1. Re: GPSDO "breathing" behavior (Bob Camp)

Re: GPSDO "breathing" behavior Bob is correct in his summary, i.e. the CW12-TIM is not technically a GPSDO. The original CW25 receiver that is used in the CW12 module was, at the time back in 2007 or so, considered a "good" timing receiver due to its relatively low inherent quantization/ sawtooth error, as well as the fact that it generated a digital/programmable frequency output. The receiver was originally developed by Parthus, who just prior to completing all of the final development, sold the IP to Ceva Corp, with the intent to offer the IP only. CW/Navsync licensed the chip set, but eventually, the chip set went EOL due to inevitable fab/process closures and was unavailable after 2013 or so. The CW12-TIM was an effort to emulate the Motorola M12, which had also gone EOL in that time frame, and not intending to create a "true" GPSDO. I don't normally post to this list but I thought I would provide a file that might help illustrate the issues Bob was describing in how a GPSDO implementation performance is different from a standalone GPS receiver. I just happened to be collecting some of this information and thought I would share it. Basically, it compares three receiver's 1PPS outputs (Ublox Neo-F10T, UBlox LEA M8F (which does implement some quantization error correction) and a updated version of the original CW25-TIM (updated with a different receiver chip and architecture implementing a DPLL and quantization error correction based on a TCXO internal MCLK and a 50 mhz bandwidth setting)). Each of the 1PPS signals are fed into two Connor Winfield 1PPS locking modules: BTFC series (TCXO based with 50mHz lbw setting) and EH320TFC (OCXO based with a 3.1mHz lbw setting). The resulting timelab measurements (against a free running Cesium time base) help illustrate how these phase correction and filtering schemes affect the performance of the output in support of Bob's comments. (If there is some other way to post this .pdf file, let me know.) Dan Olp On Thu, Jan 16, 2025 at 2:30 AM <time-nuts-request@lists.febo.com> wrote: > Send time-nuts mailing list submissions to > time-nuts@lists.febo.com > > To subscribe or unsubscribe via email, send a message with subject or > body 'help' to > time-nuts-request@lists.febo.com > > You can reach the person managing the list at > time-nuts-owner@lists.febo.com > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of time-nuts digest..." > > Today's Topics: > > 1. Re: GPSDO "breathing" behavior (Bob Camp) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Wed, 15 Jan 2025 17:41:17 -0500 > From: Bob Camp <kb8tq@n1k.org> > Subject: [time-nuts] Re: GPSDO "breathing" behavior > To: Discussion of precise time and frequency measurement > <time-nuts@lists.febo.com> > Message-ID: <80716883-8961-4B77-BC6A-AA49493C9C27@n1k.org> > Content-Type: text/plain; charset=utf-8 > > Hi > > What the modules do both on the PPS and on their other signals is to > “dither” an internal oscillator. > The oscillator is *not* tuned to frequency. It free runs at some > independent frequency (and phase). > The edges of the output are shifted from time to time to drop or add one > period of the internal > oscillator. > > This is not the same thing as tuning the oscillator on frequency. That’s > why the “disciplining” term > is important in “GPSDO”. > > Bob > > > On Jan 15, 2025, at 4:06 PM, Peter McCollum via time-nuts < > time-nuts@lists.febo.com> wrote: > > > > Thanks for the comments so far folks! I am definitely NOT questioning > > anyone's expertise, but I remain somewhat confused about the assertion > that > > the CW12-TIM is "not a GPSDO". If not, then what is it? It obviously > > "disciplines an oscillator" in some way (see data below). > > > > To clarify the "breathing" behavior: > > Attached is a pic of the Lissajous, comparing the Sulzer vs the CW12-TIM > > outputs, both at 1 Mhz. I have adjusted the fine-tuning on the Sulzer > such > > that the ellipse gradually changes shape over a period of many minutes. > > BUT, over a period of perhaps 3 or 4 seconds, the ellipse goes back and > > forth between the two versions shown in the picture. As I mentioned > before, > > it is NOT a sudden change, but rather looks like a lung expanding and > > contracting somewhat. > > I interpret that to mean that there is a *temporary* change in freq, > > lasting less than 0.5 seconds or so, then the freq does the same thing in > > the opposite direction, and the ellipse is back to it's original > > shape/size. In the time frame of (say) a minute or two, there is NO > overall > > change to the ellipse. > > This behavior is only seen with the