[ARC5] Lopsided modulation

[Peter Gottlieb]

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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>