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Theory of Operation
3-20
2715 Spectrum Analyzer Service Manual
r
s
<50 Ω
47.5 Ω
20 nH
3.92 kΩ
R
t
+5 V
47.5 Ω
3.92 kΩ
82 pf
1kΩ
−8.8 V
−8.8 V
−8.8 V
0.1 f
Q1
Q2
r
e2
≈ 5.7 Ω
r
e1
≈ 8.3 Ω
82.5Ω
1.5 kΩ
Figure 3- 11: Oscillator DC Equivalent
The voltage swing required across the tank circuit must be at least equal to the
ratio of 1 V to the calculated gain, which is 0.3 V. If the bias current available in
Q1 is 3.6 mA, then the tank resistance required is calculated to be:
0.3 V
3.6 mA
= 83 Ω
Distribution. The distribution circuit represents an ECL line receiver. This circuit
receives the 100 MHz signal from the Oscillator and steers it to the following
circuits at --4 dBm.
H RF Mother Board (2nd and 3rd LOs)
H Microprocessor (Counter Section)
H CFC Board (1st LO Phase Lock)
H Port reserved for future option
H Amplitude Calibrator on the Reference Oscillator board
Amplitude Calibrator. The Amplitude Calibrator receives a 100 MHz signal from
the distribution circuit. The level of the 100 MHz signal is adjusted, and the
resultant calibrator signal is then routed to a s witch on the Step Attenuator board.
When enabled by the Microprocessor , a differential pair of transistors switch a
carefully controlled current on and off across a 56.2 Ω resistor. The resulting
signal, with a --22 dBm level, is routed to the calibrator port through an 8 dB
pad.