Whoops, looks like I forgot to send this. It got put in the wrong box. The
141T is quite able to see not only the envelope of the keying sidebands, but
the actual sidebands themselves. I've repeated some of these measurements
at 10 Hz bandwidth as well as 100 Hz and the sidebands are quite clear.
----- Original Message -----
From: "George Cutsogeorge" <w2vjn@rosenet.net>
To: <W8JI@contesting.com>; "CQ-contest reflector"
<cq-contest@contesting.com>
Sent: Sunday, March 11, 2001 2:52 PM
Subject: Re: [CQ-Contest] re:Clicks-REAL numbers.
>
> Lets look at this another way. Sometimes some analysis can put limits on
> what we should measure in the lab. This is a guide line to see if we are
> making the measurement correctly or if the equipment may not be giving us
> the correct answer.
>
> We are looking at a wave which is switched on and off at a 25 Hz rate.
> (This is like 100% amplitude modulation with a square wave.) When we do
> this, the resulting spectrum shows a carrier and many sidebands spaced 25
Hz
> apart on both sides. Lets assume for a moment that the switching signal
is
> a perfect square wave with very fast rise and fall times. This will give
us
> a limit. It represents the very worst case we could encounter.
i.e.Keying
> with no shaping at all. Now lets look at a specific offset frequency of
> about 1kHz and calculate what the sideband level would be. Since the
> modulation is a perfect square wave, there will only be odd order
harmonics.
> i.e. 1,3,5,etc. The harmonic we are interested in is 1000Hz/25Hz or the
> 40th. But there will be no energy there as it's an even order so lets
move
> to the 39th harmonic at 975 Hz. The Fourier analysis of this waveform
shows
> that this sideband will be 35.7 dB down. This is confirmed with a
> measurement on my old spectrum analyzer.
>
> This means that with the worst possible keying, the signal will be -35.7
dB
> down at plus and minus 975 Hz when a string of dots is being sent..
>
> Now lets add some shaping. Shaping will reduce the sideband levels, so
lets
> see what 5 mS rise and fall times do to the spectra. The 5 mS rise and
fall
> time is accomplished by passing the modulating signal through a 70 Hz
single
> pole low pass filter. This filter is 70 Hz wide at -3 dB. At 1 kHz this
> filter will be 23.1 dB down. This attenuation will lower the sideband
> levels in the case above by 23.1 dB. So for 5 mS rise/fall times the 39th
> sideband at 975 Hz will be -35.7 + (-23.1) = -58.8 dB.
>
> Now we have some limits Radios with keying rise times between 5 mS and 0
mS
> will have sideband levels at 1 kHz either side between -58.8 dB and -35.7
> dB. Unless I'm mistaken, this is a law of nature.
>
> Maybe some mathematical person out there could check over my numbers.
>
> For those interested, the 2000 ARRL handbook shows the line spectra of the
> exact case we are discussing on page 17-50.
>
> Note that this only addresses the amplitude modulation sidebands. If
there
> is any form of frequency or phase modulation during the keying, then the
> sideband levels will be different. Typically, if there is FM and AM at
the
> same time, the spectra will be asymmetrical as the sidebands add on one
side
> and subtract on the other.
>
> George, W2VJN
>
>
> --
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>
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