Re: New MGM Modes on Beacons


Robin Szemeti - G1YFG
 

" When the RF is passed through a frequency multiplier these spurii increase in relative amplitude as the square of the multiplication factor"

I found this "interesting" as I would have thought that if you had say 100MHz at 0dBm and 90MHz spur at -30dBm ... and then whacked the signal through a x3 multiplier, by driving some MMIC or similar hard into clipping ... I would have expected the output to be swamped by the 100MHz signal, and the "weak" 90MHz signal to be effectively blocked, and thus removed from the output,  and by the time it had been through the output filter to pick up the 3rd harmonic, I would expect it to look very clean indeed.  I had always assumed that multiplier chains cleaned up the spurs, not made them worse. I shall ponder this further ...


On Tue, 16 Mar 2021 at 16:57, John Quarmby via groups.io <g3xdy=btinternet.com@groups.io> wrote:

Alwyn

I have not got hard numbers to hand, but I can compare it with the 23cm beacon, where the beacon is approximately 95dB above noise in 2.5kHz bandwidth and results in a rise in the apparent band noise of 5db when I beam at it on 1296.200. My LO and the beacon oscillator are almost identical, so probably each contributes the same amount of phase noise. I have a crystal notch filter in the 28MHz transverter IF to reduce the beacon signal level by about 50dB going into the K3.

The 13cm beacon is about 10dB weaker, there is no change in band noise at 2320.200 when I beam at it. There are no major spurs in the SSB/CW part of the band, but they are apparent in the beacon band. When we have got it on air again I will make some proper measurements.

73

John G3XDY

On 16/03/2021 13:19, alwyn.seeds1 wrote:
Dear John and Andy,

Getting DDS spurs low enough for beacon use must be quite a challenge. At work, all of our DDS based signal generators, including the pricey ones, have quite a few spurs at the -80 dBc level- the nuisance is that they do not have easily predictable frequencies. We moan to the manufacturers but I have not seen much improvement over the last decade.

John, what sort of phase noise numbers do you get from the GB3MHZ beacon?

Regards,

Alwyn G8DOH
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England.


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Robin Szemeti - G1YFG

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