Theory of Operation
3-6 2715 Spectrum Analyzer Service Manual
This is a precision, temperature controlled, quartzcrystal oscillator that provides
the referencearound which all instrument frequency related performance
revolves. It is designed for maximalfrequency stability over temperature and
time, with absolute frequency accuracy being secondary. The instrument
firmware accountsfor any frequency inaccuracy, producing a virtually perfect
reference.
The phase gate is a high speedsampling switch that takes a very brief sample of
the 1st LO waveform once durin g each cycle of the strobe frequency. (The strobe
is a control signal supplied to the phasegate from an external source.) These
samples are later averagedby a low pass filter to eliminate ripple at the strobe
frequency,and also to eliminate other undesirable components.
When the 1st LO is phase locked to any integer multiple of the strobe frequency,
the resulting output is a phase dependent DC voltage that is used as the error
voltage in a feedbackloop to maintain phase lock.
When the 1st LO is not phase locked, the resultingoutput is a beat note
occurring at the differencefrequency between the 1st LO frequency and the
nearest integer multiple of the strobe frequency. (Actually, many beat frequencies
are present at once,but the low pass filter eliminates all but the one of lowest
frequency.)
This element is the focal point ofthe frequency control system hardware. It
accepts commands from the instrument m icroprocessor and then puts out
appropriate drive signals to the 1st LO as well as a suitable strobe signal to the
Phase Gate. The PLCFC module also uses the Phase Gate output. It also
provides counter signals and statusinformation to the microprocessor. It requires
the 100 MHz referencesignal in order to function.
Because of the variety of tasks t hat the Spectrum Analyzer is called upon to do,
it is essential to have an intelligentcontroller. This is especially true in the
frequency control context, because th ereq uired degree of control could not be
economically obtained with unassistedanalog hardware.
Instrument firmwarehas the task (among many others) of commanding the
frequency control hardware,taking feedback from it, and making readjustments
as required to obtain the desiredresult.
The interface betweenthe microprocessor system and the PLCFC module is
through a pair of serialdata lines, some latch lines, and some clock signals.
100 MHz ReferenceOscillatorPhase GatePhase Lock CenterFrequency Control(PLCFC)Firmware