The AM Forum

Greetings,

Been lurking for a while and reading the technical posts (which I find very interesting). 

A request if I may...

Does anyone have a SpecAn image of a two-tone (2T) test of an AM transmitter (plate modulated).  This sounds like the 2-T test of SSB for linearity and that's true but I wonder how a plate modulated AM signal would look, specifically the IMD products. 

The reason for asking is a curiosity how well "Envelope Tracking" would work on SSB and I think AM (plate modulated) is a close analogy.

Thanks,
 
k3Tim

Like these.

http://amfone.net/Amforum/index.php?topic=33555.msg260545#msg260545

Sorry I disappeared for a bit....


Thanks for the link but I was looking for a class A of AB amp using plate modulation.  With the two-tone test captured via spectrum analyzer.

Best Regards,

Tim

Plate modulating an Class AB amp?

Tim

The RF output spectrum, that will be produced by modulating a plate modulated AM transmitter with the sum of 2-audio tones, is not a trivial thing to predict/model... but, of course, you can measure it experimentally with a given transmitter, a given modulator, a given pair of tone frequencies and input tone levels, etc.

There are a number of assumptions and approximations (some of them involving fairly complicated mathematics) that underlie the following answer to your question.

Example: the tones are: 700Hz and 1900Hz (i.e. not harmonically related)

The spectrum will consist of the following components (at frequencies relative to the carrier frequency):

A. Linear modulation components at +/- 700Hz
B. Linear modulation components at +/- 1900Hz
C. Components due, to the 2nd order (square law) distortion of the modulator and/or the modulated RF stage, at: +/- (1900-700)Hz, +/- (2 x 700) Hz, and +/- (2 x 1900)Hz
D. Components due to the 3rd order distortion of the modulator and/or the modulated RF stage at: +/- (1900 + 1900 - 700)Hz, +/- (1900Hz - 700 - 700)Hz, +/- (3 x 700)Hz, and +/- (3 x 1900)Hz

One can contrast the above with the spectrum of an a typical SSB transmitter in the case where distortion products are primarily products resulting from the imperfections of the final RF "linear" amplifier.

In that case (USB), the spectrum will consist of the following components (at frequencies relative to the suppressed carrier frequency)

A. A linear modulation component at +700Hz
B. A linear modulation component at +1900Hz
C. Essentially no 2nd order components... unless the low level audio circuitry is introducing audio distortion products. 2nd order distortion products produced by the linear amplifier will appear at twice the RF fundamental frequency
D. Components due to the 3rd order distortion of the linear RF amplifier at: (1900 + 1900 -700)Hz, and (700 + 700 -1900)Hz  [Note, these 3rd order IMD components appear (on the spectrum display) at 1200Hz above the 1900Hz component, and at 1200Hz below the 700Hz component... and, therefore, they will appear to be symmetrically situated, relative to the pair of 700Hz and 1900Hz components].

Stu

That part of that post is true.

However the posted/linked spectrum shows an r-f carrier along with the upper and lower sidebands produced by a conventional DSB (A3A) transmitter modulated by a single ~sine-wave tone.

Applying 2-tone modulation to an A3A transmitter will produce (at least) two spectral components on each side of the carrier.

SWL

Please clarify:

How does your comment lead those of us who are following this thread toward an answer to the question that was posed by Tim at the beginning of this thread?

Stu
AB2EZ

Hi Stu,

Thanks for the detailed formulas and analysis of AM / SSB 2-T testing.  I've read tube audio amps sound better due to the 2nd harmonic distortion, or that is part of the theory.  The equations show it is presents although perhaps these equations don't map to the audio world.  None the less a *very* informative post.

I'll have to check at work if someone has an old school AM xmtr to runt his experiment.  We have a boat load (literally) of spec-an available.

hmmm..  I have a pair of mono-block Class A, (300B) tube amps, wonder how hard it would be to turn them into an AM xmtr.

Best Regards,

Tim

se 300b amp is 7 to 8 W of audio so could mod a low power tx then follow with rf amplifire ....

or could sell for outrageous $ and buy or build somphin suitable ....

or keep the 300B's and build something suitable...