CW12-TIM output - if I compare the > > Sulzer against something else that is "pretty stable" (such as an HP > > 5345A), I don't see the "breathing". > > > > Another data point: > > If I power up the CW12-TIM with its antenna shielded to limit its access > to > > GPS signals, the Lissajous "rotates" at a rate of perhaps 1.5-2 Hz (out > of > > 1 Mhz). Then, as soon as it starts to acquire sats, the Lissajous stops > > moving... The datasheet says that the freq output is reliable when it > has a > > 2-D solution. > > > > Also attached is two pages from the CW12-TIM docs. > > Is that NOT describing a "true GPSDO" ? > > I see that the doc says "full PCR MTIE performance", although I certainly > > don't understand the implications of that statement... > > > > Pete > > > > > > > > On Wed, Jan 15, 2025 at 1:06 PM Bob Camp via time-nuts < > > time-nuts@lists.febo.com> wrote: > > > >> Hi > >> > >> GPSDO = GPS Disciplined Oscilator > >> > >> They have: > >> > >> 1) A GPS module of some sort > >> > >> 2) A control loop that also acts as a noise filter > >> > >> 3) An independent oscillator that the control loop manages. > >> > >> With just the module you are missing 2 and 3. > >> > >> GPS modules have provided “chopped up” output signals for at least two > >> decades and closer to three. > >> What they provide is not the same thing as what a GPSDO delivers. > >> > >> Bob > >> > >>> On Jan 15, 2025, at 9:19 AM, Peter McCollum via time-nuts < > >> time-nuts@lists.febo.com> wrote: > >>> > >>> Azelio wrote: > >>>> The CW12-TIM is not a GPSDO > >>> > >>> From the CW12-TIM manual: > >>> "The graph below demonstrates the ability of the CW12-TIM GPS to > continue > >>> to provide a GPS disciplined output > >>> frequency with the GPS aerial located completely inside a building..." > >>> > >>> Is it just a matter of semantics, or is there a firm definition of > >>> "disciplined"? > >>> > >>> Pete > >>> > >>> > >>> > >>> On Wed, Jan 15, 2025 at 3:01 AM Azelio Boriani via time-nuts < > >>> time-nuts@lists.febo.com> wrote: > >>> > >>>> The CW12-TIM is not a GPSDO, the frequency output is not disciplined, > it > >>>> is, like the PPS, derived from the division of the free running > internal > >>>> clock, driving the divisor (someway: added/skipped cycles) to align it > >> with > >>>> the GPS. To mitigate the "breathing" there should be a sort of > sawtooth > >>>> correction also for the frequency output (not possible for that > speed). > >>>> > >>>> On Wed, Jan 15, 2025 at 8:32 AM Peter McCollum via time-nuts < > >>>> time-nuts@lists.febo.com> wrote: > >>>> > >>>>> Just curious if this is normal behavior. > >>>>> I use an X-Y display to show a Lissajous of my CW12-TIM module vs a > >>>> fairly > >>>>> stable oscillator (Sulzer 5A, or Spectracom 8161), all at 1 MHz. > >>>>> The CW12's output gives an (apparently) very solid output averaged > over > >>>>> many seconds or longer. > >>>>> But in the short term, the Lissajous does a "breathing" behavior - > >> every > >>>> 3 > >>>>> or 4 seconds (it's variable), the pattern expands a few degrees in > one > >>>>> direction, then goes back in the other direction. It's NOT a sudden > >> jump, > >>>>> rather it is very much like watching someone's lungs taking a breath > >> then > >>>>> letting it out. > >>>>> > >>>>> Is it normal, or does it imply that the "discipline" in the GPSDO is > >>>>> repeatedly needing a correction? > >>>>> > >>>>> Thanks for any comments! > >>>>> > >>>>> Pete > >>>>> _______________________________________________ > >>>>> time-nuts mailing list -- time-nuts@lists.febo.com > >>>>> To unsubscribe send an email to time-nuts-leave@lists.febo.com > >>>>> > >>>> _______________________________________________ > >>>> time-nuts mailing list -- time-nuts@lists.febo.com > >>>> To unsubscribe send an email to time-nuts-leave@lists.febo.com > >>> _______________________________________________ > >>> time-nuts mailing list -- time-nuts@lists.febo.com > >>> To unsubscribe send an email to time-nuts-leave@lists.febo.com > >> _______________________________________________ > >> time-nuts mailing list -- time-nuts@lists.febo.com > >> To unsubscribe send an email to time-nuts-leave@lists.febo.com > > > <Lissajous.JPG><p2.jpg><p1.jpg>_______________________________________________ > > time-nuts mailing list -- time-nuts@lists.febo.com > > To unsubscribe send an email to time-nuts-leave@lists.febo.com > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-leave@lists.febo.com > %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s > > ------------------------------ > > End of time-nuts Digest, Vol 249, Issue 13 > ****************************************** >