Geoff:
Your approach assumes that the tracking generator has a programmable
offset, doesn't it? RF Generator output, for example, sweeps from 7 -
7.4 MHz with the K2 tuned to 7.2 MHz, but the detector needs to sweep
5115 - 4715 KHz, if I've got the maths right. (Sweep also must be
inverted to see the display correctly due to K2's inversion on 7 MHz band.)
My VNAs don't have that feature, but I believe frequency offset is
available in some of Agilent's current production VNAs. With an
auxiliary detector, such as an HP3400 RF voltmeter, however, I could use
my HP8752B VNA's "vertical axis input is auxiliary DC voltage" feature,
taking the 0...1V recorder output from the 3400 as the VNA's auxiliary
input. That gives the advantage of a high Z detector, but the
significant disadvantage of broadband input. The other option would be
to lash up a program to command a signal generator and spectrum analyzer
via their GPIB control buses with the flexibility in frequency stepping
that provides.
Jack
Geoffrey Mackenzie-Kennedy wrote:
Hi Brett,
My personal preference when looking at the response of any filter is
to use a RF Spectrum Analyzer and a tracking signal generator. When it
comes to looking at the response of IF filters already built into a
receiver the tracking generator can work at the antenna frequency
rather than at IF, which makes the generator - pad - receiver
connection easy to make, provided that the front end selective
circuits have no effect on the measurement of the IF filter i.e.their
working bandwidth is >>> IF filter bandwidth. The penalty is that the
input of the Spectrum Analyzer needs to be connected to the IF strip
at some point before the product detector, and everything after this
point has to be disconnected unless the working bandwidth of this part
which follows is >>> IF filter bandwidth, *or* has no effect on the
measurement. One also has to ensure that the Spectrum Analyzer and its
connecting cable are transparent to the cascade under measurement, a
high impedance probe helps. AGC has to be OFF of course if the AGC
system could be active. Spectrogram does not require this fiddling,
AGC must be OFF, but being in effect a Spectrum Analyzer working at
audio it gives a limited view - about 20kHz IIRC. What I like about
using a RF Spectrum Analyzer and tracking generator is that a cleaner
picture is produced without fuzzy noise at the top, and it is possible
to sweep over a wide bandwidth to view the ears and spurious responses
of the filter. It is also usually possible to see those other unwanted
responses caused by radiated feedback or coupling via pcb traces etc.
It could be argued that Spectrogram is easier to use for routine
alignment, but has limitations for use as a diagnostic tool.
73,
Geoff
GM4ESD
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