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Theory of Operation
3-22
2715 Spectrum Analyzer Service Manual
In wide spans (5.1 MHz/div or greater), the center frequency is set coarsely by
setting a suitable bias current in the main coil. Fine increments of the center
frequency are set 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 strobe signal is not even turned on except
briefly when needed to assist in verifying the exact 1st LO frequency by
counting the beat frequency.
In moderate spans (50 kHz/div through 5 MHz/div), sweeping is done by
summing the scaled sweep signal with the drive to the FM coil. Additionally, a
noise suppressing low pass filter is inserted into the main coil circuit to reduce
residual F M 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 narrow spans (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 effected by choosing which harmonic of the strobe frequency is
used (by setting an appropriate main coil current before commanding the
hardware to lock). Sweeping and fine control of center frequency 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 C ontrol
(PLCFC) module circuitry can be grouped into the following functional areas:
H Power Supply Regulation
H 1st LO Coil Drivers for the Current Source
H Strobe Frequency Generator (VCO Module)
H 1st Phase Lock Loop and Beat Note Processor
H Signal Path and Switching Summary
H Digital to Analog Conversion
H Digital Interface
There are six regulated supplies on the PLCFC circuit board and two additional
regulators in the VCO module.
- 5 V Refer ence Supply (- 5 VREF). This is a precision reference supply for those
circuit applications where a critical frequency is directly dependent on a voltage
or current.
Power Supply Regulation