Re: Receivers.

Posted by Bill Coleman-2 on Mar 08, 2005; 12:48am
URL: http://elecraft.85.s1.nabble.com/Receivers-tp375444p375445.html


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: [hidden email]
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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