Posted by
Geoffrey Mackenzie-Kennedy-2 on
Mar 08, 2005; 9:24am
URL: http://elecraft.85.s1.nabble.com/Receivers-tp375444p375446.html
On March 08, 2005, Bill Coleman wrote:
>
> > 1) Dual vs Single conversion.
> There are a lot of up-conversion designs of this form. The biggest
> problem with them is that the early stages are easily overloaded by
> adjacent strong signals unless the first filter is very narrow. Narrow
> filters at such higher frequencies are harder to build and more
> expensive. Many design simply use a 15 kHz wide resonator.
>
I would suggest that part of the problem is driven by cost, not
only by the production cost of a narrow bandwidth first IF filter but also
that of the "post filter" amplifier and 2nd mixer which should also be
'strong'. In my experience the wide bandwidth monolithic filters often found
at the head end also often yield poor IP3in performance, compromising to
some extent overall receiver performance.
> > Narrow first IF filters are essential in double conversion for
> > "strength"
>
> The TenTec Orion and the IC-7800 use a similar strategy. However, they
> use even narrower filters -- like 500 Hz for CW. This greatly improves
> the adjacent signal rejection.
By using strong push-pull amplifiers and strong mixers right along
to the 2nd IF filter, in this particular receiver the 1.5kHz bandwidth tight
skirted 1st IF filter has not yet caused problems on CW. There is space
assigned for a narrower filter, and without question a narrower filter would
reduce the workload placed on the following stages. So far they have coped.
> > 2) Effect on Noise Floor.
>
> Noise floor isn't a terribly big deal with an HF rig. It's not hard to
> design a rig with a noise floor lower than atmospheric noise.
Absolutely agree, provided the IP3in is maintained better than
that required.
> > 3) LO purity. Both require a low phase noise LO(s),
> Lots of work has been done in the last two decades to make low noise
> PLLs, ever since the specter of phase noise raised its head with the
> new general coverage transceivers.
Some very recent work done in a country East of here shows
promise. Very difficult to get information.
> > 4) DSP. Great will be the day when a DSP unit running at
> > VHF, with a high IP3in etc, and low noise figure becomes practical.
> All these things are theoretically easy -- just expensive with current
> DSP hardware. So long as Moore's law holds, that should change in the
> future.
Indeed, I hope that they can get the noise figure, IP3in etc
right.
73,
Geoff.
GM4ESD
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