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9th Feb 2021, 10:40 pm | #1 |
Heptode
Join Date: Feb 2008
Location: Great Barr, Sandwell, West Midlands, UK.
Posts: 589
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DDS Xtal Replacement
Hi all,
Can anyone point me at a kit ideally, or a working design for a DDS synthesiser to replace HF xtals in a transceiver please. I only need one freq initially, so VCO circuitry isn't required. I have spotted VK5TM's design but was hoping something was available a bit closer to home... Many thanks, Martin G4NCE |
9th Feb 2021, 11:26 pm | #2 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,340
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Re: DDS Xtal Replacement
https://www.qrp-labs.com/ has similar in the form of 'ProgRock', look in the menu on the left of page.
You will have to send Hans an email though as I keep getting a 'Server down' message for the ProgRock page. |
9th Feb 2021, 11:52 pm | #3 |
Heptode
Join Date: Feb 2008
Location: Great Barr, Sandwell, West Midlands, UK.
Posts: 589
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Re: DDS Xtal Replacement
Thanks Terry. Small world eh?! I have sent Hans an email.
Cheers, Martin |
10th Feb 2021, 12:22 am | #4 |
Heptode
Join Date: May 2015
Location: Chatham, Kent, UK.
Posts: 964
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Re: DDS Xtal Replacement
Hi also the vfo kit both kits require a lo pass filter to get sign wave out, I have built sevral of his kits they work well Mick
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10th Feb 2021, 1:11 am | #5 |
Nonode
Join Date: Dec 2012
Location: Stockport, Greater Manchester, UK.
Posts: 2,117
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Re: DDS Xtal Replacement
Don't forget the 6V6 Electronics DDS 0-40MHz VFO Short Kit for AD9850. I built one and it does exactly as stated
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10th Feb 2021, 4:27 am | #6 | |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,340
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Re: DDS Xtal Replacement
And less places to hide
Quote:
I would recommend checking drive levels though. |
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10th Feb 2021, 4:45 am | #7 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,901
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Re: DDS Xtal Replacement
The DDS output gives quantisation distortion and a stepped approximation to a sinewave.
If you use a different look up table to get a squarewave approximation, you get jitter. Making a sinewave, lowpass filtering below Nyquist and only then using a comparator to create a square wave (if that's what you want) performs as an analogue interpolator, fixing a fair amount of the quantisation distortion and doing interpolation for you. So all DDS require an output filter (lowpass or bandpass) or else you take a hit on spectral purity. David
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10th Feb 2021, 2:45 pm | #8 | |
Tetrode
Join Date: Mar 2018
Location: Nottingham, Nottinghamshire, UK.
Posts: 88
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Re: DDS Xtal Replacement
Quote:
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11th Feb 2021, 11:19 am | #9 |
Heptode
Join Date: Feb 2008
Location: Great Barr, Sandwell, West Midlands, UK.
Posts: 589
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Re: DDS Xtal Replacement
Thanks all. Think I need to open the radio I was looking at generating the LO digitally for and get on the air (a Decca Messenger) instead of just working from the service manual, to see how much space there is inside. TBH the LPF does look a little on the overkill size at 10W to help replace a xtal... Although a boxed external variable source with an LCD does now sound an attractive, and more versatile, option.
Noted re the 3.3V/5v oscs. Martin G4NCE |