----- Original Message -----
From: "Ken Brown" <ken.d.brown@verizon.net>
>
> In order to measure the bandwidth occupied by a pulsed signal using a
> spectrum analyzer we have to integrate over a lot of spectrum analyzer
> LO sweeps. We could just send a string of dits and let it integrate for
> a minute or two, and probably get a good indication of the bandwidth
> occupied by a string of dits from that radio. Then what about rigs that
> have a different envelope shape on the first dit than on subsequent
> dits? You could use a 555 timer circuit to key the rig with delays so
> that every dit is a "first dit".
>
The FT-1000MP MK5 with the key click mod is a good example of
this. Unless you pre-bias the ALC circuit, it still clicks on leading
characters and inter-word leading characters. Looks (and sounds) really
good on intra-word characters, but the leading edge rise times on the
modified rig can be just as bad as the nominal unmodified rise.
> You can also manually do the spectrum analysis using a receiver with a
> narrow filter and an accurate S meter. The frequency resolution is
> limited by IF filters available. You might get narrower resolution using
> an audio filter and measuring the audio power output as you move the
> receiver to different frequencies. You would need the AGC to be OFF.
> The signal level would have to be low enough that no part of your system
> is going into compression or clipping, and yet high enough that the CW
> signal sidebands are out of the noise floor of the measuring system.
> There are a lot of things that can go wrong to mess up this measurement.
>
I have a 300 watt JFW attenuator that works nice for this. I pad down
the output of the exciter then run it into an HP step attenuator to knock
the signal down to something that a receiver can handle. You can even
sum in band-noise and qrm with a power combiner and an outside
antenna. Then you can listen to the signal as it would sound going out over
the air at whatever level you want (just dial it in with the step
attenuator).
For me, this is the real "proof-is-in-the-pudding" test since what I sound
like
on the air is what I care about.
As far as coming up with an occupied bandwidth number, you first have
to define your averaging time. With the modified FT1K MKV, the average
occupied bandwidth is pretty good, but the worst case instantaneous
bandwidth is pretty wide due to the leading edge clicks. That is why
listening
on a receiver is helpful since you get a sense of both the occupied
bandwidth
and the time dependency.
73 de Mike, W4EF..............................
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