Matching a 2-port (typical) SAW filter to 50 ohms - Page 1
Forum section: electronics / rf-microwave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 05, 2024, 03:37:53 pm
content: This is about how to get started developing/testing RF matching sections for a 2-port device, when neither port has a working matching network, so the "second" port is not yet properly terminated.INITIAL SPECIFICS:The device in this case is a Tai Saw TA0245A SAW filter (139 MHz, PDF datasheet attached.) The ports are not natively 50 ohms (they're basically low impedance dominated by capacitive reactance.)My questions revolve around this: Can the ports be matched separately? When the SAW filter is in passband (this device has insertion loss about 5 dB at center frequency), the getting-started condition is that neither port is matched. Does this complicate measurements "looking into" the one port when the other port is open/shorted/(what should I do here?)ADDITIONAL DETAILS:The datasheet indicates equivalent circuit for input port is 390 ohms || 300 pF.The datasheet indicates equivalent circuit for output port is 1164 ohms || 374 pF.The datasheet shows a representative input match section with 12 pF series followed by 56 nH shunt at the port.The datasheet shows a representative input match section with 9 pF series followed by 56 nH shunt at the port.I'm testing this on a small bit of double sided/plated thru FR4 (matching the device footprint, so the 6 remaining contact pads have individual vias to ground.) Input/Output paths are sized for 0603 components (shunt, series, shunt "pi" physical arrangement), but are otherwise very short (distance between the device and the match components are a few mm, then a few mm to SMA ports on the test module.)My initial construction implemented the datasheet's suggested matching section. The filter S21 (insertion) resembles the datasheet, but insertion losses are several dB higher than expected (as I'm at the upper edge of the device passband, the added several dB there make a difference; group delay there won't be an issue.)Using a VNA (calibrated at the SMA connectors of the small test module) at the device center frequency (139 MHz), with the datasheet matching sections installed, the input port yields 37 ohms || 78 pF; at the output port 16.3 ohms || 58 pF equivalence.I have made a trial run (just on PC, not building anything yet) usinghttps://www.analog.com/en/resources/interactive-design-tools/rf-impedance-matching-calculator.htmlThis has input data options that exactly mirror the physical device requirements (device port is expressed as parallel RC combination, specifying R, C and F).I'm in the "look before you leap" state right now; the analog.com page suggests match sections, but I want to be sure I'm considering (if necessary) the effect of the second device port properly (that is, within the active passband of the filter.)Thank you for any comments, questions, or requests for clarification!Dave

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 05, 2024, 05:38:25 pm
content: What power are you putting in with the VNA?The datasheet says that you can only put in at most 0 dBm (and that's the absolute maximum, so presumably you should test with -10 dBm or less).

author: RFDx
author_rank: Regular Contributor
timestamp: July 05, 2024, 05:53:42 pm
content: Quote from: wb0gaz on July 05, 2024, 03:37:53 pm390/1164 Ohm could be possible, the 300/374pF capacitive part is obviously an error in the datasheet.

author: G0HZU
author_rank: Super Contributor
timestamp: July 05, 2024, 07:26:54 pm
content: Golledge have a similar 139MHz SAW filter datasheet here:https://www.golledge.com/media/1803/ma05254.pdfGolledge are UK based and I've had good service from Golledge quite a few times over the years at work.The input impedance is given as 210R || 11pF for both the input and the output at 139MHz. If this is the same SAW filter as yours and you already have the 56nH inductors then maybe try the capacitive tap network shown below on both input and output?15pF shunt then 22pF series and 56nH shunt as shown in the diagram below.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 05, 2024, 08:06:42 pm
content: EggertEnjoyer123 - VNA output (this is a nanovna) is in the -10 dBm range. In the application, -7 dBm (0 dBm then through a mixer with ~7 dBm insertion loss) is the most the device would see. That said, I'm glad you mentioned this as I had glanced past that specification for this device (most other SAW datasheets from Tai Saw I've seen have a higher power tolerance.)RFDx - I was thinking there might be data sheet error in one respect or another, as the example given did not work as expected.G0HZU - I've used Gollege from time to time in the past (=when I was employed!), results were excellent and they were altogether helpful, however, none of the small-lot purchases (for prototyping) led to production. In this era I've not got them as a resource (they were not the provider of these parts.) That said, the MA05254 datasheet does indeed cite TA0245A, so I can consider their version of the datasheet likely to supersede (that also corroborates RFDx observation!).G0HZU - In Section D (MEASUREMENT CIRCUIT) of the Gollege MA05254/TA0245A datasheet, L is not specified. Have I overlooked something? The 160 ohm series in the path it seems would provide the SAW device ports a match from the network analyzer, but would also create non-zero additional insertion loss (which in my case might put me back where I started from, excess insertion loss vs. the datasheet specification.)G0HZU - I tried 210 ohms || 11 pF in the RF Impedance Matching Calculator page from Analog Devices (set as parallel complex load), one option is shunt C followed by series L (values given); the other option is shunt L followed by series L. It appears these seek to transform 50 ohms to 210 || 11 without attendant loss of 160 ohm series R in the matching sections.G0HZU - as I can freely change components in the matching sections (my small prototyping board has shunt-series-shunt path into each port), it isn't necessary to retain the shunt L cited in Tai Saw's original (current???, just downloaded!) datasheet.+++++++++++One follow-up question - as my last go with SPICE was some decades ago, would anyone reading this have a suggestion for an on-line passive circuit analysis tool? I'm envisioning something I can set up a few nodes, specify components (L, C or R) amongst the nodes, as sweep (in simulation) the resulting circuit.+++++++++++Thanks so much for the various and highly helpful replies!Dave

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: July 05, 2024, 10:22:08 pm
content: Without studying the problem, I'll remind you that your real matching network, using real components, will have a lot of parasitic response and so you inductors might have 1 or 2 ohms series resistance and maybe 0.05-0.25 pF shunt capacitance (depending on inductor size, maybe much more). This will move the effective values of these matching networks sometimes as much as 50% in frequency. ADS has models for SMT components, the smaller ones do have less parasitic than the larger ones. Oh, and an SMT cap might have something like 1-2 nH of series inductance as well.

author: G0HZU
author_rank: Super Contributor
timestamp: July 05, 2024, 10:58:50 pm
content: A lot depends on how accurate the datasheet is in terms of the true input impedance of the SAW filter when it is matched.I think that the matching network I put up in reply #3 should be fairly forgiving of device parasitics at just 139MHz. This assumes SMD parts and a sensible PCB layout. The neat feature of the tapped network is that you can select a fixed inductance (in this case 56nH with a typical Q of 55 at 139MHz) and then adjust the two caps to get a good match. So it should be possible to get a reasonably good match with a simple network even if one or both cap values have to be tweaked slightly.Assuming the aim is to get a reasonably good match rather than a perfect match, then it should be possible to use just two fixed value capacitors and a fixed inductance. You could also use two fixed caps in parallel for each cap value and this would allow some extra optimisation.If you want to match it as well as possible then at least one component would have to be tuneable, especially if you wanted to produce lots of copies of the circuit.

author: G0HZU
author_rank: Super Contributor
timestamp: July 05, 2024, 11:07:41 pm
content: QuoteAssuming you want something that is modern and free, then you could look at QUCS studio.For many years, the 'free' option was to use RFSIM99.This is a fairly good linear simulator that is quite intuitive to use. However, this really old program will probably not install and run under Win10 or Win11 without a fair bit of fiddling.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 06, 2024, 12:56:29 am
content: Thank you, Joel -Mention of ADS reminds me of my attempt to learn ADS given a 30-day eval license from Keysight (some years ago). The no-mentor learning curve was steep enough that I wasn't able get started before the license expired.G0HZU - Thank you for pointer to QUCS. My current task (seeing if I can simulate what I'm seeing in real life) provides a simple (enough) use case from which to learn QUCS - hopefully there is a user community forum that can help ease the start-up cycle!(Both) - Inclusion of parasitics in the SMD packages I'm currently using (0603) seems that it would be within the realm of QUCS, particularly if I can define a 2-pin component as an inductor-with-parasitics, capacitor-with-parasitics, or similar.Thanks again for the very helpful replies and advice!

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 06, 2024, 01:25:31 pm
content: Hello G0HZU et al.,Thank you for all of the kind assistance! As of today (6 July 2024) I have a successful match to the SAW filter.The key to the solution was use of the Golledge version of the TA0245A SAW filter (their part number MA05254) which provided an equivalent representation of the two ports, then use of Analog Devices RF impedance matching calculator which yielded the following matching circuit.Center of response insertion loss is now about 4 dB (better than spec); at the upper roll-off position I'll be using it tracks the datasheet response within a dB or so.I've since downloaded QUCS and will explore that tool, as this task provides a good/simple task to use as a learning project.Thank you again for the great help!Dave

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 07, 2024, 12:23:04 pm
content: Do you mean you procured the Golledge version, or you just used the Golledge datasheet to match the Tai-Saw part?Either way, it's good you seem to have something working, but these datasheets are very strange... especially the Golledge one. Under "Note 2" suggests that 210ohm || 11pF is what is seen at the ports when the other port is terminated with 50 ohms. Once proper matching networks are implemented, the input impedance would change to something else. This means that a matching network designed to match 210ohm||11pF will not give a match on both ports.Stranger still is the "MEASUREMENT CIRCUIT" section, in which they use 160ohm resistors and some unspecified inductance as some sort of matching network. I'm have no idea why they would characterize the device like this.Usually to analytically design the matching networks you would start with all the S parameters (including S21/S12). Unfortunately the datasheets don't provide that info.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 07, 2024, 12:58:37 pm
content: Thank you, mtweig - good question!My experiment is from parts procured recently (2024) from asian marketplace seller (aliexpress) that were represented with Tai Saw part number, not Gollege part number. I cannot determine if the parts I received are authentic, but I believe they are as they are working properly once the Golledge datasheet equivalent circuit was used to generate a matching circuit.By the way, my application frequency is 144 MHz, so I am using upper roll-off, which may create some variation unit-to-unit, but this is not commercial production, so the ~8 dB insertion loss I am seeing is acceptable and my application tolerates slightly more loss there (but not the >13 dB loss I was getting using matching derived from the Tai Saw datasheet equivalent circuit, which is why I posted this question in the first place.)The Tai Saw datasheet is dated 2004 (20 years ago). The Golledge MA05254 datasheet has no date information but does have markings citing both V1 and Rev 2, and includes the same Tai Saw part number.

author: G0HZU
author_rank: Super Contributor
timestamp: July 07, 2024, 01:10:12 pm
content: The strange looking test fixture isn't meant to be an application circuit, it's just a crude way to get a 210 ohm test fixture.I think the reason they use 160 ohm series resistors in the test jig is because 160 ohms + 50 ohms = 210 ohms.You can see they also suggest using two series 160 ohm resistors to 'normalise' the 50 ohm test fixture to a 210 ohm system at a 0dB level. Then you presumably insert the SAW filter and then adjust the shunt inductances to get the best frequency response. The shunt inductance will cancel out the 11pF shunt capacitance at each port of the SAW filter? This lets you measure the frequency response and the insertion loss of the SAW filter.It may be the case that both the 160 ohm resistors and the shunt inductances were initially adjustable and were optimised for the best response from the SAW filter and because 160 ohms gave the nicest frequency response in terms of passband (and overall match?) the filter was declared as being a 160 + 50 = 210 ohm filter.They probably didn't bother to quote the shunt inductance because it is implied by the 11pF given elsewhere in the datasheet. It was probably about 120nH. In this case the filter would then be declared as being 210 ohms || 11pF at 139MHz.

author: G0HZU
author_rank: Super Contributor
timestamp: July 07, 2024, 01:15:52 pm
content: If it helps, I've used the same type of test fixture (in my youth) to test unknown crystal filters. Use series adjustable resistors and either shunt caps or inductors to try and find out the optimal matching for the filter.The modern way to do it would be with a full two port VNA. I've done this many times with crystal filters. The VNA produces a two port data file (s11 s21 s12 s22) for the filter and this can then be analysed and matched in a simulator.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 07, 2024, 06:40:53 pm
content: Quote from: G0HZU on July 07, 2024, 01:10:12 pmHard to not see it as an application circuit if all the datasheet specs are based on it, and they don't provide any other design, or enough data to design an alternative.QuoteSounds like you're suggesting this is somewhat similar to how datasheets sometimes specify output matching conditions for PAs. That is, the networks don't provide a conjugate match but are still found to be optimal, in this case with regards to the filter transfer function. And thus the impedances shown on the datasheet are not necessarily the actual impedances looking into the ports, but rather the conjugate impedance of what the ports should be terminated with (meaning you will not have a conjugate match). I really dislike such roundabout means of characterizing devices personally... basically lying to the engineer in a way that may lead them to a working design.I wonder what the transfer function would look like if you made the matching networks from just LC networks which presented the same impedance to the device. Passband insertion loss would certainly be better, but who knows what would happen elsewhere... could be that they suggest lossy matching networks for a reason.I'm curious what a direct measurement of the device with a VNA would show...

author: G0HZU
author_rank: Super Contributor
timestamp: July 07, 2024, 07:53:34 pm
content: If you look at the datasheet this is described in upper case as ameasurement circuitand not an application circuit. It looks to me that this was the method used to determine that the filter is 210R || 11pF.That's why the other section shows the two series 160R resistors. This is nothing strange. This is there to initially normalise the test fixture to 0dB when using a 50R evaluation system. It's a classic (old) way to evaluate a narrowband filter that doesn't have a 50R design impedance. I've used this same method many times.With the 160R resistors inline I'd expect the insertion loss to be about 12.5dB plus the 5dB typical insertion loss of the filter (17.5dB) but if the test fixture is first normalised to 0dB (as indicated in the datasheet) the insertion loss might be about 5dB as indicated on the datasheet. The 12.5dB gets normalised out during the through calibration.Of course, the datasheet could still have errors, and the SAW filter might not have this impedance, but I don't see anything strange about the measurement method they used. Normally this method is used below about 50MHz to look at narrowband filters, but it should still be fine at 139MHz as long as suitable 160R resistors are used.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 08, 2024, 05:32:10 am
content: Quote from: G0HZU on July 07, 2024, 07:53:34 pmWhy would one add such external components when trying to measure the network parameters of a passive DUT? Especially resistors which will only increase measurement errors after disembedding the DUT. The only reason I can see is if it's actually the recommended implementation of the component (which is what I would call an application circuit). Just to be clear, do you think they're suggesting the user implement the "measurement" circuit? If not, what help is it?

author: ftg
author_rank: Frequent Contributor
timestamp: July 08, 2024, 07:42:34 am
content: Quote from: mtwieg on July 08, 2024, 05:32:10 amSo that the design engineer can see how the numbers presented were found out and if desired, replicate them.MOSFET datasheets also seem to often have some simplified schematics for the test setups used.

author: tszaboo
author_rank: Super Contributor
timestamp: July 08, 2024, 10:28:13 am
content: Quote from: G0HZU on July 07, 2024, 01:15:52 pmI was going to ask why this method is not used. VNA has a 50 Ohm output/input, the system is presumably 210 Ohm, so the reflections would be quite a lot. Then all the S parameters can be determined, and the system 50 Ohm matched. I don't know how wideband the matching needs to be, but I would guess that with L and C it would be possible to get lower insertion loss (S12) than with a resistor at the filter's frequency. If the filter is designed to handle that much power of course. Am I thinking about this right?

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 08, 2024, 11:26:31 am
content: Quote from: ftg on July 08, 2024, 07:42:34 amI'm familiar with what you're talking about wrt MOSFET datasheets. In those cases the extra components facilitate the measurements. Adding series resistance on the ports of a passive DUT does not facilitate measuring its network parameters. Only makes sense if those same resistors are actually planned to be part of the implementation, in which case they effectively get lumped into the DUT. But it's not clear if that's the intent in the case of these SAW filters.Quote from: tszaboo on July 08, 2024, 10:28:13 amAnd with the 160ohm resistors added the reflections will be even greater... definitely not "matched" in any conventional sense.QuoteRight, but it's possible that using an LC network for the "matching" might give undesirable behavior further from the passband. The datasheets don't give enough information to tell.I'm guessing a lot of these questions would be cleared up by the standard referred to in the datasheets (JIS C 6703).

author: ftg
author_rank: Frequent Contributor
timestamp: July 08, 2024, 11:34:14 am
content: Quote from: mtwieg on July 08, 2024, 11:26:31 amThat setup allows one to measure the filter passband width and overal shape.The test circuit with the resistors is not for measuring the S-parameters, it is for the filter passband.It's an easy way to determine the 3dB and 20dB bandwidths, before involving any other matching that will have a bandwidth of it's own.This way the presented passband is independent of the exact models of caps and inductors later used to implement the matching circuitry.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 08, 2024, 11:56:55 am
content: Quote from: ftg on July 08, 2024, 11:34:14 amYou seem to be claiming that the DUT has some inherent passband characteristic which is independent from its port terminations. The only way that's true is if you know the DUT's full S parameters (or Z parameters, etc), which could then be used to solve for its transfer function with other port terminations.QuoteIf the port terminations can affect the filter characteristics significantly, then why do you think they chose to characterize it with that circuit, as opposed to connecting it directly to a VNA?

author: ftg
author_rank: Frequent Contributor
timestamp: July 08, 2024, 12:30:49 pm
content: Quote from: mtwieg on July 08, 2024, 11:56:55 amI'm not claiming that the passband is somehow independent of the port terminations.What I do claim is that the resistive matching is an easy way to get that termination.It is a much more easily repeatable measurement than some LC matching circuit.It is also something that can be done to verify that you are getting the right passband shape with your matching circuit.So, for sanity checking and verification, not production use.And it does not require posessing a VNA, a spectrum analyzer with a tracking generator would have been enough.I do agree that proper two port S-parameters would be the way to go, but those seem infuriatingly rare for SAW filters.Especially for the SAW filters I would have required them for.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 08, 2024, 12:50:49 pm
content: Quote from: ftg on July 08, 2024, 12:30:49 pmOk, thanks.QuoteSorry, I just don't understand what you mean here. "That termination" refers to...? An optimal termination other than what's shown in the measurement circuit?QuoteNot sure what you mean by "repeatable" here. Making the resistors 10K would probably make the raw measurement more repeatable, but the disembedded measurement of the DUT would not be repeatable, and therefore not useful.If a device has some inherent issue with "repeatability" then that's a fundamental issue which one shouldn't willfully mask, IMO.QuoteAgain sorry, I don't understand... what do you refer to as "your matching circuit"? Wouldn't the "measurement circuit" simply supplant it?

author: G0HZU
author_rank: Super Contributor
timestamp: July 08, 2024, 06:57:19 pm
content: I suspect that the original datasheet was released in paper form in a databook about 20 years ago. At some point it was transferred to (text searchable) PDF and if you look closely at the attached datasheet in post #1 it has inconsistent fonts and some obvious errors. This often happens when the transfer from scanned paper to (text searchable) PDF isn't very successful and all kinds of strange errors can creep in.It may be the case that someone used the resistive jig technique to reverse engineer the correct Rp and Cp for this filter and this was then added to the amended datasheet supplied by Golledge.Also, if you look closely at the plots in both datasheets, the noise floor response is exactly the same for both. So clearly the same plot can't be derived from both measurement techniques as the exact same plots from the original datasheet are used in the Golledge datasheet.If you reverse engineer the original L matches shown in the original datasheet they look to be about right for about 250R || 13pF at 130MHz.The resistive test fixture in the Golledge datasheet obviously isn't meant to be an application circuit, but it will be fine to use it if you want to. It won't be matched to 50R and the insertion loss is going to be about 17.5dB though...

author: G0HZU
author_rank: Super Contributor
timestamp: July 08, 2024, 08:44:56 pm
content: Here's a quick youtube demo I put together showing the resistor method in action. You can see that there is also a normaliser network used, just as in the datasheet for the SAW filter. In this case the normaliser is between ports 3 and 4.The software is able to auto normalise by computing s21 - s43 and plotting this with the blue trace.With the normaliser, you can predict the true insertion loss of the filter when it is matched. In this case the blue trace shows 2.04dB.Once the R and C is optimised for best match (960R and 23pF) the video then shows the response when tuned with an adjustable complex source. The passband response and insertion loss are virtually identical.So the series resistor method is a valid way to estimate the Rp and Cp (or Lp) of a narrowband filter.

author: G0HZU
author_rank: Super Contributor
timestamp: July 08, 2024, 09:08:57 pm
content: I believe that the above KVG 9MHz LSB filters were designed to be 1000 ohm devices and a small amount of external capacitance is required to match them properly.960 + 50 = 1010 ohms which is very close!The filter I have here is the same as the one tested by someone else in the link below:https://awsh.org/80m_superhet_receiver/By using a couple of trimmer resistors and a couple of trimmer caps it should be possible to accurately reverse engineer the optimal Rp and Cp required for this filter using this classic method. There is no need to use a VNA, just something with a 50R generator and some sort of tracking receiver with enable the operator to quickly establish that this is a 1000R filter and there also needs to be about 23pF added in shunt to it.If this is done correctly, you can see how flat the passband can be. There's hardly any ripple at all. This is a great way to find out how good the passband response can be if you reverse engineer the correct combination of Rp and Cp for the filter. Sometimes Cp is negative and therefore a shunt inductance is needed for some filter types.

