W1EZ:
>I believe there are significant misconceptions here that should be addressed
by someone closer to the Flex Radio product than myself. As I understand
it, there is no need for a "roofing filter" in the SDR-5000 because of its
direct conversion architecture - the same thing that makes my K2 so good
without a roofing filter. Yet I have now seen shots taken at the SDR-5000
in several places for its lack of a roofing filter. Am I ignorant of the
facts, or are they? Also, making the decision to purchase one radio over
another based on a perceived shortfall in the 20 kHz dynamic range when the
close in 2 kHz dynamic range of the SDR-5000 is superb, and possibly the
best in the industry, seems wrong.
Rich, SDRs using QSD are fundamentally different beasts
than rigs like the K3 or Orion, even though those have DSPs at
the final IF stage. You will see things like IMD, BDR and
Phase Noise be constant with frequency in SDRs, unlike the
more classical rigs with roofing filters (and yes, even the K2
has a roofing filter at it's 1st IF).
I feel we need to be agnostic about the basic
design (i.e. roofing filter vs QSD) and focus on the
fundamental metrics like IMD, BDR and Phase Noise. We
also need to look at these at all spacings, and not only
close-in. SDRs will often be better at narrow spacings,
but they do not improve at wider spacings which can be a
disadvantage. Interfering signals can cause problems
even at very wide spacings where an SDR may be at a
relative disadvantage to classical rigs.
As an example of this, take a look at Phase Noise:
Rig 1kHz 2 10 20 50 100 1M
K3 -110 -119 -136 -140 -143 -144 -150
5000 -123(flat)*
*(Sherwood) "Phase noise does not fall off at 6 dB per octave as expected.
Flex believes the present phase noise limitations are caused by A/D
clock jitter". ("Clock jitter" is another word for phase noise).
In this case the K3 has the advantage, since its phase noise
integrated over the entire spectrum will be lower. The 5000's relatively
higher phase noise will likely be an issue, especially for VHF users.
Bottom line: I would look at all 3 sources of spurious
signals integrated over a wide range of frequencies.
73, Bill W4ZV
_______________________________________________
FlexRadio mailing list
[email protected]
http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz
Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/
FlexRadio Knowledge Base: http://kb.flex-radio.com/
FlexRadio Homepage: http://www.flex-radio.com/
