Dave Edwards writes...
> The 756 Pro scopes work great...better than the old Heath and Kenwood ones.
> In fact, a nice feature....if you see a low, open/dead spot on the band, you
> can put a marker there.
Just thinking about bandscopes has led me to several questions for
those who know. They reflect my current bias, but maybe I'm missing
something important.
1. Isn't the real-time bandscope in a rig like the 756Pro made
possible by the wide bandwidth of the first IF? To make that display,
signal strength with respect to frequency has to be measured
somewhere. I would think a radio with a really strong front end and a
good amount of selectivity would run into bigger problems with a
real-time bandscope than a radio that is more wide open.
2. I would think the selectivity problem would be overcome by using a
second receiver to scan the band to measure activity that is displayed
on the bandscope. I do that in a simple way with my RX-320, for which
Carl's control software provides a bandscope feature, though it does
take a little time to perform the sweep (and the receiver isn't
otherwise usable at the time). Does the Orion use the secondary
receiver to sweep the vicinity to generate the bandscope display?
3. In a radio where most or all selectivity is provided in software
(like the Icom pro-series rigs or the Flex SDR's), it seems to me a
bandscope is just a matter of evaluating the digitized IF stream in a
different algorithm in addition to the main algorithm used for
reception. But the RF had to be allowed to get all the way through the
digitizing process before subjecting it to selectivity, which seems to
me to open the door for selectivity problems such as desensing from
strong, close-in signals. The Flex seems to get around that, but I
don't know how. Am I missing some basic point?
4. One of the things I like about my Omni V is the strong RF filtering
in what amounts to a preselector and the strong (and very narrow
compared with rigs whose IF is in the VHF range) filtering going into
and within the first IF. These are the tools needed to prevent strong
signals on nearby frequencies from desensing the receiver or causing
mixing products that interfere with the desired signal. I would think
that all this filtering would make it impossible to provide a
real-time bandscope without using a second, unfiltered receiver path.
Again, am I missing something?
I'd personally rather have the selectivity for weak-signal work and
for contesting, where a bandscope isn't so useful. The sweep-style
bandscope on my RX-320 is good enough to identify pileups and strong
QSOs in progress. And with it, I can just click on the peak and the
RX-320 is retuned to that frequency. If it's something I want to
explore further, I can hit the "To Omni" button, moving the frequency
to the Omni V's VFO, and listen to it with all that selectivity I paid
for. The bandscope on a 756 is pretty, but at what price do we get it?
(Considering what I paid for my Omni V and RX-320, I would say that
the price of that bandscope is something in the range of about $1200,
and I ended up with better receiver performance.)
Rick, KR9D
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