On Sun, 08 May 2005 10:44:27 -0700, Tim Wescott
wrote:

The basic difference is that with a digital system you either end up
with a clean signal or a useless signal. In an analog system the
character and purity of the signal must be carefully guarded, at least
until you manage to digitize it. This means that there will be a much
greater chance that adding a new card to the radio will degrade not only
the function of the new card, but the function of all the other cards.

Second, the PC market is a huge one, with great advantages to be derived
from common equipment and software, and much smaller advantages to be
derived from commonality. This is the exact obverse of the radio
market, including homebrew radios. To make a "card" radio would be to
define a basic radio architecture, probably down to the IF frequency (or
at least to the point of forcing you to match your IF and front end).
While improvements could be made within this structure an independent
experimenter couldn't play around with such things as direct-conversion,
different IF schemes, etc., without extensive modification.

I agree that it would be quite hard to make a good quality radio with
some common backplane structure. However, connecting various
functional modules with 50 ohm input and output impedance could be
used to make quite different radios with good specifications. For
instance Mini-Circuits also makes various diode ring mixers,
amplifiers and apparently also VCOs that are boxed and have BNC or SMA
connectors. With each functional module in a metallic enclosure,
controlling the spurious radiation between modules is much easier. I
don't know that anyone would make filter modules, which would be
required to build a complete radio. Also SSB-Electronics sold separate
amplifier, mixer, frequency multiplier and crystal oscillator modules
mainly intended for a 10 GHz transverter.

Unfortunately the cost of these modules is quite high, apparently due
to low production volumes and large amount of manual labour needed to
assemble them. If there would be a large demand for such modules, it
would make sense to design them to require less manual labour to
assemble them and hence get the price to more affordable levels.

While a backplane would not be suitable for running the RF signals, it
would be a good idea to have a common control interface standard. This
might be some sort of serial interface or perhaps a CANbus interface
as used on some AMSAT satellites.

Paul OH3LWR