Theory of Operation
3-20 2715 Spectrum Analyzer Service Manual
rs<50Ω
47.5Ω
20nH
3.92kΩ
Rt
+5V
47.5Ω
3.92kΩ
82pf
1kΩ
−8.8V
−8.8V −8.8V
0.1f
Q1
Q2
re2≈5.7 Ω
re1≈8.3Ω
82.5Ω
1.5kΩ
Figure 3- 11: Oscillator DC Equivalent
The voltage swing required acrossthe tank circuit must be at least equal to the
ratio of 1 V to the calculatedgain, which is 0.3 V. If the bias currentavailable in
Q1 is 3.6 mA, then the tank resistance requiredis calculated to be:
0.3 V
3.6 mA =83 Ω
Distribution. The distribution circuit represents an ECL line receiv er.This circuit
receives the 100 MHz signal from the Oscillator and steers it to the following
circuits at --4dB m.
HRF Mother Board (2nd and 3rd LOs)
HMicroprocessor (Counter Section)
HCFC Board (1st LO Phase Lock)
HPort reserved for future option
HAmplitude Calibrator on the Reference Oscillatorboard
Amplitude Calibrator. The Amplitude Calibrator receiv es a 100 MHz signal from
the distribution circuit. The level ofthe 100 MHz signal is adjusted, and the
resultant calibratorsignal is then routed to a switch on the Step Attenuator board.
When enabled by the Microprocessor, a differential pair of trans istors switch a
carefully controlledcurrent on and off across a 56.2 Ωresistor. The resulting
signal, with a -- 22 dBm level, is routed to thecalibrator port through an 8 dB
pad.