Hi The mixer you are using will give you a sine wave output *if* it’s properly filtered. A mixer is a mixer.
Bob On Oct 11, 2014, at 6:31 PM, Simon Marsh <[email protected]> wrote: > I (mostly) understand this when considering an analogue mixer, but I'm lost > on whether there are any similar effects going on with a digital signal ? > > TBH, I'm not really sure 'mixing' is the right phrase in the digital case, > and my apologies if I got that wrong. > > What's actually going on is sampling one (digital) signal at a rate close to > the signal frequency. This gives a vernier effect and the result is a purely > digital set of pulses at the beat frequency, aligned to when the signal and > sample clock are in phase. It does not have a high frequency component to > filter out. > > Cheers > > > Simon > > On 11/10/2014 21:11, Bob Camp wrote: >> Hi >> >> Your glitches are (in part) coming from the 20 MHz (10 + 10) component on >> the mixed signal. Since they have no direct relation to the beat note, >> filtering them after limiting is not a simple task. It is far easier to keep >> filter the signal pre-limit than to do so post limit. >> >> The other component of the glitches is related to the limiting process. The >> paper by Collins is a good one to read for information on gain, bandwidth >> and the limiting process. Again, there is very little you can do “post >> limit” to sort things out. None of the zero crossings you are getting may >> be “correct”. It’s not simply a process of picking one out of the group. >> >> —————— >> >> Some math: >> >> You have two 10 MHz signals and a (say) 10 Hz beat note. You are looking for >> 1x10^-13. You get 1x10^-6 from the downconversion. You need to get 1x10^-7 >> out of the beat note. >> >> Put another way, 1x10^-13 at 10 MHz is 1x10^-5 Hz. >> >> If your beat note is 3 V p-p, it will cover 6V every 1/10 second. It’s about >> 1.2X faster than a triangle wave as it zero crosses (memory may be failing >> me here), so that makes it equal to a 7.2V triangle excursion. >> >> 1x10^-6 of 7.2V is 7.2 microvolts. >> >> That’s how accurate your limiter / filter combination needs to be, >> pre-limiting. >> >> It can be in a fairly narrow bandwidth, so it’s not quite as daunting as a >> radio front end. >> >> Since you have a very large signal, and very small noise, the normal >> “dithering will help me” effect of the noise can not be counted on. >> >> The thing you *want* to come up with is essentially a random signal (ADEV), >> so massive filtering will not do the trick either. >> >> Bob >> On Oct 11, 2014, at 3:33 PM, Robert Darby <[email protected]> wrote: >> > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
