"Ian White, G3SEK" wrote in message
...
Avery Fineman wrote:
In article , "Ian White, G3SEK"
writes:
[...]
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.
I think it does much better than just give general indications. It certainly
has opened my eyes to a whole new world inaccessible to me before having the
analyser. It's comparable to the change in my hobby life when I aquired my
first oscilloscope. You still enjoy the hobby with no oscilloscope, but when
you get one you feel like you'd been blind before, and can't imagine life
without it.
The other day I looked at the 39.5MHz IF in an old television tuned to a UK
TV station signal. I could see the video and sound subcarriers clearly
identifiable. Tuning the TV I could move the signals out of the Surface
Acoustic Wave (SAW) filter skirts and get a good idea of the shape of the
filter's response curve. Before I might have read about that stuff in
theory, now I can actually see it!
Under many cases it will be possible to identify any spurious signals
generated within the analyser itself. The input attenuator (in my case
0-63dB in 1dB steps) allows quite accurate measurement of the magnitude of
frequency components, and also allows reduction of strong signals. In many
cases this can be used to eliminate many of the spuri.
Or, if the signal being monitored is a VFO output for example, or another
variable frequency source, changing the frequency will often identify the
spurious signals. For example I might increase the VFO frequency, causing
the fundamental frequency peak to move left to right on the screen. If I see
some other peak move in the opposite direction, or if I see a peak moving
across the display very quickly, I know I've seen a spurious response.
Much of the spurious response problems come at the high end of the frequency
coverage range. For lower frequency work one could insert an additional
easily-constructed low pass filter and eliminate much of the problem.
So yes, no doubt the more effort you put in and the more complex the
spectrum analyser, the better the results obtainable. But even with a simple
analyser there's so much you can do.
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.
I could be wrong but I thought the FCC requirement was 40dB minimum harmonic
suppression, or 30dB for transmitters with output powers 5W or less? That's
well within the range of even a simple spectrum analyser. I doubt I'd have
any problem ensuring an HF QRP rig met FCC requirements with my analsyer.
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.
I'm already working on my Mk2 analyser. Not because I'm unhappy with the
measurements of the Mk1, but because I had so much fun with it, ending up
building it for its own sake, and not ready to stop having fun yet.
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.
Not sure I agree completely - if the EUT is *supposed* to be in your zone 3,
but is not functioning properly, then even a simple spectrum analyser could
help you find the fault. It wouldn't help you make precise receiver
performance measurements on it when you corrected the fault, but would help
with diagnosis.
[...]
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.
Don't forget the W7ZOI analyser covers 0-70MHz only. You can double that
with a NE602 design. Nevertheless I liked the W7ZOI analyser a lot.
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).
Sounded interesting, but a google search as described turned up 0 results.
In fact a search for S57MV on its own got 0 results. Do you have any other
references?
73
Hans G0UPL
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek