3948	2010-01-05 09:55:33	Chris Trask	Spur-Free Synthesizers
Message	Date	From	Subject
Recently, someone made mention of a 6m synthesizer project that failed due to excessive spurious outputs. Here's how you design synthesizers that are basically spur-free: http://www.home.earthlink.net/~christrask/Paper023.html Chris ,----------------------. High Performance Mixers and / What's all this \ Amplifiers for RF Communications / extinct stuff, anyhow? / \ _______,--------------' Chris Trask / N7ZWY _
3949	2010-01-05 19:39:27	Tim	Re: Spur-Free Synthesizers
I appreciate you showing us the actual construction of your BCD DDS, that's pretty cool. For many ham applications, isn't it generally true that any currently technologically achievable straight DDS without cleanup via a PLL will generate objectionable spurs for at least some programmed frequencies? I was playing around with the AD 400MHz 14-bit DDS's a few years back and of the entire zoo of possible programmed frequencies, some mathematically "round number" frequencies made really blatant spurs nearby that were not easily filterable, and other nearly numerically equivalent (differing by a tiny fraction of a Hz) frequencies had no objectionable spurs anywhere nearby. It struck me that a really clever designer could probably use a DDS without much in the way of cleanup loops, if he could somehow know in advance which frequency control words would make the nasty spurs and choose a nearby (fracti
3950	2010-01-06 06:41:38	Chris Trask	Re: Spur-Free Synthesizers
> > I appreciate you showing us the actual construction of your > BCD DDS, that's pretty cool. > > For many ham applications, isn't it generally true that any > currently technologically achievable straight DDS without > cleanup via a PLL will generate objectionable spurs for at > least some programmed frequencies? > Possibly, though I didn't test this one at every frequency to see if the spur levels were consistent. I did make a measurement of the spurs at one frequency with a sawtooth output and then the trianglewave output, and saw that they were down at least 6dBc without the 3-pole lowpass filter. That was an unexpected benefit of adding the column of XOR gates to produce the trianglewave output. I just wanted to get rid of the even-order harmonics so that I could get a clean output without having to resort to the usual PROM/DAC arrangement. It may still be attactive to use an 8-bit DAC instead of the R2R/FET arrangement I've used. > > And as long as we're talking about clever clean synths, I am in > awe of the Elecraft K2's LO for its simplicity and cleanliness > (although it breaks your rule Chris by using a microcontroller). > I compare it with the fancy-pants NRD and WJ radios which use > literally a hundred times as much circuitry and aren't as clean. > The "rule" that I set was a result years ago of being confronted by a uP/uC programmer who claimed that RF design was a dinosaur and would eventually die. That later inspired my dinosaur ASCII art signature. Back then, I took him and a dead QAM encoder chip out in the parking lot, leaned back, and threw it as hard as I could. "That's how far your encoder can communicate without a radio, but I can make a simple radio that will bounce signals off the moon without it." He was not amused. Getting back to my approach, if you look at Fig. 1 you'll see that there is plenty of room for using a uC or PC to operate one of those really nice National Semiconductor LMX series PLL synthesizers for the reference together with an equally nice Analog Devices DDS and come up with a spiffy high-frequency fine-tuning signal source that will fit on a small PCB. But, I had a number of goals that I wanted to achieve with this: 1. Show how to make a spurious-free signal source with DDS fine tuning that does not require taking out a loan. 2. Show how to get rid of DDS even-order harmonics by generating a trianglewave output using a simple bank of XOR gates instead of a PROM lookup table. 3. Show how to make a BCD parallel-programmed DDS using discrete logic devices so that a uC interface is not mandatory. 4. Show how to make a suitable replacement for the obsolete Motorola parallel-programmed PLL devices using discrete logic devices, and improving their phase detector. And I'm serious about having someone step forward and reduce at least the DDS and 3-digit BCD priority encoder to an FPGA device. I'm absolutely certain that there would be at least a small market for such a device, and many designers would love to have a replacement for the obsolete parallel-programmed Motorola PLL devices, especially with the BCD interface. Chris ,----------------------. High Performance Mixers and / What's all this \ Amplifiers for RF Communications / extinct stuff, anyhow? / \ _______,--------------' Chris Trask / N7ZWY _
