On Mar 7, 2005, at 8:51 AM, Geoffrey Mackenzie-Kennedy wrote:
1) Dual vs Single conversion. Both can be a disaster if the IFs are not
chosen properly With the IFs chosen properly, and the receivers built
properly, up-conversion to the first IF with the LO on the high side,
there
are far far fewer spurious responses to be found in a dual conversion
than
found with a single conversion receiver, and "weaker". The RF
preselector
in a "Dual" has something more to add to attenuation. 'Built properly'
is
very important.
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.
Narrow first IF filters are essential in double conversion for
"strength"
reasons, selectable by reed relays not diodes. Here I use VHF 12
poles, 6,3
and 1.5kHz.
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.
2) Effect on Noise Floor. Assuming the use of strong mixers (+50dbm)
and
strong low noise figure IF(s), the difference can be zero.
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.
3) LO purity. Both require a low phase noise LO(s), free of spurs.
Until a
cheap low phase noise PLL appears, I'll stick with premix systems
running at
VHF. Much more work, but worthwhile.
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.
4) DSP. I fully agree. I see little point in having a ho-hum filter at
the
front end of the IF, letting a cocktail of signals romp down the IF to
be
dealt with by a DSP module. 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.
Bill Coleman, AA4LR, PP-ASEL Mail: [EMAIL PROTECTED]
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
