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Old 9th Feb 2021, 9:40 pm   #1
Sparky67
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Default 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
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Old 9th Feb 2021, 10:26 pm   #2
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Default 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.
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Old 9th Feb 2021, 10:52 pm   #3
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Default Re: DDS Xtal Replacement

Thanks Terry. Small world eh?! I have sent Hans an email.
Cheers,
Martin
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Old 9th Feb 2021, 11:22 pm   #4
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Default 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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Old 10th Feb 2021, 12:11 am   #5
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Default 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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Old 10th Feb 2021, 3:27 am   #6
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Default Re: DDS Xtal Replacement

Quote:
Originally Posted by Sparky67 View Post
Small world eh?!
And less places to hide

Quote:
Originally Posted by mickm3for View Post
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
In most cases, a square wave is fine for xtal replacement purposes as long as the rest of the set is properly set up/aligned.

I would recommend checking drive levels though.
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Old 10th Feb 2021, 3:45 am   #7
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Default 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.

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Old 10th Feb 2021, 1:45 pm   #8
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Default Re: DDS Xtal Replacement

Quote:
Originally Posted by Julesomega View Post
Don't forget the 6V6 Electronics DDS 0-40MHz VFO Short Kit for AD9850. I built one and it does exactly as stated
Be aware though that some of the eBay AD9850 modules are fitted with 3.3v crystal oscillators. Their life on 5v can be short and hot.
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Old 11th Feb 2021, 10:19 am   #9
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Default 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
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