Original thread on DDMTD in 2008: https://www.febo.com/pipermail/time-nuts/2008-December/034955.html
Later comment on using a shift register to minimise metastability issues: https://www.febo.com/pipermail/time-nuts/2011-August/058648.html Bruce On Sunday, October 12, 2014 12:14:27 AM Robert Darby 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 <subscripti...@burble.com> 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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.