My first little crystal filter. - Page 1
Forum section: electronics / rf-microwave

author: purpose
author_rank: Frequent Contributor
timestamp: March 08, 2021, 05:10:53 pm
content: Afternoon all,I've been novice filter building in the hope of isolating the 10MHz harmonic of a 2MHz square wave.A foot long, 6th order Butterworth BPF, with inductors wound on some plastic plumbing that I found in the loft, worked fairly well, but had a 2MHz bandwidth.Then I found out that crystals could be used for a very narrow bandwidth, so W2AEW's video on characterisation of crystals was watched and frightened the life out of me with something called mathematics.Nevertheless, ebay sent me the requisite 50 cheap Chinese gizmos to select the magic pair, or more.Two crystals were randomly (ish) picked from the pile and nailed to the pcb and I now have a rather splendid 10MHz BPF.Chasing the insertion loss is above my pay grade, so can live with it, besides, that's what amplifiers are for.Would you be happy with my result?

author: fourfathom
author_rank: Super Contributor
timestamp: March 08, 2021, 06:51:25 pm
content: That looks very useful! Have you seen "Experimental Methods in RF Design" (an ARRL publication)? This has a pretty good section on crystal filters, more practically-oriented than mathematically rigorous. Chapter 3.4 in my copy.This reminds me of a 51 MHz tripler I re-designed. We needed to get 3x (155.52 MHz) from a 51.85 MHz clock. The previous designer had started with a class-C-type multiplier and then had to filter out all the even and odd harmonics. Since the input clock had a clean 50% duty-cycle (virtually no even harmonics), I redesigned the thing, using a 155 MHz SAW filter and a post-amplifier/squarer. Way cleaner output that way. The goal was to minimize jitter. The previous design added a bunch of even harmonics, only to have to filter them out.[edit: Redesigned by throwing away the old design, taking the 51 MHz digital clock, impedance-matching that signal to the SAW filter (probably just resistors, don't recall) and squaring up the SAW output. Square-wave in, third harmonic out.]

author: purpose
author_rank: Frequent Contributor
timestamp: March 08, 2021, 07:29:59 pm
content: That looks very useful!Thank you. I thought so too.Have you seen "Experimental Methods in RF Design"I have not, but might get a copy if I can get rid of my other kidney.Joking aside, that looks like a tome of might and worth a punt for a little light reading.My copy of "The Art of Electronics" has seen far too little page turning too.Your last paragraph will need decrypting, but I believe contains many nuggets of usefulness.I really should read more.Cheers

author: LaserSteve
author_rank: Super Contributor
timestamp: March 08, 2021, 07:54:21 pm
content: EMRD gets far, far more reading at my place then AOE.Get the second or later edition.Beautiful first unit.Steve

author: MikeK
author_rank: Super Contributor
timestamp: March 08, 2021, 07:57:45 pm
content: "Experimental Methods" is a great book full of great circuit ideas. I bought my copy before the used book vendors went off the charts into insane price fantasyland.* Keep looking. You might try finding/asking local hams, or posting a wanted ad in amateur radio forums such as qrz.com.* And those prices have to come down eventually, since nobody is going to pay $200 for that book.

author: purpose
author_rank: Frequent Contributor
timestamp: March 08, 2021, 07:59:03 pm
content: Quote from: LaserSteve on March 08, 2021, 07:54:21 pmThanks Steve.

author: purpose
author_rank: Frequent Contributor
timestamp: March 08, 2021, 08:00:39 pm
content: Quote from: MikeK on March 08, 2021, 07:57:45 pmI'll keep em peeled, Mr. K.Cheers.

author: MikeK
author_rank: Super Contributor
timestamp: March 09, 2021, 12:00:27 am
content: By the way, the crystal oscillator that Alan (W2AEW) uses in video #123 comes straight out of "Experimental Methods...". Although, Alan excludes the switch in the book's circuit that allows you to measure/calculate motional capacitance and motional inductance of the crystal. I build it, but haven't created a crystal filter yet.What is your 10MHz crystal filter going to be used for? I think it may be too low for 30-meter amateur radio?

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 12:41:52 am
content: Mr. K,What is your 10MHz crystal filter going to be used for?I've been faffing around with a fake Chinese U-Blox gps module in the (probably vain) hope of making a gpsdo-ish unit.When putting out a 10MHz signal, both fake and real U-Blox have horrendous jitter, but 2MHz seems solid, hence the harmonic isolation.I've found that the better the filter I make, even 10Mhz is cleaning up nicely.Others who really know what they're talking about have pointed out that I'm an idiot, they may have a point, but I'm just passing the time tinkering, not looking to conquer the realm of atomic time.Peter

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 12:51:28 am
content: Added bonus... the foot long Butterworth filter.Go on... have a laugh at my expense.

author: fourfathom
author_rank: Super Contributor
timestamp: March 09, 2021, 01:02:09 am
content: My prototypes look just like that filter board, and I used to get paid for this stuff. How are you cutting the grooves in the PCB? I use a woodworking "gouge" chisel, or sometimes use my CNC mill.I got lucky and bought my EMRD book a couple of years ago when the ARRL did a reprint run. $49 (!)Now *here's* a breadboard:Allegedly Bob Pease's breadboard of the LM131 V-F converter.

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 01:10:35 am
content: Now that is a weapons grade rat's nest. I would be proud.How are you cutting the grooves in the PCB?I run a blade along a metal ruler, but now I need to investigate a gouge.I got lucky£200 on ebay!.... I'll just borrow yours.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2021, 02:39:12 am
content: Yup, very sharp indeed.I made a somewhat crude one years ago, as part of a radio receiver -- seems to work very well, given the relatively poor stopband rejection from the low crystal count.More pictures:https://www.seventransistorlabs.com/Radio_20m/index.htmlNo schematics unfortunately. I really should document that properly!The filter at least is pretty self evident. The setup is... hold on, let me take this down from the shelf, what all did I do there... oh, some smart aleck also soldered a shield panel over top the 1st IF coil, I can't even see how it's tuned, let me get the soldering iron too then......IIRC, the receiver topology is:RF1: preamp at antenna connector. No input filtering (tapped coil steps up from 50 ohm antenna to grid; no tuning or matching really; as I recall it's pretty wideband and needs an inline bandpass for better image rejection)Mixer: cathode input from LO (a separate triode oscillator), grid input from RF1 (single tuned, no matching)IF1: input from mixer (double tuned, ~200kHz? BW), output to crystal filterCrystal filter: two resonant coils, two crystals. Coils around 32uH (j800 ohm at 4MHz), tapped at 28%.Note, tube amps need high impedances; probably plate load is ca. 10kohm here. Crystals, the bandwidth depends on Zo, where a lower Zo gives a wider bandwidth, at some expense to insertion loss (because ESR is modest, ca. 100 ohms, and obviously fixed for a given part). So, that puts the crystal impedance near 1kohm.The crystals are arranged in an extended pi network, as per usual. It wasn't found necessary to use series tuning caps (which can partially correct for mismatched center frequency between crystals). It was found helpful to add parallel inductors, which resonate out mounting/holder capacitance, giving a more symmetrical passband.Input side: 10pF shunt, series 4MHz crystal (with 330uH in parallel), shunt 22pF (middle node), series 4MHz crystal (with 220uH in parallel), no output shunt cap oddly; and finally, tapped coil up to following stage grid, which I think is the same or at least very similar to the first coil.The input and output are single tuned (a few fixed caps and a variable, including plate output and grid input capacitances respectively, presumably around 50pF).IF2: input has 10kohm grid leak (also setting load for the crystal filter -- grid admittance is quite small at this frequency), single tuned plate load into detector (6AL5). Audio stuff after that, etc.Well... er... now you know why a schematic isn't being drawn of this project... it's the Rick and Morty scene "hey pal, fuckme" "no way man, fuck ME", IRL...Anyway, as for performance? The stopband attenuation isn't too bad, it's around 40dB for two crystals, give or take what the tuning coils around them are doing. The bandpass has a double-skirt profile, with good asymptotic attenuation for >>100kHz away from center, but a bit of a shelf inside there, until the few kHz passband. There are spurious modes, due to the higher order resonances of the crystals themselves -- which aren't very well matched (just two random junkbox crystals), which I guess you might make the argument that, because they're mismatched, the spurs might be expected to be arbitrary and non-overlapping; alas, I don't know how true that is in practice -- but they are at least lower in magnitude than the primary (and intended) passband.The spurs are a bit interesting to play around with. Most of them are low gain, if obviously still plenty strong to hear. Many of them are incredibly narrow, some just a few 10s or 100s of Hz! I recall testing them by setting up my vintage Eico 322 RF generator, tuning near one, then tapping on its enclosure -- the mechanical vibration of the oscillator coils and other mechanical parameters, induces AM and FM in the generated tone / harmonics. What you hear, sounds very much like audio down a very long steel pipe -- the resonance is sharp and prominent, and yet what you are hearing is not at all the acoustic reflections within a meters-long pipe, but the acoustic waves travelling through, or over, a tiny plate of crystal! Oscillations that are going on at many times the audible rate (i.e., ~4MHz), and yet after downconverting, sound just like this, and this is by no accident, of course!So, anyway anyway, a good design lesson is to use many crystals, when you need to use them that is -- there's good reason why SSB filters are normally constructed with six or more. The stopband ratio is pretty gross per stage, maybe 10 or 20dB -- this is due to the effects of the series-parallel transformation*, and the holder capacitance. Holder capacitance in particular, causes a zero in the transfer function, which boosts the upper stopband -- a single stage might give say 10dB upper stopband attenuation, and 20dB lower. So, many stages are needed just to get the stopbands down to reasonable levels, which has the bonus effect of making the passband really, really steep. At some expense to insertion loss, but this is put in the IF strip where gain is effectively unlimited, and input-referred noise is essentially irrelevant (namely, that's set by the mixer or RF preamp).*That is, a crystal is a series or parallel resonator (depending on which mode you use). It can be transformed from one to the other, by stringing it up between capacitors or inductors -- this works with lumped components, much the same as a 1/4 wave transmission line acts as a reciprocal transformer. So, we can use series-resonant crystals, in series branches between capacitors, as if they were parallel resonant tanks. Or even more to the point, whereas a canonical bandpass has alternating parallel and series resonators -- we can in turn transform that to all-parallel or all-series, using the same strategy, and that is why the extended-pi network, with all shunt C's, and all series crystals, works to implement a bandpass response.As for bandpassing harmonics -- crystals shouldn't be necessary for that, but as you've demonstrated, they can certainly do a good job. As long as the color you want is black, so to speak. I mean, you can custom-order crystals of pretty much any value, but you'll pay form them, too. You're otherwise limited to commercial selections, so I hope you only need harmonics at, whatever frequencies crystals are commonly made for.(The IF strip is an easy one, the RF and LO are to some extent, free variables -- just use whatever's handy. Strangely I only had the two 4MHz crystals at the time, so, that's what I did...)And as for bandpassing harmonics without crystals -- another project I did has that, a single stage pentupler to 100MHz. Not great because the sidebands are pretty close, takes quite a high Q to do that. I... hrm, do I not have any schematics online? I swear I did... apologies, I can't seem to find any.Well, I'll describe the circuit: it's a 20MHz crystal oscillator, into two cascaded diff pairs, intended to sharpen and amplify a square wave. They have sharp tuned loads, to select the 100MHz harmonic, giving both limiting (clipping distortion to reduce amplitude modulation -- put another way, the nearest pairs of harmonics (80 and 120MHz, 60 and 140, etc.) get mixed together, tending to cancel them out) and tuning (to brute-force select the desired harmonic). The tuning is only single (LC resonator). Well, after those two stages and some tuning, I've got sideband suppression of about 40dB -- not terrible, it certainly looks alright on the scope -- but what's left is still noticeable, a percent or two wobble in amplitude, and on the spec, there's no mistake about what's going on. More limiting seems difficult to come by (with the MMBTH10s I'm using), and I may resort to a double or triple tuned (coupled resonator topology) bandpass just to be done with it.Which, an LC lumped-equivalent or helical resonator is absolutely doable, it's just a bit tedious to construct and tune. It's not exactly going to be off-the-shelf, right? Though, ceramic coaxial resonators are commercially available, and may be usable over the same frequency range -- a production-ready possibility.Tim

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 10:14:40 am
content: Quite the essay, Tim.Thank you.I barely know the difference between capacitors and chocolate, so a great deal of the terminology contained therein are words and phrases that I've never heard before, but now have to look up. Pentupler? A very angry tube?I only got a few hours in last night, so forgive me if I have to re-read that a few times before attempting to understand even 10% of it, or comment.I love the construction of home made gear, so your pics were a treat.Back after some reading.Peter

author: joeqsmith
author_rank: Super Contributor
timestamp: March 09, 2021, 12:40:01 pm
content: Does the open source software or firmware for the original NanoVNA automate these measurements? If you made a fixture for it, the VNA's hardware is more than capable.