3952	2010-01-07 06:36:17	richj_focus	Re: Spur-Free Synthesizers
Chris, i do asic/fpga for a living and i could have the code done and simulated in a day. Another day to get it loaded
3953	2010-01-07 07:26:39	Chris Trask	Re: Spur-Free Synthesizers
Rich, It certainly would be worth the effort. Having a DDS that is BCD-programmed would undoubtedly be welcome, as would having improved replacements for the Motorola parallel-programmed PLL devices. It amazes me that nobody has ventured forth to provide these, especially when you see the prices that those parts command on auction sites and surplus dealers. What size PLCC to you envision the full DDS being in? I can get inexpensive PLCC sockets with 0.1" spacing from Digi-Key and Mouser. I'm going to see what I can squeeze out of Texas Instruments and Analog Devices for some AD/TLC7524 DACs as one person has recommended doing away with the R2R ladder so as to further improve the spurs, and I'm all in favour of improvements. Chris Trask N7ZWY WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/
3954	2010-01-09 03:33:36	ha5rxz	Re: Spur-Free Synthesizers
Chris Your paper states that "With the excepti
3955	2010-01-09 05:26:31	Chris Trask	Re: Spur-Free Synthesizers
Peter, Thank you for bringing that to my attention. I'll make mention of it when I revise the document. Chris ,----------------------. High Performance Mixers and / What's all this \ Amplifiers for RF Communications / extinct stuff, anyhow? / \ _______,--------------' Chris Trask / N7ZWY _
3956	2010-01-09 16:13:40	hanssummers2000	Re: Spur-Free Synthesizers
Hi Chris > Thank you for bringing that to my attention. I'll make mention of it > when I revise the document. Also, you might be interested in a DDS project I built some years ago, with an AD9851 DDS programmed by 24 DIP switches and with the additional assistance of a couple of 74HCxx logic chips. The project was later replicated by Armand ON4TL. He's eighty years old but evidently undaunted by the surface mount aspect! 73 Hans G0UPL http://www.hanssummers.com
3957	2010-01-10 07:04:23	Chris Trask	Re: Spur-Free Synthesizers
> > > Thank you for bringing that to my attention. I'll make > > mention of it when I revise the document. > > Also, you might be interested in a DDS project I built some years > ago, with an AD9851 DDS programmed by 24 DIP switches and with > the additional assistance of a couple of 74HCxx logic chips. The > project was later replicated by Armand ON4TL. He's eighty years > old but evidently undaunted by the surface mount aspect! > I like what you've done with the AD9851. I had thought of something similar using the 74HC244 instead of the diodes. To take what you've done one step further, if you were to use a 21.47483648 MHz clock, the AD9850/51 would have a resolution of 0.005 Hz, and from there you could implement some form of BCD programming, though it will take a fairly elaborate FPGA priority encoder to implement that so as to take full advantage of the potential. Both Digi-Key and Mouser have 21.47727 MHz crystals which might be possible to pull. Chris Trask N7ZWY WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/
3958	2010-01-10 08:42:30	w4zcb	Re: Spur-Free Synthesizers
what you've done with the AD9851. I had thought of something similar using the 74HC244 instead of the diodes. To take what you've done one step further, if you were to use a 21.47483648 MHz clock, the AD9850/51 would have a resolution of 0.005 Hz, and from there you could implement some form of BCD programming, though it will take a fairly elaborate FPGA priority encoder to implement that so as to take full advantage of the potential. Both Digi-Key and Mouser have 21.47727 MHz crystals which might be possible to pull. Chris Trask N7ZWY WDX3HLB Senior Member IEEE Yea, but it'd be damned difficult to keep it there. W4ZCB [Non-text portions of this message have been removed]
3959	2010-01-10 09:07:16	Chris Trask	Re: Spur-Free Synthesizers
