In article , "Ian White, G3SEK"
writes:

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.

Okay. Then allow me to state that I've been a professional in radio
and electronics for 51 years AND an electronics hobbyist in my
free time for longer. I've not "paid any dues" in the latter area except
a lot of hours spent having fun in my workshop..

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.

? What is wrong with that?

An NE602 or SA602 is just a means to an end. It's a nice little IC
that allows some balanced mixing AND as the active device for the
local oscillator. A single IC, dual function. Used within its limitations
it is AS ACCURATE AS ANY OTHER DEVICE.

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.)

I have NO idea of what is "completely obvious" to anyone. Everyone's
mileage varies.

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.

Let's not lump all the "non-pro" analyzers into the "fun-toy" category,
okay?

Sorry, but I can quickly categorize/calibrate/check ANY analyzer as
to whether or not whatever it shows is "real" or internal. There's lots
of written information available on how to do that...and a bit of thinking
will reveal the very same thing.

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.

Sorry, you've drifted outside of specific spectrum analyzer use. The
harmonic content of ALL transmitters can be measured WITHOUT
any spectrum analyzer, as accurately (in most cases moreso) as with
an analyzer. A spectrum analyzer is much much faster in that
application, but harmonic content measurement is not dependent on
its use.

Good operation and good specifications in homebrew designs are as
good as the designer, as good as the other equipment for calibration
and characterisation that is used. It's all together. It's only "damned
hard to guarantee" if there is nothing available to ascertain operation
and calibration. That can exist equally well for an HP 141 plug-in or
one that you term a "fun" NE602.

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!

Good grief! There's plenty of instances in metrology where actual
measurements are done to a higher level than what each piece of
test equipment can do. Time and frequency measurement is a good
area for examples in that, other areas' explanations will take more
time to describe than I have at the moment.

EVERY single piece of test equipment MUST be used within its
limitations. That INCLUDES your "HP 141" racks, Anritsu analyzers,
etc., and whatever else is there.

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.

Nonsense. Each and every "user" had damn well pay close attention
to using whatever they have within its equipment's limitations. That
holds true for the best of Rhode & Schwarz wares AND for the "fun"
SA602/NE602 analyzers.


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.

You're going to have to explain that better..."clean" is highly subjective
and subjective judgement in any metrology effort is a no-no.

The absolute standard of comparison is the perfect spectrum analyser
with no spurious responses.

Okay, show me one. They must be in storage with bottles of the
"universal solvent" and other fine ideas...


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?"

That's nice. But you are drifting towards extreme categorization of
equipment. Simple, low parts count home projects are "fun" (in your
terms, like toys or something for recreation. The only "GOOD" gear
is semi-pro, expensive, etc., etc., etc.


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.

In MY workshop, nearly everything I do and use is pure FUN. If I use
them within their limitations, then they give me REAL numbers as
valid as anything I use from a small pro lab's $400K capital collection.


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.

So...bottom line is that spectrum analyzers with NE602s in them are
kiddie toys useable only for entertainment and recreation?!?

Not a good characterization, that. Used within its limitations, the
"kiddie" instrument is as accurate as anything else. Input level
displayed linearity is a direct function of the successive detection
logarithmic chip used in the back end...has very little to do with the
front end. Resolution is again a back-end concern and is a direct
function of the filtering of the final IF in the analyzer...and the sweep
rate and sweep span (all related). If you want linearity in the
horizontal as to frequency, then you have to pay a lot of attention to
VCO (or equivalent) control and that has little to do with messing
about with the front end mixer.

The front end of any spectrum analyzer is the place where your
spurious responses are generated, if any at all. If you are going for
the very best in higher-level inputs then you absolutely need a high-
IM-rated mixer, which also probably needs a high-level LO. Both of
those cost a great deal of money...and the higher input level free of
spurious responses may or may not be worth it. It may not be
worth it when one of those "fun" analyzers don't exhibit any false
signals or spurii below a specific input level.

Len Anderson
retired (from regular hours) electronic engineer person
formerly an Associate Editor with HAM RADIO Magazine

PS: I once or twice used an HP 141. It always had "ordinary"
oscilloscope plug-ins in it to make it overall an oscilloscope. Shrug.