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 03:33:48 pm
content: Was that directed at me, Mr. Smith?If so, I'm unaware of any automation, but the 'nanoVNA SAVER' software certainly gives a great deal of info.I ran all 50 crystals on the vna and screen captured them all, then flicked between similar ones to get the best match.example

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 05:55:49 pm
content: Something very strange is going on.I finally stuck the filter on the gps module, outputting a 3.6Vpp 10MHz signal and it was very jittery, so fiddled with the series trim caps and got a jitter free signal at17.2Vpp. I hadn't expected that.Is there some perfect voodoo resonance going on?The filter shape giving me the gain.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 09, 2021, 06:01:23 pm
content: That's too bad as it certainly seems capable of making these measurements. I played around a bit comparing results against my other analyzers as well as the simple oscillator and my RLC meters. Eventually got to a point were the numbers were coming out close to one another.Thread is here if interested:https://www.eevblog.com/forum/rf-microwave/fun-with-crystal-filters/msg3036196/#msg3036196I bought the V2 Plus VNAs and hoped to clean up the code enough to release it to the public but it turned out that this VNA is pretty limited for making these measurements.Link to video showing how it would have worked.https://youtu.be/XaYBpPCo1qk?t=2346

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 06:11:26 pm
content: I'll get busy watching and reading.Cheers.

author: fourfathom
author_rank: Super Contributor
timestamp: March 09, 2021, 06:25:14 pm
content: Quote from: purpose on March 09, 2021, 05:55:49 pmCan you post your test fixture circuit? There will be no power gain in a passive filter (or any passive network), but if the input and output impedances are different then there will be voltage or current gain (assuming a lossless system). It sounds like you are driving the filter with a low-impedance source, and measuring the output with a high-impedance load. You had to tweak the capacitors because the source and/or load impedances weren't the 50 Ohms of the VNA ports.

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 06:58:28 pm
content: Can you post your test fixture circuit?What I used to individually test the crystals? If so, just a couple of sockets on a bit of pcb. I thought that I'd get some sort of results and the ones that were sameish would be my picks.There will be no power gain in a passive filter (or any passive network), but if the input and output impedances are different then there will be voltage or current gain (assuming a lossless system). It sounds like you are driving the filter with a low-impedance source, and measuring the output with a high-impedance load. You had to tweak the capacitors because the source and/or load impedances weren't the 50 Ohms of the VNA ports.There was certainly an impedance mismatch (out of the filter into the scope), but I've never seen that kind of 5/6X gain.I thought I had invented the wheel.

author: geggi1
author_rank: Frequent Contributor
timestamp: March 09, 2021, 07:09:40 pm
content: Quote from: purpose on March 09, 2021, 12:41:52 amAny frequency that is an even number of the 48Mhz clock in the GPS will give a cleaner signal.

author: fourfathom
author_rank: Super Contributor
timestamp: March 09, 2021, 09:59:11 pm
content: Quote from: purpose on March 09, 2021, 06:58:28 pmThe maximum voltage transformation is limited by the "Q" in the network (Q is related to loss). Crystals have an extremely high Q, so large voltage gains are possible.

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 10:49:46 pm
content: Quote from: geggi1 on March 09, 2021, 07:09:40 pmWhole number. Odd or even, but I know what you meant.I think the mathematicians call it an integer.

author: purpose
author_rank: Frequent Contributor
timestamp: March 09, 2021, 10:51:49 pm
content: Quote from: fourfathom on March 09, 2021, 09:59:11 pmThat is the most important thing that I learned today.Many thanks.

author: MikeK
author_rank: Super Contributor
timestamp: March 10, 2021, 01:34:52 am
content: Quote from: purpose on March 09, 2021, 10:49:46 pmWhat odd number fits into 48MHz with no remainder?

author: purpose
author_rank: Frequent Contributor
timestamp: March 10, 2021, 02:42:20 am
content: Quote from: MikeK on March 10, 2021, 01:34:52 am3 springs to mind.

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 05:59:12 pm
content: Evening all,For the first time I decided to check the signal's duty cycle.Infinite persistence shows the 10MHz wobbling and veering from 47-53% duty. Not good, I thought.Then I did the same with an (admittedly old) OCXO and got similar results.Threw on the Siglent sig gen and pretty much the same.Is this common, or is my Rigol scope (doing the measuring) a pile of poo?It could of course and more than likely be that I'm a moron.Peter

author: joeqsmith
author_rank: Super Contributor
timestamp: March 11, 2021, 06:16:48 pm
content: No idea without sitting over your shoulder. Maybe make a video clip showing how you are making these measurements. Details are important.

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 07:42:34 pm
content: Quote from: joeqsmith on March 11, 2021, 06:16:48 pmMr. Smith,I possess not one bit of gear that captures any decent video. Unfortunately a 25 year old digital cam that lets me record 10 seconds of mush will not help. Besides, it would take me hours to tidy my workbench, as I wouldn't want you finding out what a slovenly specimen I truly am.I've long subscribed to your youtube channel and it was your recommended video that made me purchase a nanoVNA in the first instance (£23 on ebay UK) and re watching your work has helped me understand the instrument immensely.Your software is most impressive.PRESS SORTI can see being a Godsend to those who make these things on a regular basis.I digress.Details... 2/10MHz 3.6Vpp out of gps module, through coax to filter, through 50R terminated coax into Rigol DS1054.Infinite persistence 60% intensity.10MHZ OCXO through coax to Rigol. Same termination and settings.Sig gen same.Solid frequncy, but all displaying the same wobbly behaviour.Peter

author: fourfathom
author_rank: Super Contributor
timestamp: March 11, 2021, 08:27:53 pm
content: Scope triggering setting problem?

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 08:52:22 pm
content: Quote from: fourfathom on March 11, 2021, 08:27:53 pmEdge trigger, with minimum persistence gives me a solid waveform.Infinite persistence gives it the shakes.I'm just hoping that its the scope inaccuracy.Cheers.

author: fourfathom
author_rank: Super Contributor
timestamp: March 11, 2021, 09:44:26 pm
content: Apologies if this is too obvious, but is your trigger point near the 50% (vertical) point?

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 09:53:45 pm
content: Smack bang.Just had suggested to me to increase the size of the waveform, so the edge is more steep. That worked a treat.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 11, 2021, 10:41:10 pm
content: Quote from: purpose on March 11, 2021, 07:42:34 pmYou want to try it?

author: purpose
author_rank: Frequent Contributor
timestamp: March 11, 2021, 10:50:11 pm
content: Of course. I would love to.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 12, 2021, 12:49:42 am
content: First thing you would need to do is try and install and run the software for the V2+. If you can get that to work, at least your PC is up to the task. The manual has a section on measuring crystals. You would need to construct a fixture. This is the basic 2X 3dB attenuators and 2X transformers. Because your working at very low frequencies, the test jig's construction won't be too demanding. Documentation for the jig can be found in that link I provided. If this seems doable on your part, I'll have a look into the software side of things.****Also, while I am thinking about it, do you have the programmer for the Nano? When I was playing around with it, many beginners were hacking the open source code and releasing their changes. The firmware was anything but stable. It was so bad I had started to put together a regression test.Most of the changes people were working on have no benefit to me as I would run it headless and use the PC to run the calculations. I would have much rather a very simple protocol and cleaned up, bug free code than having a low battery indicator. You could imagine if things don't work, people not knowing if the problem was the nano's hardware, firmware, software, the os, drivers...... You could waste a lot of time pointing fingers.I think to make the software available, the firmware will need to be stable as well. I would most likely qualify a version of firmware and document that. You may need to reprogram your Nano as part of this. If you brick it, you may want to have a programmer for it on-hand.

author: purpose
author_rank: Frequent Contributor
timestamp: March 12, 2021, 01:42:56 am
content: Mr. Smith,First thing you would need to do is try and install and run the software for the V2+It installed without issue and I can switch between functions, no problem.You would need to construct a fixtureLooking forward to it. The fixtures I have planned for it keep me up at night.do you have the programmer for the Nano?I do not, but shall look into it. I know it's a nanoVNA-H, based on edy555's firmware. I'm sure I've seen instructions on firmware update and such.You may need to reprogram your Nano as part of this.I'm game.If you brick it, you may want to have a programmer for it on-hand.I'm willing to take a punt.Many thanksPeter

author: joeqsmith
author_rank: Super Contributor
timestamp: March 12, 2021, 02:02:08 am
content: Ok, first problem is you have the -H. I don't know anything about it. Based on how unstable the firmware was/is for the original Nano, it's very possible that my software would not support it.Do you know if ALL of the commands are identical between your -H and the original Nano? If you are a member on some of the groups, you may want to just ask the people who are coding the open source software to see if they had to make any changes to support it. I think we need to sort that out before anything else.It's really too bad that the V2 Plus couldn't handle these basic narrow band measurements or I would just suggest buying the V2+4. That unit is an RF experimenter's dream come true. Hope the next low cost one I look at supports narrow band with even higher BW.If you need help finding the cores to make the transformers, let me know and I will see if Digi-key or another distributor has any stock. You will need to build that up regardless.

