Hi The same “small fraction of a degree” and “small fraction of a db” issues that plagued the analog SSB generation process still get into this approach. For “good” ADEV you need spurs down below the -130 dbc range (and likely much lower). This only gets you to 60 db or so ….
Bob > On Jul 1, 2020, at 5:38 PM, glen english LIST <glenl...@cortexrf.com.au> > wrote: > > Hi Bob > > I imagine in physics there are times when you want an oscillator to move a > few Hz for offset, and the oscillator is fixed due to some physical / atomic > property. > > yes, the whole thing will be phase locked, so no issue with freq error. For a > fixed frequency operation , +45 and -45 deg networks for the HF will be > accurate enough. I'll actually look at generating it out of the micro that > is already on the board (as it loads the PLL and also indicates to the user > presence of 10 MHz and input level) . see what the jitter calculates to be. > > Anyway, thats one way to do it I guess if you have a source that is not quite > where you want it but otherwise good. > > hi hi , an SSB generator ha ha who'd thought I would be back in analog SSB > generators after my years in SDR... > > g > > On 2/07/2020 12:14 am, Bob kb8tq wrote: >> Hi >> >> There is a NIST paper (somewhere) that has an example of doing this. Like >> any image >> reject mixer approach, it only does just so well. It’s no different than >> generating SSB >> the same way. You get a spur that is 40 to 60 db down at the “image” >> frequency. You can >> tweak this or that to get it to the 60 db point, how long it will stay there >> …. that depends … :) >> >> Since you are summing a very low frequency signal with a very high frequency >> one, the >> accuracy of the low frequency signal does not need to be very good. A 1% >> accurate >> 5 Hz will be off by 0.05 Hz. Your combo also will only be off by 0.05Hz. For >> a lot of >> Ham sort of stuff, that’s plenty good enough. >> >> Indeed, the noise of the 5Hz and the mixer setup does get into the act. That >> may >> rule out a simple R/C oscillator as the source of the 5 Hz “tone”….. >> >> Bob >> >>> On Jun 30, 2020, at 9:36 PM, glen english LIST <glenl...@cortexrf.com.au> >>> wrote: >>> >>> Hello group >>> >>> I have an idea that might work, and I wanted to discuss with likeminded >>> that might already have experience with the problem. >>> >>> Shifting a fixed oscillator a few Hz using a image reject mixer. >>> >>> background : From time to time I (and others) make lock boards for ham >>> gear, pulling the internal VCXO (vary from 11 to 55 MHz ish - which are out >>> by a few Hz ) in against a 10 MHz input. Frequency accuracy is required for >>> narrow band modes, and low phase noise 10kHz-200kHz is required as not to >>> desense your ham neighbours. >>> >>> I use fast LVDS diff receivers to square stuff up and ADF4157 high res >>> fract and about 10Hz BW. That's all fine. ....That aside, there are a >>> bunch of radio that have only XOs, no control facility. Varying the supply >>> voltage as a means of control is one way, but that doesnt work for the >>> ovenized nor internal regulator types. >>> >>> Other people to solve this problem by applying an external oscillator that >>> has been disclipined - usually like crappy SiLabs spury synthesiser chips >>> that can produce the oddball reference frequencies like 31.28234MHz ! >>> Actually they are pretty good for what they are, but they are certainly not >>> as clean close in , and particularly poor in spurs far away. they are what >>> they are. >>> >>> Of course the great way is a DDS , and run something like a 400-700 MHz >>> VCO/SAW/BAW clock. One needs to go that high to get decent oscillator Q , >>> and of course the DDS needs the high clock. The clock is of course pulled >>> to the 10 MHz with something like a ADF4002 etc integer synth running high >>> BW like 200 kHz to kill to close in VCO noise. But that's alot of stuff >>> >>> *** I thought in the shower this morning of inserted a block, and shifting >>> the internal radio oscillator (running at say 31.28234MHz the few Hertz >>> either side it needs to move.) >>> >>> - by using an image reject (full complex) mixer with a +/- 5 Hz oscillator >>> applied. >>> >>> - by using a analog or digital variable delay line to remove or insert >>> delay as to strecth or contract the period . almost like a phase modulator, >>> but I think it is going to wrap and cause trouble. Hmm if I play with the >>> high and low period separately, I might be able to fix it when it wraps. >>> But that technique will likely insert noise for any soet of small easy >>> implementation. >>> >>> - alias to close to baseband using another oscillator (fixed) and then >>> alias back up. Aliasing technique are very cheap and useful in DSP. Hmm >>> that might be soemthing I do in DSP for other signal processing tricks, but >>> not on a small board . >>> >>> The cpx mixer is the 1st thought : >>> >>> Perhaps a commutating HC-CMOS switch quadrature DBM (like HC4316) with the >>> complex LO +/- 5 Hz coming from something I can dream up. >>> >>> For a single frequency, I would be able to get the quadrature matching at >>> least -60 over temperature in my experience with something like this.. >>> >>> That would be the unwanted sideband down that far. Of course with square >>> wave drive, the mixer will be sensitive to the harmonic series, but the >>> input is squeaky clean, so that's no issue. ALTHOUGH hmm the close in noise >>> say + /- 10 Hz would get a say as it would be aliased in, but the 10Hz >>> noise on those XOs is usually prety good, and for this purpose, it is the >>> noise at offsets of 10kHz up to 300kHz that are the most important. >>> Control bandwidth only has to track thermal drift in the radio, have be >>> fractions of a Hz, so the system could spend some time calculating and >>> generating the LO. >>> >>> Anyone tried this (IE shifting the frequency of the source) ? Comments ? >>> >>> Glen >>> >>> (VK1XX, AI6UM) >>> >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@lists.febo.com >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
