The DDS has outputs at DC + desired frequency and clock - desired
frequency of equal amplitude. Then there can be many others depending on
the quality of the D/A and the number of bits in the D/A and waveform
data rom. Sometimes the amplitudes of those other spurs will depend on
the precise count of the desired frequency.
Years ago, the DDS principle caught my eye and I decided I could clean
up a 1 bit D/A (e.g. take the MSB raw for my output) with filters for a
narrow band application. That proved to be wrong as there were many
wandering spurs as I changed frequency slightly. It does take the
sinewave creating D/A and ROM to clean it up and that isn't perfect.
I believe the Japanese DDS radios use DDS for the fine tuning and a
coarse PLL for the bigger steps, like 10 KHz. I think some use the DDS
for the mixing step between the VHF first IF (with 15 kHz wide roofing
filter) and do the upconversion with 10 KHz steps in the PLL. Some use
the DDS to move the reference frequency of the 10 KHz PLL for fine
tuning. The first scheme will show up with asymmetrical close in
intermod or blocking measurements when the second mixer is its typically
inadequate design. That asymmetry will be frequency dependent repeating
each major PLL LO step. On a quiet band (like VHF SSB) with just one
signal per major PLL step, there are often clicks in the audio as the
multiple PLL switch over their ranges and the receiver sweeps over the
signals in that band segment. I'm going VCXO for fine phase noise of my
next VHF equipment.
Multiple conversion radios are always full of spurs, more so when the
shielding is skipped, like the Tentec Corsair II.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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