At 06:46 PM 1/26/2006, you wrote:
would that increase the number of spurs but lower in amplitude?
KD5NWA <[EMAIL PROTECTED]> wrote:
- Here is another item that will tweak your sense of reason.
- A DDS can have all of it's spurs cleaned up if when you take the sine
- wave value out of it's lookup table and add a small random number to
- it, then that value is sent to the DAC and output.
- The reason is a follows;
- The imperfections in the Phase accumulator, the sine table, and the
- DAC are highly predictable, hence every time particulars frequency
- sine waves are produce the errors are roughly in the same place.
- Depending on the reason, they repeat over and over adding power to
- that spur. When you randomly change the sine wave you make those
- spurs occur in different spots so the power does not add up.
- The result is not necessarily more desirable since the spur energy
- has to go somewhere, what happens is the noise floor goes up a few
- decibels but the output is free of spurs.
- Raising the noise floor lowers the MSD ratings of the radio, but you
- have the consolation that you are not picking up unwanted signals.
- It is important to have the front end be as tight as possible and not
- allow wide areas of RF to come in, so there are no signals out of
- band to mix with the spurs and produce an output.
- At 01:39 PM 1/26/2006, you wrote:
- > >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
- >
- >
- >
- >
- >
- >Yahoo! Groups Links
- >
- >
- >
- >
- >
- >
- >
- >
- >
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- Cecil Bayona
- KD5NWA
- www.qrpradio.com
- "I fail to see why doing the same thing over and over and getting the
- same results every time is insanity: I've almost proved it isn't;
- only a few more tests now and I'm sure results will differ this time ... "
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Cecil Bayona
KD5NWA
www.qrpradio.com
"I fail to see why doing the same thing over and over and getting the same results every time is insanity: I've almost proved it isn't; only a few more tests now and I'm sure results will differ this time ... "
YAHOO! GROUPS LINKS
- Visit your group "soft_radio" on the web.
- To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
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