.4kΩ
2.5Ω
−
+
−
+1kΩ
Figure 3- 9: Crystal Heater and 15 V Regulation Circuit
Oscillator. The oscillator generates the 100 MHz referencefrequency that is used
by the instrument firmware to enhance frequ ency accuracy. Figure 3--10 shows
the AC equivalent for the oscillator.At resonance, the tank inductor and
capacitor Lt and Ct exhibit an opencircuit condition, and the LC equivalent of
the crystal (Lm and Cm) exhibit ashort circuit condition. Thus, at resonance, the
open loop gain is calculated as:
A=Rt
(re1+re2+rs)
This gain must be greater thanunity to ensure oscillation.