author: purpose
author_rank: Frequent Contributor
timestamp: March 12, 2021, 02:14:09 am
content: Do you know if ALL of the commands are identical between your -H and the original Nano?I do not, but shall research in the morning. Its 2AM and this old boy is flagging.If needs be, I'll track down the correct unit.It's really too bad that the V2 Plus couldn't handle these basic narrow band measurements or I would just suggest buying the V2+4I would have bought the V2 +4 until in your video you mentioned the narrow bandwidth issues. Good job they were out of stock.If you need help finding the cores to make the transformers, let me knowI've not read the manual yet, so I'll let you know.Thanks.

author: purpose
author_rank: Frequent Contributor
timestamp: March 12, 2021, 01:51:52 pm
content: Morning Mr. Smith,I'd hold your horses on any programming for now.The page relating to the firmware in my unit ishttps://github.com/hugen79/NanoVNA-HVersion 0.4.5-1-gfbbceca Build Time Dec 26 2019 - 19:32:31Still searching for hardware differences, but this was the cheapest one on ebay, so wouldn't be surprised if they cut corners.If I find anything pertinent, I'll let you know.Peter

author: purpose
author_rank: Frequent Contributor
timestamp: March 12, 2021, 06:09:29 pm
content: It would appear that I have "worse clone". Certainly no shielding, but believe all commands are the same as original.I'm hoping that the delay in getting the V2+4 available, is down to them addressing the bandwidth issue.Then I'll grab one.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 12, 2021, 06:28:24 pm
content: Quote from: purpose on March 12, 2021, 01:51:52 pmIt's easy enough to continue to do nothing on my part.The plan would be to rewrite the software for the original Nano to use a share a common code base with the V2 Plus and my other VNAs. Besides making it easier to maintain, another advantage is they would share a common user's manual. The various flavors of firmware and hardware would certainly complicate matters. Not a trivial task but certainly doable.

author: purpose
author_rank: Frequent Contributor
timestamp: March 12, 2021, 06:54:56 pm
content: It's easy enough to continue to do nothing on my part.Put your feet up, Mr. Smith, but before you do, can you please tell me which toroids you used for the xtal fixture.Might as well get on the case.

author: fourfathom
author_rank: Super Contributor
timestamp: March 12, 2021, 07:24:01 pm
content: Quote from: purpose on March 09, 2021, 12:41:52 amBack to this, have you characterized the 10 MHz jitter? Again, probably some obvious observations on my part, but your filter won't suppress any jitter components that are lower in frequency than half your filters bandwidth. Also, sharp filters often have significant phase shift near the transition region, and this will show up in the phase of the filtered clock output. A static phase shift probably doesn't matter in your case, but a sharp filter may also have enough temperature sensitivity to give you (say) diurnal phase modulation as the environment heats up and cools down.It may be easier to use a clean-up PLL to handle the lower-frequency jitter, as it's *much* easier to get a narrow filter bandwidth and may have less temperature-associated phase shift variation. Again, the best approach will depend on your needs.

author: purpose
author_rank: Frequent Contributor
timestamp: March 12, 2021, 08:34:04 pm
content: Mr. Fathom,have you characterized the 10 MHz jitter?I've been very rude to it, but I don't think that's what you meant.your filter won't suppress any jitter components that are lower in frequency than half your filters bandwidthThe -3dB bandwidth is around 400Hz. Is that good or bad?diurnal phase modulationYou're just showing off now. And making me look like an idiot.It may be easier to use a clean-up PLLThat is something that I looked at, but couldn't make head nor tail of it. Decided to try filtering.CheersPeter

