On Jul 9, 2007, at 4:44 PM, Brian Lloyd wrote:
Well, you have just described the product offerings from Flex Radio.
Not exactly what I had in mind -- there's way too much hardware
involved.
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.
No. You don't "hope" the A/D has enough dynamic range. You have to
design in that kind of dynamic range. Which means you've got to use
an A/D converter which has lots of bits -- very expensive today.
Elecraft gets rid of the undesired signal by using tight roofing
filters.
Yup, that's one approach. It also is very cost-effective for a single-
signal type receiver.
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.
Consider the Yaesu FT DX 9000. This design has a 40 MHz first IF that
has roofing filters of 3, 6 and 15 kHz.
Similarly, the Icom IC-7800. This design has a first IF of 64.455 MHz
and roofing filters of 3, 6 and 15 kHz.
Both of these designs have competitive IMD and BDR characteristics.
It is possible to do this with wider roofing filters in an up-
conversion design. However, it isn't as cost-effective as the
Elecraft design. (I can buy several fully-loaded K3s for the price of
one FT DX 9000 or IC-7800)
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.
I totally agree. When I was first licensed, one of my dreams was to
design and build a state of the art homebrew transceiver. For a
number of reasons, I never quite did that, but with the Elecraft
K2/100, I've come close. (I didn't design it, but I certainly built it)
Bill Coleman, AA4LR, PP-ASEL Mail: [EMAIL PROTECTED]
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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