>The chances are that one product,
>somewhere, will come out unpleasently close to the wanted output, and it is
>quite illuminating to see just how quickly the spurii change for even a
>modest shift in output frequency.
OK, herein lies the rub, I think::>
It seems to me that a real world receiver will have it's performance
significantly degraded when running with a DDS LO.....
We aren't talking a few spurs here that we can steer clear of (by
design), are we?
Let's say we have a conventional 7 Mhz double conversion receiver.
The first IF is 10.7 Mhz, the second IF is 455 KHz. The front end is
tuned, but not overly sharp, but lets say it admits out of band
signals from 5 to 15 Mhz. Our DDS operates at 17.7 Mhz (10.7 + 7
Mhz). For the sake of argument, let's assume our mixer is a 'good' mixer.
The DDS operates at 17.7 MHz, but it has hundreds (or thousands) of
sprurs, in the 70 to 90 db down range (relative to the desired 17.7
MHz desired output). Some of these spurs happen every few hundred
Hz, some happen every few KHz (on average). While the spurs are 70
to 90 db down, there are many of them. Each spur allows the receiver
to receive a small amount of interference, which mixes with many 30
to 60 over S9 signals within the range of the front end tuned circuit
(5 to 15 MHz).
While spurs down 70 to 90 db sounds rather harmless, the end result
is a disaster because there are so many of them and because out of
band signals within the range of the front end tuned circuit are very
strong and numerous as well. You end up with hash, all across the
desired listening frequency that really isn't there.....and your
ability to copy weaker signals is gone!
Clearly, any DDS used as a local oscillator should be de-spurr'd (is
that a word)?
Is my assessment of the overall impact on the receivers ability to
copy weak signals based on the example above accurate or am I missing
something? If the receiver's ability to copy weak signals is
compromised by using DDS technology, shouldn't we (as a group) be
incorporating de-spuring countermeasures every time we hook up a DDS
for use as a LO in a receiver?
The point I'm trying to make here is that we aren't just concerned
with the spurs that happen near the desired listening frequency that
cause us grief!!!! It's ALL the spurs mixing with ALL the signals
within the bandpass of the front end filter that we need to be concerned with.
Correct, or incorrect?
Regards,
Art
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