The AM Forum

For grins I've built a cathode modulator for the Heathkit AT-1. Good audio, my friends say. But how good, or how bad is it really? How should I measure the distortion? The percent modulation? The bandwidth? The hum?

I have a reasonable handle on the percent modulation from my cat's meow REA Modulation Monitor, hardware version. I can look at bandwidth using a Flex radio as a poor man's spectrum analyzer with an audio sweep or white noise generator. I've done that before.

But what about distortion? I thought maybe using an HP 303A with a pick off from a high quality detector. The one in the REA might be good enough or I could borrow an HP 8901A.

Does this make sense? What else should I look at?

http://amfone.net/Amforum/index.php?topic=33555.msg259656#msg259656
-Interesting read!-

-A scope and REA monitor will absolutely tell the % modulation story.
-A "professional" distortion analyzer is great, however as the above link demonstrates, you can do quite well with your Flex and the proper test tones.

Ah, quantitative vs. qualitative measurements.

I did plenty 'audio proofs', or EPM's for AM broadcasters.  The hardest part was always getting a true demodulation of a complex carrier wave into a distortion analyzer.

Sometimes a simple swamped diode fed attenuated RF did the job, other times the audio out from the mod monitor was the ticket.  All in all, it was a paper chase to please the FCC inspector if he ever set eyes on it in the stations' files.

Just numbers.

For the best tonality, out came a good set of phones and a Streisand LP.  Maybe Placido Domingo.  Both of them had extremely rich voices, fragile voices that could turn to mush if ANYTHING from the turntable thru the detector was not right.

Fresh stylus.  Headroom.  Balanced modulator current.  Audio bandwidth.  RF bandwidth.  Good tubes throughout.  Is it below audibility with no program?

All could be summarily judged in real time by good ears.

Afterwards, every time, managers and program directors were pleased and always wondered just 'what did you do?

So try pumping some hi-fi audio into that mic jack and see what sounds like on a good radio.  You can't ask for more than that... :D

73DG

Ralph, w2wme(SK) said that 'piano' musick was a good test for distortion. I E, the ability to reproduce the music with litle audio distortion.




Klc

This is an excellent topic.  Good answers so far.

Over here, I use a few measures to prove out audio quality.

The first one is the triangle wave test.  I feed triangle waves at various frequencies into the transmitter and look at the envelope at 100% modulation.  It should be absolutely straight at all reasonable audio frequencies from 20hZ to at least a few k.  Few transmitters will actually pass this although the one described in the first post should.

The 2nd test I personally do is the square wave test - similar to the triangle wave test - square waves at various audio frequencies.   I expect a good square wave (particularly at low frequencies) with no tilt or overshoot.

The last test is an extension of the first test (triangle waves).  I feed the triangle wave into the transmitter and purposely overmodulate the transmitter to test linearity of the modulation above 100% positive.  This cannot be done with all transmitters - only those with negative peak limiters.  This is a very good test because you can see where the positive peaks start to fall apart or clip.

An alternative test to the one described above would be a waveform with a smaller duty cycle in the positive direction than the negative.  I use this one as well.

And of course sweep the frequency response with a sine wave to test for flatness and a reasonable high frequency cutoff.

Anyway, that's how I test.   Those triangles sure will tell the tale!

Regards,  Steve

Triangle waves work great. Hard test to pass if the rig has any problems at all - especially when the triangle is less than 100 Hz..

Also, once you have memorized what a perfect sine wave should look like by studying one on the scope directly from a good signal generator, you will be able to see VERY small distortion. The human eye is very good at seeing distortion as low as 0.1% on a sinewave. It's a matter of practice.

Using a negative peak limiter, look carefully at the sine wave's rounded peaks and crossover areas as you push the positive peaks above 100%. This is a very telling test.

Be very paranoid testing with a square wave if you are using modulation iron, especially below 100 Hz or so. Keep the % modulation very low.  Square waves can also generate overshoot/spikes in the high end of the slope.   Probably best used with class E rigs and balanced modulator type rigs.

T

I expect this super simple transmitter to flunk all the tough tests but I'll do them anyway, at least the triangle test. I will definitely do the plain old play music too. Since this transmitter and modulator design are from the early to mid '50s, I'll find something era appropriate to play.

Thanks everyone.

Is the design more or less direct coupled (or with very good resistance coupling)?  Most of the cathode modulators I've seen don't use transformers.

Easy to apply neg peak limiting too!

Your results may be very, very good - hopefully they are.

Regards,  Steve

I had typed a triangle wave post as well.

I first made sure my modulator passed them 20 to 20khz.  Once that was done,  it was time t hook the amp to it.

Amazing how chokes made with too small wire can distort a triangle when feeding a collector or drain!

--Shane
KD6VXI

Or, one could use a bi-polar or fet to modulate the cathode, in lieu of any inductive component, as Cunningham did here:

Phil - AC0OB