Hi With *all* of these “drop to a lower frequency” approaches, the theoretical resolution is very good compared to the useful resolution. A straight mix to 1 Hz into a 5370 is a great example. The filter / limiter is the thing that sets the useful resolution rather than the theoretical 1x10^-17 the setup provides.
Bob On Oct 12, 2014, at 12:14 AM, Robert Darby <[email protected]> wrote: > Bob Camp, > > Bob, Simon is talking about the sampler versus a true mixer. This is the > idea I asked you about some months ago when I asked about how the digital > filter functions. You were kind to explain the filter method in terms of > buckets. You are of course correct that the resolution is low, 100 ns for a > 10 MHz DUT with a 10 Hz frequency offset but the hetrodyne factor takes the > theoretical resolution to 100 fs. That's not shabby for a very low cost > DDMTD. And of course, the actual noise floor will not be close to this but > potentially it's better than a 5370 and a lot easier to maintain. :o) > > Simon, > > I have a 4 channel 1 ns tagger "working" but I can't successfully link the > FTDI library to a c program so doing this in hardware looks far more > attractive to me. Here's how I see it at this point: > > -- Objective: > -- A four channel DDMTD with 44 bit time tags delivered over the USB > port > -- At least 100 Hz beat frquency on each channel > -- The hardware is capable of much higher rates but increasing the > beat frequency offset > -- degrades resolution and realistically the device will probably > be used at 5 or 10 Hz > -- > -- Additional Hardware Required: > -- A "wing" with three or five LTC6957-1 low phase noise buffers to > convert sine inputs into > -- high speed low-jitter square waves using LVPECL differential > outputs > -- Either an oscillator offset by the beat frequency or a DDS > frequency generator > -- A USB equipped computer > -- > --Architecture > -- Differential inputs are fed to the master clock, thence to the D > flip-flops clocks > -- Differential inputs for each channel are fed to the data inputs for > each flip-flop > -- The master clock drives a 44 bit counter which is common to all > four channels > -- Each channel has two independent counters, provisionally 14 bit, > designated high and low > -- The low counter first establishes a low state without transitions > i.e. it times out > -- After the low counter times out, the flip-flop is armed > -- The first high output at q resets and starts both high and low > counters - whichever counts depends on whether q is high or low > -- Every time the high and low counters match we store the 44 bit > count; each new match replaces the previous one > -- At some point (2^14 highs) the high counter will roll over - > hopefully low will have stopped counting much earlier > -- The highest stored match should meet the equal count criteria as > described in the P. Moreira and I. Darwazeh paper > -- Since there are four channels it will be necessary to multiplex the > time tags into the fifo > -- The multiplexer will add 1 bit per channel for one-hot channel id > coding > -- The 48 bits will clock into a 48 bit to 8 bit fifo thence to an 8 > bit USB port > > I believe you can have multiple points where the two counts match but I don't > have any data to confirm that. I played with this in excel and when you feed > it ones and zeros in a distribution that "looks" like the typical output out > of a digital sampler it is possible to get multiple matches. My intention is > to go with the last crossing and the scheme mentioned above does this rather > trivially. Unless, of course, I'm missing something and I usually do. > > I've got a Pipistrello board and it has the option of an asynchronous fifo > USB interface; since I've already paid my dues on that I'll just use that > code again. The data rate is so low that snail mail would work. The > computer gets a series of time tags and your program has to pair up the > channels to get the deltas. Getting time tags lets you compare three or four > devices simultaneously and facilitates three-cornered hat calculations. I > suspect that's a lot easier to say than do but we'll cross that bridge if we > ever get there. Also time tags permit continuous sampling; there's no counter > dead-time which I think can be an issue when it causes variable data sampling > rates. > > Bob Camp mention Collins low jitter hard limiters but I suspect that's much > more of an issue on the very shallow slopes you see on 5 or 10 Hz mixer > outputs. The LTC6957 is probably overkill on 10 MHz inputs but I believe > they're a tad better than a 74AC gate, but then again maybe not all that much > better. Lot more expensive. Bob C discussed sine to square conversion in a > recent post (IIRC) perhaps in connection with 5V to 3.3V conversion, and for > a low cost solution the 74AC gate looks pretty good and they're easy to dead > bug. > > I'm out of spit. Later > > bob > > > > > On 10/11/2014 9:17 PM, Bob Camp wrote: >> Hi >> >> Ok, a little more data: >> >> You can hook your flip flop up as a sampler or as a full blown mixer. Hooked >> up as a full blown mixer, you get the 20 MHz and 10 Hz signals. You also get >> more resolution on the 10 Hz. Either way, the 10 Hz is still a beat note. In >> the case of a sampler, the filter is there for edge jitter. >> >> With a sampler, your data is only modulo 100 ns. With a 100 ms beat note >> period, you only get 1x10^-6 at best. That’s very different than what you >> get with the same chip used as a mixer (or an XOR gate). The true mixer >> connection gives you data the instant the edge changes. The sampler goes to >> sleep and lets you know up to 100 ns later ... >> >> 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. >> > > _______________________________________________ > 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.
