Re: Instrument questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Matt D. wrote:

> Is there any relatively easy way to Kludge together a sweep
> generator and a dual trace analog scope to make a poor man's
> spectrum analyzer?

You could try this for frequencies above audio:

                     input
                       o
                       |
               rf      |
  |sweep gen|--------[mixer]------[audio amp]----[scope Y]
       |
       |      ramp 
        -----------------------------------------[scope X]  

The mixer to consist of just a diode. As you can see, this
is just a crude, swept-frequency, direct-conversion receiver.

The audio amp makes a nice narrow band lowpass filter, providing
it's a cheap and cheerful one!  Use only low sweep rates.

You can make a nice simple logarithmic amp out of an op-amp and
a another diode or two.  Well worth the effort.

Ought to work for low frequencies, eg those used by TCs.
For UHF, yank a vco tuner module from an old VCR or TV and take the
video output to the scope Y.  Apply ramp to vco and scope X.

Just take a diode, a long wire antenna, your sweep gen on 'manual'
put them together and see if you can get any audio across the
diode as you tune around - doesn't matter if it's horribly
distorted. If necessary use an RC or two between mixer and amp
if the resulting spectrum looks too busy.  Adjust osc level for
best results, then connect scope and turn on sweep.

For better performance, replace the single diode with a ring of
four and drive with a little center-tapped toroidal xformer so
as to make a balanced detector - much cleaner.

Or even better, use one of those analogue multiplier chips for
the mixer.

The nice thing about this direct conversion approach is you can
start off simple and improve the performance in little steps as
and when you need: RC filter, log amp, balanced mixer, lower
noise amp, etc.  Great project for the electronics novice, and
an interesting sunday afternoon for the expert.

Maybe it could be combined with a little circuit to ping a TC
resonator (pri, sec, or pri+sec) to give a tobacco-tin style
TC analyser?  Forget the scope, use a manual frequency sweep,
and an earphone or speaker to hear the audio frequency beats.
Ping the coil at a rate of say 200Hz, and you should hear this as
rough tone as you tune past each of the coil's resonant modes.

Would make a good way to measure the effective k of the coupled
resonators - if you have a freq counter hung off the oscillator:
just tune for the upper and lower beats and note the frequencies.
Potentially very accurate indeed, especially at low k.  For anyone
wishing to determine the difference (if any) between k measured at
60Hz, and the effective k at resonance, this might be one way to
go (hint!).

I guess it could also double as a bat detector (replace pinger
with microphone) or as a VLF receiver (replace pinger with long-
wire). 
--
Paul Nicholson
--