Posted by
Jack Smith-6 on
Mar 17, 2007; 7:51pm
URL: http://elecraft.85.s1.nabble.com/Filters-and-Spectrum-Analyzer-tp444864p444868.html
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
>
>
>
>
> _______________________________________________
> Elecraft mailing list
> Post to:
[hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>
http://mailman.qth.net/mailman/listinfo/elecraft
> Help:
http://mailman.qth.net/subscribers.htm> Elecraft web page:
http://www.elecraft.com>
_______________________________________________
Elecraft mailing list
Post to:
[hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help:
http://mailman.qth.net/subscribers.htmElecraft web page:
http://www.elecraft.com