"Ian White, G3SEK" wrote in message
...
Hans Summers wrote:

The W7ZOI homebrew spectrum analyzer article is not to be missed:

http://www.qsl.net/n9zia/wireless/pdf/9808035.pdf (part 1)
http://www.qsl.net/n9zia/wireless/pdf/9809037.pdf (part 2)

You'll find this design vastly superior to the "poor man's spectrum
analyzer" projects out there that are based on CATV tuners.

The W7ZOI project is very interesting. I'm interested in what ways you
think
it's superior to the poor man's spectrum analysers based on CATV tuners?
The
latter surely have greater frequency coverage. But in what ways are they
inferior

A spectrum analyser is simply a frequency-swept receiver with a
dB-scaled output to a screen. To give meaningful results, that receiver
must have a very high dynamic range with very low spurious responses.

The problem is that real incoming signals and the analyser's spurious
responses all look very much the same on the screen. When you can't
trust what the analyser says, it becomes very hard to understand what's
really going on.

As with lots of beginner-level test equipment, it sometimes needs an
expert to understand it!

CATV tuners and low-level NE602 mixers are simply not the building
blocks for a high dynamic range receiver. The W7ZOI design uses much
more appropriate building blocks so its readouts are much more
trustworthy.

I used two SA602A mixers in my design (see
http://www.hanssummers.com/electroni...ctrumanalyser/) and get
about 60 or 70dB of on-screen dynamic range. The SA602A approach does have
the advantage of similicity, at the cost of reduced performance. I use one
SA602A with the on-chip oscillator as the VCO swept from 145 to 290MHz with
145MHz IF output. The 2nd SA602A has 153MHz fixed on-chip oscillator and
mixes down to the 8MHz 2nd IF.

Spurious responses in my analyser from strong signals aren't usually a
problem except at the upper end of the frequency coverage. The W7ZOI
analyser covers 0-70MHz, mine 0-140MHz. It would be very interesting to
compare my design against a W7ZOI analyser over the same frequency range,
i.e. place a 70MHz low pass filter ahead of my analyser. I suspect spurious
reponses would be comparable, but on-screen dynamic range somewhat less in
my analyser.

My analyser Mk2 is already under construction and will aim to provide masses
more features, greater coverage, and improved dynamic range performance.


and can you quantify it?

Very easily, in the same ways as you test a receiver for strong-signal
handling.... but most graphically by looking at the same spectra with
two analysers side-by-side. The one showing fewest signals is the one
you can rely on most.

What if you don't have another analyser ;-) Part of my intention with my
simple analyser was to build something straightforward but still extremely
useful, and do so without access to much other test equipment. When I have
the Mk2 up and running it will be interesting to compare.

73, Hans G0UPL