author: fourfathom
author_rank: Super Contributor
timestamp: March 13, 2021, 01:39:17 am
content: Quote from: purpose on March 12, 2021, 08:34:04 pmRe. characterizing the jitter, you had mentioned that a derived (?) 2 MHz output had much less jitter. I don't know anything about the UBlox design, but this is an interesting observation. Is this 10 MHz clock generated from a higher frequency clock via a NCO or other divider? These will have an intrinsic jitter of one high-speed clock cycle, unless the division ratio is carefully chosen -- and sometimes you have no choice anyway. The jitter components will depend on the divider structure.a -3dB filter bandwidth of 400 Hz will begin attenuating jitter components above 200 Hz (assuming a perfectly-centered filter). Whether this is good or bad depends on the jitter. If it's 60Hz jitter (unlikely) thin it won't do a thing.Diurnal phase modulation: This is just the kind of info you pick up along the way during your career. When I was designing telecoms gear that operated in (-- wait for it --) plesiochronous networks, we had to deal with phase shifts in the transmission lines as they heated up and cooled down on a daily cycle. Your design probably doesn't have to deal with that, but the crystals in your filter probably have something like a +/- 10 ppm frequency shift over a 0-50 deg-C temperature range. 10ppm at 10 MHz is 100 Hz. If your center frequency isn't quite close you may see significant phase shift over a typical temperature swing. Probably not enough to cause problems, but it's worth at least thinking about.Again, depending on the jitter and your requirements, you may want to consider a clean-up PLL using a VCXO. On the other hand, your jittery 10 MHz clock may not be an issue at all. I've *added* jitter to input signals in timing synchronization designs - the jitter can eliminate deadband and actually increase timing resolution.Since I don't know what your design goals are I'm just throwing stuff at the wall to see what sticks. It's a fun area to play in. When considering jitter, I find it helpful to remember that the time-domain and the frequency-domain are just two ways of looking at the same phenomena. I've got serious weakness in the traditional analytical approach, but I've been able to apply frequency domain tools and arrive at some very workable solutions that eluded some who only consider the time-domain.

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 02:07:09 am
content: Mr.Fathom,Is this 10 MHz clock generated from a higher frequency clock via a NCO or other divider?Indeed it is. 48MHz. Whole number divisors create the best waveforms on these jobbies.a -3dB filter bandwidth of 400 Hz will begin attenuating jitter components above 200 Hz (assuming a perfectly-centered filter). Whether this is good or bad depends on the jitter. If it's 60Hz jitter (unlikely) thin it won't do a thing.OK. That makes sense.plesiochronousYou just had to, didn't you.The rest of that paragraph makes perfect sense.Again, depending on the jitter and your requirements, you may want to consider a clean-up PLL using a VCXO.I've seen the price of some of those jitter cleaner ICs and whilst tempting, I wanted to keep this gizmo 'ghetto'.On the other hand, your jittery 10 MHz clock may not be an issue at all. I've *added* jitter to input signals in timing synchronization designs - the jitter can eliminate deadband and actually increase timing resolution.Interesting. I'm sure ublox knows what they're doing.I've been able to apply frequency domain tools and arrive at some very workable solutions that eluded some who only consider the time-domain.You've got the job. You started yesterday.I started another thread (ghetto 10MHz gpsdo) in the test equipment forum last month and on page 3, reply 57 is some clever chap's analysis. Might be worth another clever chap's eagle eye.Peter

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 01:32:09 pm
content: Mr. Smith,I've just squirted the latest firmware into the nano and there's a whole slew of different commands.Some, I would think, very useful to someone who knows what they're doing.Bandwidth selection. I don't remember seeing that.Up to 2GHz. Not that I see myself using it.I'd say writing software that covers a range of nanos would be way too much to ask.Again... put your feet up.PeterP.S. The saver software doesn't like it.

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 02:34:05 pm
content: Getting closer.Feeding 10MHz from the gps module into the filter. The best I've seen so far.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 13, 2021, 02:52:53 pm
content: Quote from: purpose on March 13, 2021, 01:32:09 pmI was afraid of that. Because I run them headless, I doubt there is much added that would be a benefit. We have a PC with much more computing power. We just need to reliably get the data into the PC. Lots of data and fast. The V2+4 does a fair job of this.I asked the group for a recommended stable version of firmware for the old hardware, not a single response.

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 02:58:55 pm
content: I saw that.Let's hope that the next batch of V2+4s will address the narrow bandwidth issue and then that'll be the one.Peter

author: joeqsmith
author_rank: Super Contributor
timestamp: March 13, 2021, 04:15:57 pm
content: It seems like it would be a fairly major change to the design. Let's hope that they have considered this for the new 6GHz model OWO mentioned.

