[AMRadio] Frequency Response


Gary Blau gblau at w3am.com
Sun Sep 26 21:58:36 EDT 2004


By 'tilt' I mean the observable tilting of the top and bottom of a
square wave (or resulting modulated envelope) caused by low frequency
roll off and non constant group delay through the transmitter in
question.
Ideally what you want is waveform fidelity, what goes in should come
out.
The closer you can get to this ideal the better.
Square wave tests are very useful as they will tell you a lot of info in
one picture, and many hams have function generators available, (and if
you operate AM you should already have a scope).   

This tilt will increase the peak voltage of the incoming waveform.  This
in turn will force you to reduce your average modulation level to keep
the increased peak level from overmodulating the transmitter.
The same result happens from any ringing or overshoot the system may
have.

Where this becomes a significant issue is if you try to drive this poor
performing xmtr with waveforms that have flattop components, typically
produced by audio clipping.  You end up fighting yourself as you just
increased your average level with the clipping, but are then forced to
turn it down again to some extent by the inability of the xmtr to pass
the waveshape.  Since some degree of peak control is necessary to
prevent negative overmodulation (and the resulting splatter and
unneighborly excessive occupied bandwidth), this all conspires against
you to produce a less than fully modulated signal.  There are some low
level audio gimmicks that can be applied to partially compensate for
this, but it's always best to go after the root cause and improve that
first.      

Granted this is only ham radio and it's not a big deal if everything
isn't optimized.  But it's interesting to look at all this and try to
improve things, and it gives us something else to talk about on the air
other than politics!  

I'm looking for some helpful pictures for you but haven't located any at
hand.  
FWIW, you might look at Fig. 2-9 on page 2-22, and Fig. 2-10 on page
2-24 of the operating manual for the Orban AM Optimod 9100B, which is on
their FTP site here:

ftp://ftp.orban.com/9100B/9100B_Manual_Section_2.pdf

That may help make it more clear.

g


Byron Lichtenwalner wrote:
> 
> Gary
> I have read several of your writings that refers to "tilt".  What is it, or
> where can I go to learn more about it?
> Byron W3WKR




More information about the AMRadio mailing list

This page last updated 16 Dec 2017.