[time-nuts] PM-to-AM noise conversion

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Jan 17 19:50:14 EST 2008


Bruce Griffiths wrote:
> John Miles wrote:
>   
>> Hmm, I didn't know that.  Wouldn't that effect have suppressed the 2nd lobe
>> almost entirely on the baseband measurement, though, rather than making it
>> appear 6 dB too high?  I should probably repeat that measurement to make
>> sure of what I'm doing.
>>
>> Much appreciate the advice, by the way!
>>
>> -- john, KE5FX
>>
>>   
>>     
> John
>
> In a linear system the corresponding USB and LSB sideband component
> amplitudes are identical so that on translation to baseband by the phase
> detector all the even harmonics cancel and the odd harmonics add.
> However in a non linear system the USB and LSB components will not have
> equal amplitudes. Assymmetric filter passband response will also
> attenuate one sideband with respect to the other.
>
> If the phase detector LO phase shift isnt exactly 90 degrees wrt to the
> carrier the even harmonics no longer cancel exactly.
>
> In an I + Q demodulator FM USB and LSB components have the following
> characteristics:
>
> I channel:
>
> for odd harmonics (of the modulation frequency) USB and LSB components add
>
> for even harmonics (of the modulation frequency) USB and LSB components
> cancel.
>
> Q channel:
>
> for odd harmonics (of the modulation frequency) USB and LSB components
> cancel
>
> for even harmonics (of the modulation frequency) USB and LSB components add.
>
> Bruce
>
>
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>   
Whoops reverse that:


Q channel: (phase detector)

for odd harmonics (of the modulation frequency) USB and LSB components add

for even harmonics (of the modulation frequency) USB and LSB components
cancel.

I channel: (AM detector)

for odd harmonics (of the modulation frequency) USB and LSB components
cancel

for even harmonics (of the modulation frequency) USB and LSB components add.

Bruce




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