One way of simplifying this whole process is to move the radio into
the computer. This is in line with a prediction I made way back in
1995 on the CQ-Contest list -- that the radio of the future would
be inside the computer. In 1995, processors were just appearing
that had sufficient DSP capability to do this. 12 years later, it
would take a relatively insignificant portion of the main CPU (or
just a portion of a few cores, as multi-core machines are now common).
What you'd end up with for the "receiver" would be a Mixer and
clean DDS, followed by a high-speed, wide-range A/D converter.
Everything else would be done in the host computer. The
"transmitter" would go the opposite way, a D/A converter followed
by a mixer fed by a DDS. Power amplification could be external to
the computer (if the transceiver were a card).
The interesting part of this approach is that we can re-define what
we mean by a receiver. The detection portion of the radio need not
resolve to the width of an audio channel. Consider a receiver that
can decode every CW signal in a 50 kHz portion of the band.
Simultaneously. How useful would that be?
Well, you have just described the product offerings from Flex Radio.
They are certainly interesting competitors to Elecraft. They are a
completely different approach to constructing the radio. I am not
convinced that their approach is better than Elecraft's but they are
certainly interesting.
It also would be good to sell the receiver and transmitters
separately. That way, obtaining the two receiver, one transmitter
configuration needed by SO2R operation could be inexpensively
obtained.
Or they could be multiple boards in the same chassis.
Of course, to achieve the IMD and dynamic range of the K3, the
mixer and A/D would be pretty marvelous pieces of equipment.
Elecraft has exactly the same issues in the K3.
My concern over the Flex Radio SDR approach compared to Elecraft's
approach in the K3 is that, in order to be able to receive multiple
signals simultaneously, e.g. like we do in demodulating PK31, you
have to accept all the noise and cruft in the wider passband. If
there is a strong signal in there you have to pass it through to the
A:D and hope that the A:D has sufficient dynamic range to deal with
the difference between the desired signal and the undesired signal.
Elecraft gets rid of the undesired signal by using tight roofing
filters.
Basically you make you choice and accept the limitations. Elecraft
has optimized for reception of a single signal. Want the ultimate in
CW reception? I think that the K3 is probably the winner. Want the
ability to demodulate several signals at once or do some new wideband
mode? I think that you have to look at the Flex Radio offerings. But
you give up performance in one area to get performance in the other.
There ain't no such thing as a free lunch.
Let me put it another way: one of the reasons that the Elecraft
receivers work so well is that they do fewer conversions and use
lower IF frequencies so that they can put good filtering as far
forward in the chain as possible. This gets rid of products that
could cause IMD in later stages.
It's still possible to get good IMD characteristics with an up-
conversion general-coverage receiver. There are some $10,000 radios
on the market that do exactly this.
But all of them upconvert to something like a 70MHz 1st IF. You
aren't going to find a 200Hz roofing filter there. That means you
aren't going to get the good close-in (1KHz spacing) IMD and BDR
performance. So to get general coverage receiver performance you give
up close-in BDR and IMD performance. Again TANSTAAFL.
Elecraft has apparently mastered the art of offering high-
performance gear at an excellent price point.
I agree. Elecraft should be receiving the order from the ARRL for our
school's K2. I plan to let the kids (4th-8th grades) build the rig
under my guidance. I think that the K2 will perform a lot better than
the other rigs that they were offering us, e.g. Icom IC-706, and I
think that the kids will understand and appreciate the radio better
if they have a hand in building, testing, and calibrating it.
(Besides, it will dovetail nicely with my "this is how a radio works"
section in science class.)
I want a whole boatload of demodulators there in the K3's DSP with
access coming out to me in some convenient fashion -- like on an
ethernet connector.
Sounds like what you really want is something more like the 1995
pipe dream.
Well, it is not a pipe-dream anymore. You can have it. It all depends
on what parameters you want to optimize for.
Bill Coleman, AA4LR, PP-ASEL Mail: [EMAIL PROTECTED]
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
73 de Brian, WB6RQN
Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com
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