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Model 8902A
Operation
Frequency Offsetting.
The IF frequency can be offset by
one
of the following three methods:
0
offsetting the R F input signal.
0
offsetting the external LO, or
0
offsetting the internal LO using the
fi
kHz or
kHz keys.
An increase in the R F input signal moves the IF signal down the lower skirt of the IF bandpass filter.
An increase in the LO frequency (either external or internal) moves the IF signal down the upper
skirt. Refer to
Meuwrcmenf Tcchniqire
on page 3-125 for an illustration of this.
The measurement specification may require that the frequency offset be made from the
6
dB corner
frequency of the IF bandpass filter.
Finding the IF
Peak.
For maximum accuracy, the frequency offset should be relative to the frequency at which the I F
sigrial is peaked in the IF bandpass filter. (IF filter ripple can be as high as 2 dB.) Automatic tuning
brings the IF clo$e t o the peak, but the instrument controller does not specifically check for an
I F peak, Finding the
IF
peak is most easily accomplished by tuning the
RF
input signal (if this
is
possible) or by tuning the external LO while Special Function 24.1, 24.3, or 24.5 is active. The R F
input signal or external
LO
is tuned until the displayed IF voltage is at its peak. Finding the I F peak
by incrementing the internal LO is similar, but the active special function must be re-entered after
each frequency increment. Tuning requeires a range of only a few kHz.
In all the measurements, you can tune the input signal instead of the LO. This is the easiest way if
the source is tuneable. However, you must tune the source opposite to the direction you would tune
the LO.
For
example, if
yoti
tune the LO down, you would tune the source
u p
in tiequency.
3-127
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