Joe,
I plan on trying some AM. I remember the old dedicated station monitors
of the past. I've seen some RF samplers & AF demodulators. These seem
to be in the $200 + range, along with the used older dedicated CRT type
station monitors.
There is retro fun in playing with them - but currently we are in the 21st
century! There are much better ways to monitor a signal.
I'm curious as to what folks are using to monitor modulation.
Most use nothing, and many apply the "all knobs fully clockwise" rule, which
results in the kind of signals we all have heard more than often enough. But for
the good hams who want to monitor their modulation, your computer is your best
friend.
The sound chip of an average computer has a bandwidth of 20kHz and a dynamic
range of about 80dB. Together with some free software, this makes a very
powerful and accurate measuring instrument. Very much better than any old
station monitor! You just have to bring your signal down into the frequency
range the sound chip can manage.
And that's quite easy: Simply build a diode ring mixer, or use a ready-made
SBL-1 or similar one. Get or build some RF signal generator. On eBay you can
find cheap, very usable Direct Digital Synthesizers, assembled or in kit form.
Or maybe you happen to have some signal generator - just make sure it's
reasonably stable, because it's a chore touching up the tuning of an old tube
type signal generator all the time. Drive the diode ring mixer from the signal
generator, connect its output via an ultra simple RF-bypassing filter (can be as
simple as a single capacitor!) to the computer's audio line input, and use a
piece of wire, about one meter long or so, to pick up some of your transmitted
signal and put it into the mixer. You can adjust the length and position of that
wire to get the desired signal level into the mixer and computer.
While operating, set the signal generator roughly 10kHz away from your operating
frequency, preferrably on the lower side, to avoid reversing the sidebands. This
will place your signal in the middle of the sound chip's passband. Then use any
software you want, to do clever things with that signal. You can use Spectrum
Lab (free) to look at the transmitted spectrum and measure the IMD of your
signal, the noise level, etc. That software can display a spectrum waterfall,
and it can average the signal over long times, among many other tricks. Or you
can use SDR software such as Dream, or many others, to demodulate your AM or SSB
signal and then watch its waveform, if that's what you want. I'm pretty sure
that HDSDR can do the same tricks, and look nicer. You can also losslessly
record the signal while you transmit, and then analyze it calmly after the QSO
has ended. Most software allows to analyze a recorded file just as well as a
live signal.
I plan on using a 30 MHz dual trace o-scope, it is a Hitachi V-355.
The computer is a MUCH better tool! I have a Tektronix scope here, but I never
use it for modulation checking. I did that several decades ago, but not now!
It's hard to see distortion below the -30dB level on a scope, let alone measure
it. Instead with the computer you can accurately measure it down to at least
-60dB, and usually beyond -70dB. That's with a cheap sound chip residing on the
computer's mainboard! With an external high quality audio interface you get much
better resolution, but for monitoring a ham signal, even an excellent one, a
cheap sound chip is plenty.
Manfred
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http://ludens.cl
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