2231	2008-10-12 00:25:00	Ashhar Farhan	Audio DDS as a reference for PLL
Message	Date	From	Subject
If we consider a 7MHz VCO fed to divide-by-1024 counter, we would get a 6.83KHz output, that is in the audio range. We can then program a PC card to generate a sine wave of 6.83KHz (using it as a DDS) and use it as a reference to lock the VCO. The current day PC sound cards can be used to generate frequencies of 22KHz and more with resolution of 16 bits and more. My specific issues are: 1. How will the phase noise of the audio card effect the VCO? can we use an active band-pass filter to clean up the sound card output before feeding it to the phase detector? 2. Though tuning can be very fine, is there a way to theoretical predict the in-band spurs of such a system? using a non-dedicated dds for frequency control is interesting for me as a pc is available in every home lab. here is a link (though i have no idea of how well it performs, onno - are on this list?) http://www.geocities.com/pa2ohh/04dds1.htm - farhan
2234	2008-10-12 03:35:39	Lasse	Re: Audio DDS as a reference for PLL
Ashhar Farhan skrev: > 1. How will the phase noise of the audio card effect the VCO? can we > use an active band-pass filter to clean up the sound card output > before feeding it to the phase detector? > I am sure you need a lot of filtering to get a resonably clean reference. With a divisor of 1000 you get a 60 dB multiplication of phase noise and spurs from the reference to show up witin your loop bandwith. Outside of your loopfilter the VCO will set the noise. > 2. Though tuning can be very fine, is there a way to theoretical > predict the in-band spurs of such a system? > See above, with-in the loop filter the spurs from your reference will be mulitplied 60 dB, but outside of the filter you will mainly have the reference leak through every 6 kHz. The amount of this is a function of what phase detector, filter and sheilding you have. TANSTAAFL or There Ain't No Such Thing As A Free Lunch
2235	2008-10-12 16:49:38	Murray	Re: Audio DDS as a reference for PLL
Hi Farhan I've built a version of Onno's PLL using his discreet phase detector but a different PIC for the DDS. My reference runs 120khz - 230khz or so. I use this to give me a stable VFO for an old FT-101 so I can experiment with the various digital modes. I did (do!) have issues with noise from the DDS and/or the phase detector but because I don't need to change frequencies quickly I can damp them out of the control loop. While it wouldn't work for break-in, the VCO settles in less than a second. I'm reasonably confident that any noise phase or otherwise I suffer is just normal oscillator noise in the VCO. Cheers Murray VK3ACF Melbourne Ashhar Farhan wrote: > If we consider a 7MHz VCO fed to divide-by-1024 counter, we would get > a 6.83KHz output, that is in the audio range. > We can then program a PC card to generate a sine wave of 6.83KHz > (using it as a DDS) and use it as a reference to lock the VCO. > > The current day PC sound cards can be used to generate frequencies of > 22KHz and more with resolution of 16 bits and more. My specific issues > are: > 1. How will the phase noise of the audio card effect the VCO? can we > use an active band-pass filter to clean up the sound card output > before feeding it to the phase detector? > 2. Though tuning can be very fine, is there a way to theoretical > predict the in-band spurs of such a system? > > using a non-dedicated dds for frequency control is interesting for me > as a pc is available in every home lab. > > here is a link (though i have no idea of how well it performs, onno - > are on this list?) > http://www.geocities.com/pa2ohh/04dds1.htm > > - farhan > >
2236	2008-10-12 16:49:41	Gheorghian Romeo	Re: Audio DDS as a reference for PLL
There will be no significant spurs, at the output VCO if the final divided frequency will be like 6kHz or better more in a PLL HF system. The PLL loop filter will suppress to a insignificant level any spur if the frequency at the fq and phase comparator will be more than 1 KHz, better 6 or much better 12kHz. There are very good PLL synthesizers made in the past years using VCO's which frequency was divided using programmable CMOS or ECL&CMOS dividers (like MC145170...) to 1 kHz or around and have very high performance, see the K2 project made by Elecraft. I see no reas
2237	2008-10-12 16:49:41	Gheorghian Romeo	Re: Audio DDS as a reference for PLL
Correcti
2238	2008-10-12 22:01:33	Ashhar Farhan	Re: Audio DDS as a reference for PLL
the software required to generate this reference shouldn't be too difficult. a few years ago, I had written a dsp shell to experiment with the dsp algorithms of emrfd, it can be easily used to generate the reference. this is intriguing enough to 'throw code at the problem'. i'll finish the pc end this week and post results soon. - farhan On 10/13/08, Gheorghian Romeo <yo8can@yahoo.com> wrote: > Correcti