On Wed, 2009-02-11 at 19:50 -0700, pfizenmayer wrote:
> Bob - I think your thought about 2nd harmonic may be a very significant
> factor. I know that in my original Orion 565 I was trying to do a AF freq
> response watching the RF amplitude on a meter - Things looked screwy at low
> frequencies up to about 100 cycles -Then I hooked up my scope and looked at
> the RF envelope and there I could see the generation of very significant 2nd
> harmonic with AF input freqs under 100 cycles . Above 100 cycles that went
> away. I did not try to figure out where it was coming from as I was not
> really interested in USING the Orion with AF inputs under 100 cycles -it
> just showed up in testing . I have not looked at my OII to see if it does
> the same .
When you inject a tone below the cutoff frequency of a high pass filter,
often it takes very little distortion to make the second or third
harmonic show up in the pass band of that high pass filter. And the
audio/SSB selection filter of any radio acts like a high pass filter at
the low side of the passband. Successful measurement of a steeply
skirted high pass filter requires a perfect sinewave with NO distortion.
In a hardware radio the filter skirt may change 20 dB per 200 Hz, or 10
dB per 100 Hz so to not show that second harmonic of the 100 Hz input
the second harmonic has to be at least 10 dB down, or more. In the
digital radio that skirt may have a slope of 50 dB in 100 Hz which makes
it necessary for that test signal to be really clean.
73, Jerry, K0CQ
>
> The ne thing that went thru my mind is that the ESB guys should love that -
> it was a trick the old AM radio designers used to use to make the bass sound
> better .
>
>
> 73 de Hank K7HP
>
>
>
> > In thinking about this a bit more, if the slow sine modulation is seen
> > while
> > using the PSK mode then I would suspect the 2nd harmonic, being 62 Hz, of
> > the PSK signal beating against the 60 Hz line freq producing a 2 Hz
> > difference and modulating the PSK signal. Now the real question, if
> > indeed
> > it is the 2nd harmonic of the 31 Hz PSK signal, then where is it being
> > generated?
> >
> > Typically 2nd harmonics come from a single ended stage that is incorrectly
> > biased or is being over driven. Sound card, interface box, input stage to
> > the radio are all suspect and good places to start looking.
>
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