Theory of Operation
2715 Spectrum Analyzer Service Manual 3-19
Rt
82.5 Ω
Cm
Lm
Cp
Lt
Ct
Q1
Q2
+5V
221Ω
re1≈8.3Ω
rs=50Ω
re2=5.7Ω
Figure 3 - 10: Oscillat or AC Equivalent
At oscillation, the loop gain is unity and the phase 0°.If the phase delay of Q1,
Q2, or the tank changes, the operatingfrequency must also change to maintain a 0°
loop phase shift. Becaus e of this phase/frequency shift dependency, the Q of t he
parallel tank circuit is made as low as possible to minimize the tank phase change
as the values of Lt and Ct drift.
The component values of the tank circuit are determinedby the voltage swing at
the collector of Q2 (Figure 3 --11). The co llector of Q2 drives an ECL line receiver.
Therefore, the collectorvoltage should swing 1 V peak to peak about 3.8 VDC
(3.8 V being the ECLVbb bias voltage). The gain of Q2 is calculated as:
A=Rt
(re1+re2+rs)=3