Theory of Operation
3-22 2715 Spectrum Analyzer Service Manual
In wide spans (5.1 MHz/div or greater), t he center frequency is set coarsely by
setting a suitable bias current in the main coil. Fine increments of the center
frequency areset by appropriate currents to the FM coil. In this mode, sweeping
over the span is accomplished by summing a suitably scaled analog sweep signal
with the drive to the main coil. The strobesignal is not even turned on except
briefly when needed to assistin verifying the exact 1st LO frequency by
counting the beat frequency.
In moderate spans (50 kHz/divthrough 5 MHz/div), sweeping is done by
summing the scaled sweep signal with thedrive to the FM coil. Additionally, a
noise suppressing low pass filter is insertedinto the main coil circuit to reduce
residual FM and phase noise due to noise from the main coil driver.Otherwise,
the setup is the same as for wide spans. In this mode, the strobe is unused except
when counting the 1st LO frequency.
When operating in narrowspans (20 kHz/div and less), the 1st LO is phase
locked to a harmonic of the strobe frequency.Coarse control of the center
frequency is effectedby choosing which harmonic of the strobe frequency is
used (by setting an appropriatemain coil current before commanding the
hardware to lock). Sweeping and fine control of centerfrequency are done by
controlling the strobe frequency.When phase locked, the FM coil is used by the
loop to adjust the 1st LO to zero phase error.
PLCFC Module Functional Blocks.The Phase Lock Center Frequency Control
(PLCFC) module circuitry can be groupedinto the following functional areas:
HPower Supply Regulation
H1st LO Coil Drivers for the Current Source
HStrobe Frequency Generator (VCO Module)
H1st Phase Lock Loop and Beat Note P rocessor
HSignal Path and Switching Summary
HDigital to Analog Conversion
HDigital Interface
There are six regulatedsupplies on the PLCFC circuit board and two additional
regulators in the VCO module.
-5 V Reference Supply ( -5 VREF). This is a precision reference supply for those
circuit applications where a criticalfrequency is directly dependent on a voltage
or current.
Power Supply Regulation