[ARC5] Lopsided modulation

[Bob kb8tq]

Hi

They seem to be reasonably close. Just as with phasing SSB, both 
amplitude and phase matter as you head for a full null (total sideband 
suppression). With some sort of weird generator that allowed you to 
AM modulate independent of the FM, you could play with that.

One way to think about the effect:

If I have two 1V p-p 400 Hz tones and sum them:

1) I can get 2V p-p if they are in phase

2) I can get 0V if they are 180 degrees out of phase

3) I can get anything in-between depending on what the phase is …

There’s no real guarantee that the FM and AM will be “in phase” when 
you are depending on strange interactions in a transmitter to generate
the FM….

The FM sidebands *will* be out of phase with each other. That is pretty 
much a given. It’s the phase between the AM and FM that is a “that depends”
sort of thing.

Bob

> On Mar 2, 2018, at 10:17 PM, Peter Gottlieb <kb2vtl at gmail.com> wrote:
> 
> I arrived back home and just ran an experiment.  I have a HP 8643A signal generator which can do simultaneous AM and FM modulation.  I also have a HP 8566B spectrum analyzer.  Both are reasonably in cal as far as I know, and here are some readings:
> 
> I set up the following:
> 4.000000 carrier
> -10 dBm
> 
> AM only modulation
> 400 Hz audio tone
> 80% modulation
> LSB:  -8.0 dBc
> USB: -7.9 dBc
> 
> Now I add simultaneous FM:
> Deviation:  40 Hz   (10 PPM)
> LSB:  -9.3 dBc
> USB:  -7.1 dBc
> 
> Deviation:  80 Hz   (20 PPM)
> LSB:  -10.6 dBc
> USB:  -6.1 dBc
> 
> Deviation:  160 Hz   (40 PPM)
> LSB:   -14.1 dBc
> USB:   -4.4 dBc
> 
> Trying to find a null, there seems to be a minimum LSB at 328 Hz deviation (82 PPM):
> LSB:   -37.1 dBc
> USB:  -1.0 dBc
> 
> Are these readings consistent with the math?  They certainly show the effects of FM on the sideband amplitudes (whether it affects receive audio fidelity is a separate question).  I'm using the signal generator default modulation phase shift between AM and FM which is presumably zero.  What it is in the transmitter is going to depend on the transfer function of the coupling between the PA modulation current and the VFO frequency.
> 
> Peter
> kb2vtl
> 
> On 2/26/2018 9:12 PM, Bob kb8tq wrote:
>> Hi
>> 
>> Ok, since you asked:
>> 
>> Carrier at 4 MHz
>> Modulation tone at 400 Hz
>> Deviation 20 ppm (not unreasonable for a VFO being pulled by a load)
>> First sideband at -12 dbc
>> Second sideband at =30 dbc
>> 
>> So how far down is the AM sideband? …. that depends
>> 
>> The FM sideband subtracts from one AM sideband and adds to the other one.
>> If The AM is at -12 dbc, you get a null on one side and 2X amplitude on the other side
>> 
>> Yes, phase also matters …
>> 
>> Bob
>> 
>>> On Feb 26, 2018, at 8:57 PM, Mike Feher <n4fs at eozinc.com <mailto:n4fs at eozinc.com>> wrote:
>>> 
>>> Rather than commenting, I suggest some of you review the mathematics behind modulation theory. To me, so far, none of the expiations offered make any sense. 73 – Mike
>>> Mike B. Feher, N4FS
>>> 89 Arnold Blvd.
>>> Howell NJ 07731
>>> 848-245-9115
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>> 
>