Avery Fineman wrote:
In article , "Ian White, G3SEK"
writes:

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.

"Meaningful results" are subjective to the hobbyist.

I'm an amateur, not a professional, but have paid my dues in this area.
I have helped develop, and have built and used, two generations of HF (0
- 100MHz) spectrum analysers of the NE602 persuasion. Then a
basket-case Texscan 0-1GHz box came along which I rescued and used for
VHF work. That was followed by the HP-141 series analyser which I'm
currently using (and can now be bought for much less than the price of a
new rig). I also developed a 0-1.5GHz tracking generator for the 141
boxes. Through friends, I have some limited access to modern
professional equipment, and know how to use it.

My experience with the NE602 type is that it's great as a "fun"
analyser, but you can very quickly outgrow it. Yes, it will give you
general indications, like "Is that oscillator working?" or "Is that
final output stage generating harmonics?" Those indications even have
some numbers of MHz and dB attached.

NO analyzer means NO results.

Of course I'd rather have that much information than no analyser at all.
(Len, if you happen not to mention something that is completely obvious,
I still assume that you're completely aware of it. Kindly extend the
same courtesy to me.)


But once you have a "fun" analyser giving you basic indications, it
inevitably draws you into asking more searching questions, like "How
much of a harmonic problem do I really have?" That's where fun analysers
come unstuck, because you don't know how much of those indicated
harmonics are real, and how much is being generated in the analyser
itself.

An important practical crunch point for amateurs is "Does this
transmitter meet the FCC requirement for 60dB minimum harmonic
suppression?" Now you discover the big difference between 60-70dB
on-screen dynamic range, and 60-70dB *spurious-free* dynamic range. The
first is easy - the second is damned hard to guarantee in a HB design.

If you get to that point - and many people will - then you'll wish you'd
taken the extra trouble to build an analyser that you could trust a
little more. The NE602 type categorically will not hack that kind of
problem, because the equipment under test is probably much cleaner than
it is! The W7ZOI type may just do it, with care, and the same is true of
the 141 generation. Only the best modern analysers will ace the problem
without needing careful attention from the user.

The simple rule of thumb is:

1. If the performance of your test equipment is better - in whatever
ways are important for the measurement in hand - than the performance of
the equipment under test (EUT) then the results will be reliable enough
for amateur work.

2. If your test equipment is comparable with the EUT, you'll get useful
indications.

3. If your test equipment is worse than the EUT, what you're actually
testing is the testgear itself! You'll learn nothing useful about the
EUT, and may actually be misled.

So aim for zone 1, settle for zone 2 if you must, but stay out of zone
3.

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.

Have you priced the used spectrum analyzers lately? Do you expect
others to have ready access to "another" spectrum analyzer?

That comment was mostly to make the point that the cleanest display is
the most real.

The absolute standard of comparison is the perfect spectrum analyser
with no spurious responses. That is an *idea* to which anyone has ready
access, any time.

Before I ever had a real spectrum analyser, I got a lot of good mileage
out of thinking "Now if I had a spectrum analyser, what would it
probably be showing me here?" That isn't empty dreaming - it's a highly
*practical* way of focusing your thinking on a problem in the
here-and-now.

When I built the first "fun" analyser (which had to wait until the
hardware collection had expanded to include a good oscilloscope) I found
it quite disappointing, because it still wasn't showing me reality. I
was still having to think "Now if I had a *good* spectrum analyser, what
would it probably be showing me here?"

Over the years, more and more of the mental testgear has been changed
into hardware, but the mental spectrum analyser still gets switched on
quite frequently - it has instant warmup and it also works very well in
the bath and, er, the other places where I do my best thinking!

Out in the workshop, the real hardware adds real-life numbers to the
ideas, and also checks that there isn't anything happening that I hadn't
thought of.



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

I disagree. There are any number of application notes free for the
downloading on the Internet, from Agilent they are copies of older
(two decades at least) FREE paper application notes. Agilent also
has free application notes on the basic building blocks within an
analyzer and much information on the characteristics of those blocks.

That's not the point. You are talking about the availability of
information, which is not in dispute. I am talking about understanding
and applying it.

By the time you've absorbed the available information - especially the
information about the *imperfections* of spectrum analysers, so you can
understand how much of what you see on-screen is real - you're not a
beginner any more. It will have been a useful learning experience... but
perhaps not the one you'd have chosen right then.

Being a beginner in any area is hard work. When you're ttrying to learn
what it's all about, the last thing you need is to have to fight poor
equipment as well. Musical instruments are very much the same -
so-called "beginner" instruments are actually quite hard work for anyone
to play; in contrast, a better-quality (read: expensive) instrument
feels almost like it's playing itself.

Unfortunately, budget reasons mean that beginners will almost always
have to learn on instruments (test or music) that are hard to use. We
have to accept that, but for heaven's sake let's not pretend to make a
virtue of it!

In this particular case, there does exist a very good, reliable,
second-level design (the W7ZOI). It's a little more expensive to build
than the entry-level stuff, but not significantly more complex. It's
very well designed, so while you're building it you'll learn some very
useful things from a top amateur (and former professional) RF designer.
It's also modular, so you can build it in stages, and upgrade various
parts as and when you want.


Bottom line: having played with the entry-level NE602 stuff myself, my
advice would be:

* If you only want a "fun" analyser, then go for it and have fun.

* But if you want the analyser as a tool to help you develop good radio
gear, go direct to the W7ZOI design.

For a VHF/UHF HB design - which is obviously going to be a lot more
complex, and really is an advanced project - search Google for: "S57MV
spectrum analyser" (without the quotes, and also look for the American
"analyzer" spelling).


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek