Fun with crystal filters - Page 1
Forum section: electronics / rf-microwave

author: joeqsmith
author_rank: Super Contributor
timestamp: April 26, 2020, 10:32:59 pm
content: In response to one of my videos for the NanoVNA, I was asked about making a video about crystal filters. From the attached, they are planning to attempt to make an 8-pole filter using 12MHz crystals. They want to use the DISHAL software with something called the G3UUR method. I thought rather than make a video, I would invite them to join the EEVBLOG and work together with them in the forums.I have downloaded the DISHAL2052 from here:http://warc.org.uk/?page_id=387

author: 9aplus
author_rank: Regular Contributor
timestamp: April 27, 2020, 07:09:25 am
content: 8 pole can be the real challengeOnce upon the time, built this 5 pole 1,6 MHz LSB xtal filter for SEA222 marine transceiver...The adjustment was nice experience...Big can is original USB filter, relays switching USB/LSBOn exit of 5 pole filter one stage of IF amplification to make equal level to original USB filterTesting wihtin transceiver

author: joeqsmith
author_rank: Super Contributor
timestamp: April 27, 2020, 11:06:20 am
content: Nice work.Without more details from the OP on their particular requirements and not having a need for this filter myself, I may just put something together. I glossed over the help files included with the DISHAL software. I have the 2015 edition of the ARRL handbook and read the section on crystal filters. They offer a few simple circuits to evaluate the crystals.A few other references I glossed over:Ladder crystal filter design, J.A. Hardcastlehttp://warc.org.uk/wp-content/uploads/2014/01/RadCom-Feb_1979.pdfCrystal Ladder Filters for All, Jack A. Hardcastlehttp://www.arrl.org/files/file/QEX_Next_Issue/Nov-Dec_2009/QEX_Nov-Dec_09_Feature.pdfQuartz Crystal Parameter Measurement, Jack Hardcastlehttp://warc.org.uk/wp-content/uploads/2014/01/QEX-Jan_Feb_2002.pdf***This paper may be of some help. They talk about the G3UUR Colpitts oscillator and offer what they consider an improved test jig. The OP plans to use the G3UUR technique and that is most likely what I will do.http://jvgavila.com/other/Crystal_Test_Set.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: April 29, 2020, 02:06:04 am
content: I was hoping the OP would join. They did provide one key bit of info. It seems they only have a NanoVNA for test equipment. While I wait to see if they plan on expanding their lab, I made some progress.Attached is a circuit from the 2015 ARRL handbook. I spent some time tonight wiring up most of it, taking liberty to sub values for what I had. The multimeter stage is missing. The software seems to allow you to enter the values used for the oscillator. It looks like it will require a fair amount of time to measure a single crystal. It takes a few minutes for the temperature to settle after handling the part.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 29, 2020, 11:25:25 pm
content: Finished assembling the oscillator and picked up 16 X 12MHz crystals. Next comes the boring part of measuring them.Most of my equipment, including the counters use a GPS reference. I will be using the counter and meter to make these measurements.It seems the OP wants to try and only use the Nano which I'm not sure why they mentioned the G3UUR method. After I measure them, I am try and see if the Nano would be good enough to directly measure them. I've never looked to see what sort of resolution it has or how stable it is.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 01, 2020, 12:23:06 am
content: Sorry about the old software. I have yet to rewrite the program for my counter to support Windows 10. The counter uses an HP GPS receiver as its reference. This runs 24/7. The counter can resolve down to 1Hz.I've asked the OP what BW they required but have not yet heard back from them. The software states no more than +/-2% of the BW when matching the crystal's frequency. At 2KHz, that's 40Hz of 12MHz which seems tight even 60Hz for a 3KHz BW would seem difficult. Assuming the parts are screened on the bench, I need to make sure they are stable before taking a reading. The first thing I noticed is that they require a very long time to settle after being handled.1: showing a 3.6865MHz crystal in the test jig running 5 minutes after about a half hour warmup. The jig seems stable within 1Hz.2: showing the drift after handling a second part while installing it. This is roughly 8 minutes. It's a good 4 minutes for it to become stable within 1 count.3: showing a 12MHz crystal after handling. We can see it required even more time to settle.4: I held onto the next 12MHz crystal for 20 seconds or so to get a better idea on the how long I need to wait. The 12MHz crystals seem sensitive to air currents so I tossed my calibrated beach towel over the setup which seems to have helped with the low frequency wander. The jig is using silver mica for the oscillator and the offset caps. It seems that the drift is these higher frequency crystals.I installed a few of the 12MHz parts to get an idea how much they very part to part. The are all over the place. Same brand, date code and I assume lot. The software talks about a sub program used to compensate for the variances in the parts. They provide an example. I doubt I am going to find enough crystals in this small batch to make up the eight needed.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 01, 2020, 02:39:51 am
content: Allowing the covered 12MHz to run, it's pretty poor. The 3.68MHz parts are much better. I may need to order up some better parts.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 02, 2020, 02:12:31 am
content: Converted the Labview software to use my custom Ethernet calls to that GPIB controller. This will be nice to have.These are some 11MHz FOX parts. Again, looking at the settling time, using the calibrated beach towel. It appears their tempco is all over the place and could eat into the margins pretty quickly. Of course its not a very controlled test just handling them to install into the jig. The third part I looked at seems very stable with temperature.Digikey has some 12MHz, 10ppm parts in-stock.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 02, 2020, 02:55:14 am
content: Afterdoubleseveral more minutes, it seems to remain flat. Just a very good part from this batch. Screening them is going to be a nightmare with these long settling times.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 02, 2020, 11:43:42 pm
content: The circuit takes time to settle after throwing the switch. So it's taking several minutes per part. Basically I install the crystal, toss a towel over the setup and wait for the plot to flatten out. If I were going to attempt to run several crystals, I would automate the collection. Still without some way to autoload the parts, you have to babysit. So far, I see the little adventure as being penny wise and pound foolish. Normally the home projects I do are for the fun of it. Sorting data on crystals is no fun and so far, it looks like I would need a lot more parts to find 8 that would match.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 03, 2020, 02:06:30 am
content: After measuring all 16 parts, I went back and repeated the first and second parts. I was within 5Hz. These were made by CTS and are 11MHz rather than the OPs target of 12. The 12MHz parts I had were much worse. Data sheet:https://www.ctscorp.com/wp-content/uploads/MP.pdfI could roll an 8 crystal filter with a BW of 5.6KHz or tighten it down to 2.8ish KHz with 4 crystals. It seems the OP is not sure what they are after. They never mention details of what they require. The goal could be to find something sharp enough to make it impossible to measure with the nano. Who knows. They claim to be a beginner so I am not sure why they would be attempting to construct such a filter in the first place.I think for the little entertainment I may find, I will go ahead with a wider 8 crystal filter.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 03, 2020, 04:14:57 pm
content: Looking through my stash of caps, the 8 pole is out. The four closest crystals are within +/-24Hz. Rather than dealing with the match, adjusted the BW to provide 50ohmish. This worked out to 1.2KHz. The caps should be easy enough to trim and it should fit onto one of my test boards.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 03, 2020, 07:33:04 pm
content: The ripple is about 3dB, BW is about 1540Hz. With a 10KHz span, like the simulation, the rolloff is not near as good.See posts below.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 03, 2020, 08:00:55 pm
content: In order to get this sort of narrow band resolution, I had to change the scale on my software.Shown with a 10KHz span and at 2KHz.There's a fair bit of error between the two system but we are talking about a $50 unit being compared with in the 1997 HP Test and Measurement Catalog listed for $24,750.There was a problem with my setup. See the following posts.****I have a very old HP8754A VNA. This system can run down to 4MHz but there is no way it can make a narrow sweep like this. When I have done it in the past, I have used a separate signal generator.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 04, 2020, 03:33:23 am
content: For a comparison, an old SHOWA 9MHz 2.2KHz filter, model SF0922B.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 04, 2020, 05:11:42 am
content: I was thinking about why there would be such a difference between the simulation of the 4-pole filter and what I measured, especially for the ripple. The Nano actually seems closer to the simulation than the HP. I wondered if I had selected the wrong impedance but what it turns out, I was overdriving the filter. Backing down the source and things smooth right out.The Nano and HP are now looking pretty close.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 09, 2020, 12:53:23 am
content: Another good article:http://www.rfcafe.com/references/electronics-world/crystal-filters-electronics-world-april-1969.htmShowa filter along with matching transfomer.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 09, 2020, 01:24:20 am
content: I attempted to make a 10-pole 12MHz filter but the results were less than impressive. As you can see, it is not very flat.Looking at a commercial filter, it was less than $30 and I would guess much higher performance.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 09, 2020, 01:59:36 am
content: RBW still at 150Hz, allowing it to average for 100 sweeps.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 09, 2020, 02:20:10 am
content: My software could use some improvements (higher res for pretty much everything, more cursor features...). Still, we can see we have something very similar.

author: SilverSolder
author_rank: Super Contributor
timestamp: May 09, 2020, 01:44:38 pm
content: I have an old HP 3571A Tracking Spectrum Analyzer here, from the 1970's, a boat anchor that I like to play with sometimes...It uses a 5 pole crystal filter to do its magic. Each section of the filter is on its own plug-in board, dripping in gold and oozing crazy quality goodness as you'd expect.It goes down to a bandwidth of 3Hz and is still accurate today, as far as I have been able to ascertain.Crystals... gotta love them!

author: joeqsmith
author_rank: Super Contributor
timestamp: May 09, 2020, 05:23:44 pm
content: That's a nice old bit of equipment. I have a few old systems myself still in use. I've had a lot of fun playing with the little NanoVNA. Attached shown with the 10 pole filter but using a low noise amplifier with a step attenuator to get a bit more range from the Nano.That 3589A network analyzer can go down to 0.0045Hz when using the narrow band mode. The UI is well thought out.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 09, 2020, 08:40:00 pm
content: I added a narrow band plot feature to subtract off the center frequency. Here I have let the Nana sweep the filter with a 10KHz span for roughly an hour. The plot is +/-5KHz from the center. Again, using the low noise amplifier. The Nano seems stable enough.I'm not sure why the OP mentions the G3UUR method if they wanted to use only the Nano. It seems the next step is to try and measure a few of these crystals using the Nano.

author: SilverSolder
author_rank: Super Contributor
timestamp: May 09, 2020, 10:19:46 pm
content: The Nano VNA does look interesting - I don't have room for more boat anchors, so maybe it's time to look at some of the things that are possible in 2020AD!Where is a good place to buy a good Nano VNA?

author: mark03
author_rank: Frequent Contributor
timestamp: May 10, 2020, 12:22:19 am
content: It would be cool to implement a curve-fitting crystal measurement scheme like this for the NanoVNA:https://ieeexplore.ieee.org/document/1539973(I would post a sci-hub link but I'm not sure if it's allowed.)

author: joeqsmith
author_rank: Super Contributor
timestamp: May 10, 2020, 12:27:00 am
content: Quote from: SilverSolder on May 09, 2020, 10:19:46 pmPersonally, as much as I like the little Nano you may want to have a look at the Nano V2. As I understand it has a wider dynamic range, doesn't use harmonics and a much higher operating frequency. I have yet to get one so I can't comment on how well it performs.There's a decently sized group dedicated to the original Nano. You may want to check that out and see if it's worth your time to join up.The original Nano is much better than I would have ever guessed but as you can see from various posts, it has it's downside. I just choose to ignore the shortcomings, because it's $50.I think Flipper bought them off Amazon.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 10, 2020, 12:32:10 am
content: Quote from: mark03 on May 10, 2020, 12:22:19 amSorry but I don't have an account.I have not looked into what the current standard practices are. What is the advantage of this method? Can you summarize the concept?

author: joeqsmith
author_rank: Super Contributor
timestamp: May 10, 2020, 12:43:04 am
content: A couple of other links showing various fixtures.http://pages.suddenlink.net/wa5bdu/crystal_slide_show.pdfhttps://www.nonstopsystems.com/radio/pdf-ant/antenna-article-xtal-1cqp.pdfI would have thought that the miniVNA would have been a squarewave like the Nano and they would have had a filter as part of that jig.***A few morehttps://www.hongkongcrystal.com/wp-content/uploads/images/AppNotes/ANENG-EQP-000.pdfhttps://oliverbetz.de/cms/files/Artikel/Quarzoszillator/technicalintro_crystals_e.pdf

author: mark03
author_rank: Frequent Contributor
timestamp: May 10, 2020, 01:28:31 am
content: Quote from: joeqsmith on May 10, 2020, 12:32:10 amYou can copy and paste the DOI into sci-hub (google it) if you don't have institutional access or don't want to pay whatever ridiculous fee they are asking now. Legality depends on where you live, but IMHO the morality does not. I say this as a published IEEE author (same journal, even!).Quote from: joeqsmith on May 10, 2020, 12:32:10 amHere's an interesting summary and comparison of different measurement schemes:https://www.mikrocontroller.net/attachment/195160/Crystal_Motional_Parameters.pdfI haven't studied this enough to give you a good summary, but AFAIK the "admittance circle" method is the current standard and is based on that IEEE paper. It uses data taken at many different frequencies to curve fit the parameters, which seems like common sense nowadays. Most of the older methods use only "landmark frequencies" (-xx dB points, 45-degree phase-shift points, series and parallel resonance frequencies, etc.), and some do not assume that vector measurements are available. So for a modern approach I think the 1987 paper might be a good starting point.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 10, 2020, 04:47:33 pm
content: Quote from: mark03 on May 10, 2020, 01:28:31 amGoogled as suggested.https://scholarlykitchen.sspnet.org/2018/09/18/guest-post-think-sci-hub-is-just-downloading-pdfs-think-again/I would rather make sure the people involved with creating the content are paid. If you find where the original author has placed it into the public domain, I will have a look. The DISHAL software that the OP plans to use only supports the G3UUR and 3dB methods. I have not spent a lot of time looking for other crystal filter software. There may be others available that use the method you are asking about.I like that last paper you linked. When I was measuring the crystals with that home made oscillator jig, I mentioned once I was done, I remeasured the first few parts. I was always plotting the frequency and watching it settle. Many papers talk about waiting < minute. Some talked about not touching the parts when installing them. Some talked about the parts heating as they were ran causing some initial drift. I know my setup was fairly stable with the beach towel and waiting several minutes until the parts settled. Your paper makes no mention of it and I wonder how repeatable their setup was.Seeing their standard crystal, I had a look at Saunders site. I wasn't able to find where they sell these standards but noticed this paper:https://www.saunders-assoc.com/resources/datasheets/article/Load%20Resonant%20Measurements%20of%20Quartz%20Crystal.pdf

author: mark03
author_rank: Frequent Contributor
timestamp: May 10, 2020, 07:27:13 pm
content: I respect your position and won't belabor the point; I will only mention that none of the money paid to access papers goes to "content creators." The content creators (as well as the editors and the peer reviewers) are all volunteers, working for free. There was a long discussion of this issue (open-access vs closed-access academic publishing) here:https://www.eevblog.com/forum/oshw/the-problem-i-have-with-closed-(source)-papers/This is a completely different moral question than, say, pirating music and movies. One may certainly come down on the same side (the side of the law) in both cases, but not because the moral dimensions are comparable---they really aren't.Anyway, sorry for the OT rantI'll keep on eye on this thread. You have encouraged me to develop a crystal-measurements script for my VNA, if I ever find the time. Work is still busy, which I know I should be thankful for. Sometimes I do envy those with the freedom to work on hobby projects however.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 10, 2020, 07:30:56 pm
content: Using the Nano, output to a lowpass filter, to a step attenuator, to an LNA, to a 10dB atten, to the DUT, to a 10dB atten, to the input....Span set to the minimum allowing to see the 3dB from the peak. I let it run for a while then gently squeezed the crystal with my fingers. With a 100 points and just looking at the data, you can't see a shift. The plot below is holding a heatgun to the part for several seconds.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 10, 2020, 07:38:36 pm
content: Quote from: mark03 on May 10, 2020, 07:27:13 pmNo matter what arrangement the supplier has with the authors, I see no reason to not pay for their services if I want to use their product. If the authors want to make their papers open to the public free of charge, that's up to them. There is certainly many ways to do this today. I would imagine there is some prestige for the author, or the company they work for to have certain organizations publish their papers. You may find the authors or the companies they work for are paid members of these same organizations. No matter, it has nothing to do with playing with filters.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 10, 2020, 07:53:32 pm
content: Zooming in with a 500Hz span. We can now clearly see the ripple. Notice that the heatgun caused a fairly large change in the amplitude, but with the 100 points, its still difficult to see the frequency shift.If I ran a segmented sweep (multiple 101 point sweeps over the span) to increase the resolution and fit the data, maybe we could tease out some numbers. I am not sure from reading the DISHAL help file if one method is better than the other but using the counter, seems like it at least can at least resolve the signal to use it. That 2% of the BW for a narrow band filter and looking at using 12MHz crystals, seems like the counter is a better solution than trying to use the Nano.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 10, 2020, 08:39:49 pm
content: Lowering the gain to help with the noise. S11 isn't too bad.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 10, 2020, 09:01:08 pm
content: Going from the Nano to a 3dB attenuator, to the DUT, to a second 3dB attenuator. Touching the crystal mid run, we can clearly see the shift now with the lower noise. Knowing they will shift about 10Hz and that we can see it, seems like it could work even with a 101 points. The match isn't very good this way. Time to make up a set of pads.Added plots.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 12, 2020, 01:08:53 am
content: Sorted some resistors and built up a quick jig for the 3dB method. Basically the same old same old. I used the pot method to measure the series resistance as it seems to be the most common method.Just a sanity check, using the old cal, match appears much better. Lm is off a bit between the two but close enough it seems we are in the ballpark.The Nano software is doing a fit to get the centroid the going 3dB down from that. It uses interpolation to determine the frequency. Basically trying to tease out the information from the 101 data points. Next I plan to measure some of the crystals to see how they vary compared with using the counter.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 12, 2020, 10:21:42 pm
content: Having to insert a pot, tweak it, pull it and finally measure it with a meter seems like a poor idea. I made up a set of standards to characterize the jig. There is a SOL but also another resistor that is 12.23 ohm to use as a sanity check. I tried this setup out using that same crystal and get very close to the number I had using the pot. The next step is add support to the software to automatically make these measurements.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 13, 2020, 03:01:48 am
content: I tried a few different fitting and filtering methods to tease out the crystal's series frequency. In the attached plot, in the upper right I am plotting the calculated Fs. It has an error of 50Hzish. Consider it was a 1KHz span or roughly 1 sample every 10Hz, it's not too surprising. The 50Hz could very well be more than the entire error budget for a filter.It's certainly possible to sweep in segments like I demonstrated when using the Nano to derive a model of a crystal for SPICE. However the Nano is VERY slow.We also need to consider how wide the span needs to be to handle to parts we are trying to measure and the amount of time it will require to perform each sweep. We may want to use multiple sweeps to track the drift or average the data as well. As much as I like the little Nano, it seems like a really poor choice of tools for this particular task. But again, it's $50 so no surprise.Another thing to consider is the Nano has no reference clock. If all you have is the Nano, you have no way to know how accurate yours is. You could spend days collecting worthless data.Of course, the other thing to consider is the cost of buying an actual filter you need. While the OP never did post their requirements, it looks like a 12MHz 8-pole crystal filter with a few KHz BW will set you back around $30.The second plot is using the segmented sweep. The span of each segment is 100Hz. The total span is a 4KHz. Even with the added data, I am seeing about a 25Hz error. Of course 12 sweeps is not much to go on. Note that the first sweep is off the screen because I handled the part. It may be good enough in some cases however, it takes so long to collect the data, it seems rather pointless. A simple counter and GPS is not very expensive, would be much faster and it seems more accurate (than using the Nano).Maybe there is a better way to leverage the Nano for narrow band filter prototyping. If you have any ideas, feel free to chime in.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 13, 2020, 11:13:59 am
content: I let the NanoVNA run overnight, continuously sweeping the same 12MHz crystal and plotting the calculated series frequency. With the 4KHz span, it recorded 120 data points in roughly seven hours. The crystal and fixture are covered with the calibrated/certified beach towel during this test. The first data point after touching the crystal being tested was ignored to not bias the data.Shown is the histogram of the data. It appears to have a somewhat gaussian distribution. 40Hz p-p noise is poor.

author: SilverSolder
author_rank: Super Contributor
timestamp: May 13, 2020, 12:09:16 pm
content: What is the noise from - the DUT or the NanoVNA?

author: joeqsmith
author_rank: Super Contributor
timestamp: May 13, 2020, 12:43:02 pm
content: Quote from: SilverSolder on May 13, 2020, 12:09:16 pmHard to say at this point. I thought one test would be to use a lower frequency crystal. The results were not what I was expecting. It looked so good after the first to sweeps, I stopped the test and changed from the segmented sweep back to a single 500Hz span. With the fitter, we can now easily see the initial drift and the p-p noise, including the drift is 4Hz. Of course the absolute accuracy is. Without a some sort of frequency reference we have no idea.So at least it's not a total bust. Depending what you want to do, it's starting to look possible that we could maybe pull it off with just the Nano. I would like to try some other brands of 12MHz crystals. I'm not sure why the OP chose that particular frequency. It seems high for a narrow band filter.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 14, 2020, 12:35:53 am
content: Going from 3.7MHz to 5.6MHz seems to make a small difference in the noise but it seems very usable.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 14, 2020, 12:39:47 am
content: At 7.8MHz, we are now at roughly 13Hz p-p noise.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 14, 2020, 12:45:49 am
content: With an 11MHz part, we are getting close to 40Hz p-p. At around 500 samples into the run, I installed a home made 5 pole 12MHz LP filter to see if it had any effect. Sadly, it doesn't.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 14, 2020, 01:22:32 am
content: Looking at an old JK Products crystal. It's interesting how it warms up.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 16, 2020, 12:12:49 am
content: Another good reference: "Hewlett Packard Electrical Characterization of Quartz Crystal Units"http://hparchive.com/seminar_notes/a-119.pdfNext I plan to try measuring a handful of parts in the range that the Nano seems able to resolve when using the 3dB method. Then compare these results when using the G3UUR method.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 16, 2020, 02:10:45 am
content: Odd. The values are not even close. Maybe the serial frequency is wrong....The nano will run all the calculations automatically and store the data into a CSV file. The software can then sort them based on the series resonance and pick the best parts for the tightest BW.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 16, 2020, 05:36:22 pm
content: After some sleep, I verified my calcs against the Dishal software, then put the jig on the HP and repeated the test for one part. Plugging the data into the Dishal software, the numbers look reasonable. The 3dB full width was a mile off between the Nano using my software and the HP.After several more tests on the Nano and my software, I found the problem.The numbers are now very close. The Nano using the 3dB method is calculating an inductance of 153.7mH and a capacitance of 12.13fF. Using the same crystal with the G3UUR method, I get 165.4mH and 11.27fF.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 16, 2020, 06:40:36 pm
content: After correcting the problem, I collected the data a second time. Shown is the best set of 13 parts for a 4-pole filter. I've included the data. The columns match what is shown in the screen shot.Time to sort out the G3UUR equations so I can let the computer do it's thing.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 16, 2020, 08:02:03 pm
content: From the one paper I had linked, the which has the equations for the G3UUR method, it appears to be correct.Attached comparing the data for all thirteen Fox crystals using the G3UUR and 3dB methods. It appears there's about a 12% error between the two setups. I can believe this. Looking again at my particular Nano's clock, at 3.6865MHz where we are working, it is off by 1Hz compared with the GPS. My guess is most of the error is from my two different jigs. While I am sure I could tweak them to get the results to match, it's rather pointless. The next step would be to try and procure some sort of crystal reference standard like the one shown (aunders & Associates Reference Crystal, model 7000022).Then again, how good does it need to be?

author: joeqsmith
author_rank: Super Contributor
timestamp: May 16, 2020, 08:21:29 pm
content: Just a quick measurement of CSW in-circuit and using this in place of the 50pF appears to cut the error in half ( 6% ).

author: joeqsmith
author_rank: Super Contributor
timestamp: May 16, 2020, 10:10:43 pm
content: Measuring the crystal socket mounted into the perfboard, 1 get 1.68pF. Adding this to each crystal's capacitance brings the peak error below 3%. Interesting is the outlier is crystal #3. Notice this crystal also had double the p-p and much higher standard deviation than the other 12 parts we looked at while in the 3dB jig.With #3 in back in for a third time, I let it settle much longer. While the stddev and p-p are now on par with the other parts, the results were too close to call. Both figures of merit are calculated from a cold start, unless I reset the calculation during a run. So, they can be very wide and still have a good measurement. I should automate this part of the test.I then reinstalled the part into the oscillator and let it settle. Sure enough, it was off a few Hz. This brings the error down to 2%. Still, I have no idea about the absolute accuracy.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 17, 2020, 01:00:55 am
content: With most of the problems solved, I went back to the 12MHz CTS crystal. To try and help it along, I cut the leads down to about 10mm.First, I ran the 3dB test jig on the HP analyzer and measure -1.75dB and 690Hz 3dB down. Just to have a sanity check. I recalculated the Lm and Cm using the new capacitance values (manually adding the offset to the holder).I then tested the part on the Nano. Notice that I am still only using a single 101 point sweep. Both the p-p and standard deviation are slightly improved over what we had previously measured using a 100Hz segments.Note that peak is spot on to the HP (it should be) but the 3dB width is slightly wider on the Nano.Both Lm and Cm have under 6% error. It looks like the Nano could be used even at 12MHz depending on the filters requirements.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 17, 2020, 02:12:00 am
content: The Nano continued to collect data for the CTS 12MHz crystal for roughly an hour to get a better idea what the profile of the noise looks like.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 17, 2020, 04:19:07 am
content: Shown attaching the 3dB test jig to the Agilent with the same CTS 12MHz crystal. The noise is much lower. Obviously, if you are trying to match crystals withing say 40-60Hz, you don't want your test equipment eating up 100% of the error budget. The 689.34Hz BW is pretty much spot on to the other HP which measured 690Hz.Both Lm and Cm values are pretty much the same as we got with the Nano.I've tried to find a source for a reference crystal but no luck. I may try and contact that same company and see if they would sell me a few characterize at different frequencies. Maybe 5 and 15MHz.https://www.saunders-assoc.com/resources/datasheets/referencecrystal/6300338.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: May 17, 2020, 10:37:03 pm
content: I suspect if we were to ditch the resistive pads we could improve the 12MHz S/N. The plan is to include the attenuators on the board if it shows any promise. These were the smallest cores I had on hand that would work in these lower frequencies.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 18, 2020, 12:10:14 am
content: This is more like it. Again, just a single 101 point sweep, same span. It's obviously not calibrated as I was really only interested in seeing if it would improve the resonance frequency detection. So if you plan to use a Nano to sort your > 6MHz crystals you may want to consider something other than resistive pads for your test jig.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 18, 2020, 12:35:06 pm
content: I plan to finish building up the new jig today but I'm at a loss why it helps. The 12MHz parts have a lower loss than the 4MHz parts. The cores do offer some level of low pass filtering but when I had installed the LC filter, it had no effect. Early on I had tried to run it with an LNA, step attenuator and filter. We also know the same padded type jig works fine with both of the higher end network analyzers. Looking at the signal from the Nano with a scope and SA at 4 and 12MHz, I don't see a difference that would cause a problem. However we are looking at 0.1ppm levels.One other experiment I tied was just connecting the part directly to the Nano in series as I had shown in the first video. The results were very poor. However, part of the problem is not knowing what a good result is. For now I am assuming that with the two methods (3dB and freq shift) yielding results within 2%, they are valid but that may not be the case.So there are a few clues. If you have any ideas on the cause, I would like to hear about them.

author: SilverSolder
author_rank: Super Contributor
timestamp: May 18, 2020, 01:39:31 pm
content: How does the calibration work with one of these - do you have a short/open/load type of jig?

author: joeqsmith
author_rank: Super Contributor
timestamp: May 18, 2020, 05:04:26 pm
content: Quote from: SilverSolder on May 18, 2020, 01:39:31 pmBack a page I show the standards (loose use of the term) made to cal and run a sanity check on my software and jigs. Because the OP had asked about 12MHz, I doubt I will do any testing beyond this.https://www.eevblog.com/forum/rf-microwave/fun-with-crystal-filters/msg3064340/#msg3064340That last paper I linked from HP covers the calibration. It's not a long paper and covers the basics.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 18, 2020, 11:27:09 pm
content: Used a drop of epoxy to hold the cores to the PCB and then another drop to hold the connector to the cores. Then added the two attenuators.I placed a step attenuator between the fixture and the Nano's CH1 (port2). The first 1100 samples are with 0dB attenuation, then changing to 10dB until 1300ish samples where it was set to 20dB. Notice that the 3dB BW is very close.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 19, 2020, 12:40:19 am
content: If I cal the Nano with the standard SOLT and consider that 0dB.Gen I Resistive pad jig with a short is about -30dBGen I Resistive pad jig with a 3.686MHz crystal is about -37.3dBGen I Resistive pad jig with a 12MHz crystal is about -31.3dBGen I Resistive pad jig with a open with Nano sweeping at 12MHz is about -80dBGen II Transformer matching w/6dB jig with a 12MHz crystal is about -8.6dBThe question is why the resonate frequency of the 3.7MHz part is so stable compared with the 12MHz part when using the Nano.If I reset the Nano's internal calibration, then install a thru and scan small segments, there is something happening at 12.8MHz. I've been searching down to 4MHz and a bit over 13MHz and it only appears to show up at 12.8. I wonder if what ever is causing this is tied into our problem.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 19, 2020, 12:46:39 am
content: Running the 12MHz crystal in the Gen II jig for an extended time. Noise is now 5Hz p-p. An improvement of about 10X. The noise has a nice shape to it. Note the Lm and Cm values.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 20, 2020, 12:16:52 am
content: The Gen II jig still had some issues. The core material wasn't that great. I had two separate windings to get my turns ratio rather than a bifilar. This really hurt the performance.These are the cores for Gen III. I was originally not going to use them because of their size. The right thing to do would be to buy something but I had these left over. The magnet wire shorted on one of the cores, so Teflon it is.Looking at the 12MHz crystal with the crude setup, things look decent. The jig's 3dB point is was around 40MHz but once it was assembled it moved up to653MHz. This is much better than the previous setups.Waiting for the epoxy to cure and then we can try it out. We should be able to have a look at some higher frequency crystals now.***** UPDATE *****The original data transmission plot after assembly had a very odd shape to it. At this time, the epoxy was wet which may have been the cause. I recollected the data today and it looks smooth as you would expect (JIGIII_6).

author: joeqsmith
author_rank: Super Contributor
timestamp: May 20, 2020, 03:08:14 am
content: This 50MHz crystal is the highest I have. I suspect its a bit of a worst case condition. This is still only using a single 101 point sweep. The part was drifting a bit but the Nano seems to track it fairly well.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 20, 2020, 03:23:21 am
content: Running a 16MHz part for an extended period. 7.7Hz with a bit of a wobble at the start. The standard deviation says it all.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 20, 2020, 12:12:50 pm
content: Shown with the GenIII jig attached to the HP. I did not allow the HP or crystal to warmup and nothing was covered.As we would expect, the standard deviation is much better with the HP but the parameters are pretty much the same.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 20, 2020, 09:49:01 pm
content: I wrote Saunder's today to inquire about their crystal standard. They haven't responded but may be closed with the shutdown.With the new fixture working, the only problem now is measuring C0. The Dishal software requires this and up till now, I have used the RLC meter to measure it. The plan is to have the Nano measure it as part of the other measurements. It would be easy enough to change the setup to make the measurement but I would like to do it all in one step.First thing was to make a new set of standards for this fixture (shown). The copper block and connector is just a heatsink for soldering these.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 22, 2020, 12:26:44 am
content: This is the first time I have ever tried to use a VNA to measure the thru impedance on the backside of two impedance matching transformers. After looking at the raw data, it seemed the Nano could at least detect a change from 5.6 to 10pF.Next I measured all of my standards with my two BK RLC meters. The standards vary from 5.6pF to 100pF. Trying each part with the Nano using the Gen III jig, surprising enough, seems to work fairly well. Obviously we need to change the frequency to make this measurement but that's not a problem.Plot showing one of the 3.686MHz Fox parts. You can see it reads about 0.1pF high compared with my one BK. The RED graph is plotting the capacitance to give me an idea how stable the reading are. The stats are valid, so 0.26pFp-p error. Not too bad.I still have not heard back from Saunders about the reference crystal.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 22, 2020, 01:53:16 am
content: Measuring C0 of one of the 12MHz crystals and then installing a small 50pF trimmer. The Nano's update rate is VERY slow. This is the cause of the vertical steps, not the resolution of the calculation or the nano's hardware.***added correct plot

author: joeqsmith
author_rank: Super Contributor
timestamp: May 22, 2020, 02:22:45 am
content: Sweeping a larger trimmer capacitor.

author: SilverSolder
author_rank: Super Contributor
timestamp: May 22, 2020, 03:20:07 am
content: What's the software you use to display the charts - is it LabView or something like that?

author: joeqsmith
author_rank: Super Contributor
timestamp: May 22, 2020, 12:11:41 pm
content: Quote from: SilverSolder on May 22, 2020, 03:20:07 amhttps://www.eevblog.com/forum/testgear/national-instruments-labview-home-bundle/msg2574126/#msg2574126https://www.eevblog.com/forum/testgear/advantagesdisadvantages-ni-labview-etc/msg2348991/#msg2348991The attached table from CopperMountain shows the equations for the three types of impedance measurements. One of the papers I read, they were showing similar experiments using another low cost VNA. They comment about directly reading C0 and toss out some 80% number. I was guessing they were suggesting measuring 80% from Fs. During my experiments, I am using a fixed frequency of a MHz. This limits the low side to about 1pF as previously shown. 80% may be a bit close but I tried one part sweeping from 30-60% and this seems reasonable.Quote

author: SilverSolder
author_rank: Super Contributor
timestamp: May 22, 2020, 05:27:53 pm
content: Quote from: joeqsmith on May 22, 2020, 12:11:41 pmI am no math wiz, for sure... What is Z0, is that the reference resistor used during the test?Edit: This slide from Agilent seems to show 3 methods of measuring impedance as well - are they the same as the Copper Mountain ones?

author: joeqsmith
author_rank: Super Contributor
timestamp: May 22, 2020, 10:26:50 pm
content: Quote from: SilverSolder on May 22, 2020, 05:27:53 pmQuoteYes indeed, these are the only three I am aware of. What makes this setup unique is while it's a series thru arrangement, it has these added impedance matching transformers that have to be accounted for.The original article from Copper Mountain:https://coppermountaintech.com/measurement-of-electronic-component-impedance-using-a-vector-network-analyzer/

author: SilverSolder
author_rank: Super Contributor
timestamp: May 22, 2020, 11:47:59 pm
content: The Agilent document goes through a couple of variations on those 3 themes, basically varying the input impedance of the ports on the VNA (1Meg as well as 50 ohm options). Interesting to see how it compares.I don't understand the purpose of the common mode transformer in the Copper Mountain setup... looks like a way to prevent ground loops? Why would that be necessary, since everything is connected to the same device?Attached Agilent document for reference.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 23, 2020, 12:44:32 am
content: Quote from: SilverSolder on May 22, 2020, 11:47:59 pmYes, they are trying to show you why you may want to use one method over another. It will depend what you are trying to measure. This is what the Copper Mountain document is also trying to convey.I am not sure what Copper Mountain setup you are referring to that is using a common mode transformer.Assuming you are actually asking why I am using the two impedance matching transformers with my setup compared with the resistive pads I started out with, then the answer is simple. The pads have too much loss to get reliable results with the Nano, depending on the crystal being measured. The reason that the resistive pads work fine with my older HP systems is that they have a much lower noise floor than the Nano. If you look at the graphs I've provided, the first fixture has a loss of 30dBish compared with 6 with the new fixture. When you are playing with a $50 instrument, you need to give it a little help. IMO, this isn't a big deal. It's $50. The fact I am able to pull out sub PPM data with it is impressive.When I made the videos for the Nano, I had played with a couple of crystals. One was measured using S11 (shunt) where I provided a simple example of making a model for SPICE. The other, I had one in series showing the resonance. The first thing I did was try to use the crystal directly connected and the standard deviation was very poor. The papers I attached early on are what led me to use the pad to try and improve the match.I think if you go back and read these papers, you will find I am not doing anything unusual or novel. What I have shown is we have a $50 VNA that can directly making ALL of the required measurements using a single fixture.Still no work about the reference crystal.

author: SilverSolder
author_rank: Super Contributor
timestamp: May 23, 2020, 01:14:46 am
content: Quote from: joeqsmith on May 23, 2020, 12:44:32 amAh, I actually found a different Copper Mountain article from the one you referenced... attached. Here, they use a Common Mode transformer with their VNA to get better performance. Not sure I understand how...

author: joeqsmith
author_rank: Super Contributor
timestamp: May 23, 2020, 03:41:53 pm
content: Quote from: SilverSolder on May 23, 2020, 01:14:46 amI'm not understanding why it would matter. Crystals filters don't normally have sub ohm resistances.**Just to be clear, I am not sure why there would be a need to read such low values when working with filters. I don't doubt the need to control the paths when measuring this low.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 24, 2020, 01:02:36 am
content: If we were working in the 900MHz (or even 300MHz) with the Nano, and trying to look at filters with having a high roll off, it may not work out so well.Attached are a few screen shots of a nine section 1GHz interdigial filter with a 3dB BW of around 130MHz. A long way from the 12MHz that the OP asked for.****Added narrow sweep for Nano

author: paul002
author_rank: Regular Contributor
timestamp: May 27, 2020, 11:36:53 am
content: I also use the nanovna to measure the crystals and DISHAL to design and adjust the crystal parameters. It takes some practice to really understand all the ins and outs. Also tried to use quandl to simulate the filter but somehow the parameters does not match. I use G3UUR methode to measure the crystals using 12 ohm impedance transformers and the nanovna bandpass filter option to calculate -3db points. There is also some crystal measurement software on github. But did not work for me. But for now I am happy with the result using the nanovna.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 28, 2020, 01:43:06 am
content: Their software looks like it is coming along nicely.Why do you need these 12 ohm impedance transformers for the G3UUR method? Everything I saw on it, they just used a basic oscillator. Maybe scribble a picture of your circuit.

author: paul002
author_rank: Regular Contributor
timestamp: May 28, 2020, 08:03:32 am
content: Ah I see, I don't use the G3UUR, I use the method described by WA5BDU in crystal_slide_show.pdf (slide 10 and 11). And in DISHAL use the xtal tuning function to calculate correct values. And use Nanovna-saver software to determine the -3db points

author: joeqsmith
author_rank: Super Contributor
timestamp: May 28, 2020, 11:44:10 am
content: Makes more sense. I am using something similar for the fixture.How did you validate your results? Did you use the G3UUR at one point to correllate with the Nano? Did you measure your Nano's frequency accuracy?I'm curious how your setup compares with what I have shown so far. I never did hear back about the standard reference crystal. Outside of getting an email stating that they received my request, there have been no other communications.

author: paul002
author_rank: Regular Contributor
timestamp: May 28, 2020, 12:17:10 pm
content: I also used the G3UUR method but not with a lot of success, measurements were not precise enough, but also it takes to much time. Using the Nanovna is much faster. I remeasured some crystals with my signal generator which is build in in the scope, but it is not precise enough. But I assumed that if the nanovna has a frequency error, it will make the error on all crystals. Unfortunately I don't have a GPS reference to tune the NanoVna. So yes there is a frequency error. When building the crystal filter I used a bunch of variable air capacitor to be able to adjust very easy the capacitors. After a while you get used to how the capacitors influence the shape of the filter. It is not difficult the get the passband shape, but you have to tune quite well to get a acceptable swr on the crystal filter. I think if you are able to tune the nanovna to an exact frequency this problem can also be solved. It is possible to have external clock on the si5351 maybe that is a solution. So it is science for a part but still a large part experimenting.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 28, 2020, 01:03:28 pm
content: I have no way of knowing which method is more accurate when I tried them. They both could be off a fair amount and I have no way of knowing. Seeing 2% between the two gives me some confidence in the measurement.Speed wise, for me it's a wash. The majority of the time is waiting for the parts to settle. If I were to measure several parts, I think I would make some sort of holder for them so I could pick them up easily with some plastic tweezers and make some sort of ZIF socket for the fixture. I thought about adding a small thermal controlled block that would contact the case to at least remove the effects of the room temperature.With both setups, I have to visually watch them settle. A better approach would be to automate that part as well.If temperature wasn't a problem, the Nano may have a leg up speed wise. If I were to automate the C0 measurement, it may be a bit faster to use the frequency. That Nano just crawls along. So far, using the fitter has worked well but again, no idea on accuracy. I'll take slow and tight stddev over fast inaccurate results any day.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 08, 2020, 04:22:43 pm
content: After all these months, I have never heard back from Saunder's about the standard they offer. However, at this point I am much more confident in the data I am collecting.Shown is the last filter I constructed for the fun of it.With the new low cost VNA having much better update rates (possibly 400 - 500 sample points per second), I would like to pick one up and try it with some of these filters. With it being this fast, it would make adjusting some of the mechanical trimmers so much nicer.

author: Grandchuck
author_rank: Super Contributor
timestamp: August 08, 2020, 04:35:33 pm
content: Nice Joe!Are you going to share the frequency response curve?

author: joeqsmith
author_rank: Super Contributor
timestamp: August 08, 2020, 05:03:26 pm
content: I am still thinking to make a video about the whole process which is why I built this particular filter.It could end up being more of a VNA shootout of my older higher end equipment vs the ultra low cost modern systems flooding the market. I would like to show some of the new ones that OWO is involved with. The speeds they are sweeping at exceed what I can do with my 1970's HP and some of the other claims seem really impressive.After playing around a bit above 1.5GHz, I could maybe do a decent job comparing them at higher frequencies.