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 04:33:29 pm
content: I'm sure they watch your videos and wish appease the gods.

author: fourfathom
author_rank: Super Contributor
timestamp: March 13, 2021, 06:33:52 pm
content: Quote from: purpose on March 13, 2021, 02:07:09 amI just looked at the UBLOX specs and this makes sense. To get 10 MHz from a 48 MHz clock you have to divide by 4.8. Converted to a fraction this gives a divide ratio of 4+4/5, which results in a division series of {5,5,5,5,4,5,5,5,5,4...}. The jitter here will be 1/48 MHz, or 20.8333... ns pk-pk. Since this sequence repeats every five 10MHz cycles, the jitter frequency will be 2 MHz.So the "10 MHz" clock is really shifting between 9.6 MHz and 12 MHz at a 2 MHz rate, with a duty-cycle of 80%. Since this frequency modulation waveform is an asymmetrical pulse, there will be even and odd harmonics of that 2 MHz signal showing up in the 10 MHz clock, but the jitter is still limited to 20.8333 ns, and there won't be any jitter components lower than 2 MHz. The bandwidth of your jitter filter could be widened if you like, but that isn't easy with a crystal filter.This assumes that your 48 MHz clock is perfect.

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 07:18:31 pm
content: Mr. Fathom,Thank you. That explains most everything.But.... and this is a big but, much like my own.... I'm getting a nice waveform by feeding the filter with 10MHz and not seeing anything in the way of jitter. In your opinion, should I feed it 2MHz and filter the harmonic for better performance?PeterJust tried it and not good. Wondering if my filter is amplitude sensitive.

author: fourfathom
author_rank: Super Contributor
timestamp: March 13, 2021, 08:33:46 pm
content: Quote from: purpose on March 13, 2021, 07:18:31 pmAn excellent question! A 2 MHz output will have no jitter, so neither the pure 5th harmonic. But the amplitude of the 5th harmonic will be 1/5th of the fundamental, and there will also be the nearby 3rd and 7th harmonics to contend with. So, you will still need filtering, plus amplification. With the filter you now have I suspect that using the jittered 10 MHz signal will be easier and just as clean, but I don't have the mathematical chops to prove it -- I would have to build it and see, or approach the problem from some other angle where I might have better analytical success.Your passive crystal filter should have no amplitude sensitivity, at least not until you reach the point of crystal overdrive. From your original post it looks like the ultimate attenuation of your filter is something near 60dB on the low-freq end (the xtal ladder filter devolves to a capacitive divider outside the passband). Perhaps enough of the 2 MHz fundamental and 3rd harmonic are leaking through to cause jitter?

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 08:59:37 pm
content: Mr. Fathom,An excellent question!Even a broken clock is right twice a day, even if only for a couple of nanoseconds.I suspect that using the jittered 10 MHz signal will be easier and just as cleanThat's not going to stop me knocking up a pre-filter amplifier. I have to know.Your passive crystal filter should have no amplitude sensitivityI refer you to my previous answer.From your original post it looks like the ultimate attenuation of your filter is something near 60dB on the low-freq endIncreasing the span only a little gives me flat lines at the visible minimum.Perhaps enough of the 2 MHz fundamental and 3rd harmonic are leaking through to cause jitter?Only further tinkering will reveal.Many thanks.Peter

author: joeqsmith
author_rank: Super Contributor
timestamp: March 13, 2021, 09:02:23 pm
content: Assuming you are trying to get from 48 to 10 and don't want to use a DLL or PLL, you could/4 to get 12/14 to get 2Make a simple mixer, then filter.Quote from: purpose on March 13, 2021, 04:33:29 pmI'm a small fish. I updated the manual to cover the new software and added some details about characterizing crystals with the Nano.

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 09:12:25 pm
content: Quote from: joeqsmith on March 13, 2021, 09:02:23 pmMr. Smith,That's easy for you to say.My diet of chocolate and Jack Daniel's is not permitting me to process even simple arithmetic.Will check out the updated manual.Many thanksPeter

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 09:24:16 pm
content: Quote from: joeqsmith on March 13, 2021, 09:02:23 pmYou'll always be a giant Tuna to me.

author: purpose
author_rank: Frequent Contributor
timestamp: March 13, 2021, 09:36:15 pm
content: The software for the V2 Plus series of VNAs has now been ported to support the original NanoVNA.You're spoiling us.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 13, 2021, 09:50:03 pm
content: It's pretty much done after a coding frenzy. Lots of testing to do.