> > > what you've done with the AD9851. I had thought of something > > similar using the 74HC244 instead of the diodes. To take what > > you've done one step further, if you were to use a 21.47483648 > > MHz clock, the AD9850/51 would have a resolution of 0.005 Hz, > > and from there you could implement some form of BCD programming, > > though it will take a fairly elaborate FPGA priority encoder to > > implement that so as to take full advantage of the potential. > > Both Digi-Key and Mouser have 21.47727 MHz crystals which might > > be possible to pull. > > > Yea, but it'd be damned difficult to keep it there. > What other possible combinations are available? The 6.5536 MHz crystal can provide 100Hz resolution in the discrete logic DDS that I've designed, but many people would like to see finer resolution, perhaps 1Hz. I'm also seeing that using an FPGA implementation of these discrete logic circuits may be cost-prohibitive, and keeping the cost low was a very serious design goal. However, I am going to bite the bullet and replace the R2R/FET output with a TLC7524 DAC so as to improve the output spurs, as well as rearrange the sixth 4-bit full adder in the priority encoder so as to provide the missing 128 offset pin and better enable lower base frequencies such as 200 kHz. The discussions about this both on- and off-line are giving me cause to re-evaluate certain aspects of the design and further improve it, keeping in mind the Howard Cossell maxim "We keep making it better, not more expensive". Chris Trask N7ZWY WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/
3960	2010-01-10 09:50:38	w4zcb	Re: Spur-Free Synthesizers
discussions about this both on- and off-line are giving me cause to re-evaluate certain aspects of the design and further improve it, keeping in mind the Howard Cossell maxim "We keep making it better, not more expensive". Chris Trask N7ZWY Howard obviously was not talking about professional football at the time. W4ZCB [Non-text portions of this message have been removed]
3961	2010-01-10 10:47:17	Chris Trask	Re: Spur-Free Synthesizers
> > discussions about this both on- and off-line are giving me cause to re-evaluate > certain aspects of the design and further improve it, keeping in mind the Howard > Cossell maxim "We keep making it better, not more expensive". > > Chris Trask > N7ZWY > > Howard obviously was not talking about professional football at the time. > Certainly not about the Patriots. They're behind 24-0 at the end of the first quarter. That's disgusting. Chris
4071	2010-01-23 21:33:44	Tayloe Dan-P26412	Q killing coatings
Someone was discussing the RF merits of different coating for coils. I have another vote for one to definitely avoid, Gorilla Glue. I have had two different folks use it on coils on some of the kits that I support and it appears very effective at killing RF. One was the VXO coil of a Firefly SDR, and the other the regen coil of a Scout. The regen simply refuses to run with the glue over the surface holding the turns in place. The firefly VXO barely ran. Something this good at discouraging RF has to have some useful application. - Dan, N7VE [Non-text portions of this message have been removed]
4072	2010-01-23 23:37:07	Lou Burke	Re: Q killing coatings
I've always used 5 min epoxy and haven't noticed any problems. Lou, W7JI ----- Original Message -----
4073	2010-01-24 03:28:59	ai9e_qrp	Re: Q killing coatings
I don't know if it had been mentioned in the thread, but 'JB Weld' or similar black epoxies should not be used near RF parts, especially coils. The black color comes from iron oxide. 73 - Dave NM0S
4074	2010-01-24 08:33:37	Dave	Re: Q killing coatings

4075	2010-01-24 08:36:01	Dino Papas	Re: Q killing coatings
Or if you don't want to roll your own you can still get it from Ocean State Electronics: http://www.oselectronics.com/ose_p89.htm Dino KL0S On 24Jan2010, at 11:33 AM, Dave wrote: > > The traditional material has been "coil dope". A suitable > alternative can be made from styrene. This can be obtained by > dissolving styrofoam peanuts in Acetone. > > > - Dan, N7VE > > Dave > WA4QAL [Non-text portions of this message have been removed]
4077	2010-01-24 09:23:01	bobtbobbo	Re: Q killing coatings

4082	2010-01-24 17:37:07	Pat Villani	Re: Q killing coatings
There is a message regarding their current state on their home page: http://www.oselectronics.com/ Pat WB2GBF
4083	2010-01-24 17:51:47	bobtbobbo	Re: Q killing coatings