[Amps] Transmit signal monitoring AM & SSB

[Manfred Mornhinweg]

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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