author: tszaboo
author_rank: Super Contributor
timestamp: July 08, 2024, 09:30:34 pm
content: Quote from: ftg on July 08, 2024, 12:30:49 pmNo doubt, resistive matching is indeed the easiest to achieve up to a few GHz. It's also lossy, which might or might not be a problem.I'm guessing if the matching is a Pi or similar made from LC, then there really only ever going to be one frequency where the filter is matched*. Which again, might or might not be an issue.As I understand, SAW filters are always bandpass, and narrow.I mean if the filter isn't matched and the signal bounces back from it's input as opposed to it disappearing in the filter, if it doesn't blow up your source...*Maybe more above the self resonant points for example, that's beside my point.

author: G0HZU
author_rank: Super Contributor
timestamp: July 08, 2024, 09:42:15 pm
content: It doesn't really matter that the resistive method is so very lossy because the aim of the resistive jig is to tell you the correct insertion loss and passband response for the filter once you normalise the test jig using the two 160R resistors in series between the two 50R ports.It also indicates the correct Rp and Cp for the filter if you then want to design a suitable low loss matching network for the filter.What you end up with isn't meant to be a practical application circuit. It is just a means to an end. Or, in other words, a stepping stone towards designing a low loss matching network if you don't know what the correct Rp and Cp of the filter is. Using the resistors and trimmers you can work out Rp and Cp and then progress to designing a suitable matching network.This technique is many decades old, and predates vector network analysers. Back in those days, some people would have used this method using a 50R signal generator and a 50R receiver and a pair of series trimmer resistors and a pair of shunt trimmer inductors (or trimmer capacitors). It would have taken a long time, but they would have little choice back then. If done correctly, they would be able to estimate the ideal Rp and Cp for the filter quite well.

author: tszaboo
author_rank: Super Contributor
timestamp: July 08, 2024, 09:52:27 pm
content: Quote from: G0HZU on July 08, 2024, 09:42:15 pmRight, so you match it to 50 Ohm to measure it. You don't match it this way in the application circuit.I'm familiar with the SWR meter based matching method, even though I never did it myself.

author: G0HZU
author_rank: Super Contributor
timestamp: July 08, 2024, 09:55:26 pm
content: Here's a link to someone who has used the resistor method the old school way using a sig gen and a Hi Z (low capacitance) probe.https://www.qsl.net/g3oou/filtertestjig.htmlThe aim here is to estimate the ideal termination impedance for the filter.A shunt resistor and shunt cap is used at the output because a High Z probe is used instead of a series trimmer followed by a 50R receiver.The important conclusion at the end is quoted below:Quote

author: ftg
author_rank: Frequent Contributor
timestamp: July 09, 2024, 06:53:00 am
content: Now that I really think about this and properly look at both Tai-Saw TA0245A and Golledge MA05254 datasheets, it feels like they are completely separate parts and that the Golledge part is offered as a functional replacement for the Tai-Saw TA0245A, down to the 6.5dB insertion loss.That would explain why the lossy resistive matching is presented instead of some lumped element circuit.But G0HZU's point about it potentially just being a copy paste job or a bad scan also sounds plausible.When one compares the charts and the package drawing the jpeg artefacts look the same on both datasheets.At least in these two:Golledge MA05254https://www.golledge.com/media/1803/ma05254.pdfTai-Saw TA0245A:https://www.cdiweb.com/datasheets/tai-saw/ta0245a%20_rev.2.0_.pdf

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 09, 2024, 12:32:05 pm
content: Quote from: G0HZU on July 08, 2024, 08:44:56 pmFirst of all I really appreciate the effort you put into this.Just want to check my understanding of your video.1. You're adjusting Rmatch and Cmatch with the objective of getting a good S21 "shape", while ignoring S11, S22, and overall insertion loss.2. This does not tell you anything about the "input impedance" of the DUT. But it tells you the "optimal" terminating impedances of the DUT (call it Zopt, in your case around 361 - j487 ohms).3. You would then devise impedance transformers to transform whatever impedance(s) are facing the DUT (50ohms or whatever) into Zopt. If those those impedance transformers are lossless you will get an overall insertion loss (shown in your video as S56) much better than what was seen with Rmatch/Cmatch (S21 in your video). But the return loss could also be very poor on both sides.4. On the datasheet, you would then say that the "input impedance" of the DUT is the conjugate of Zopt, assuming that this will lead designers to make matching networks presenting Zopt to the DUT.Is that correct?

author: G0HZU
author_rank: Super Contributor
timestamp: July 09, 2024, 10:07:39 pm
content: QuoteYes, but I am also looking at insertion loss via the normalised trace in the simulation. However, in a typical setup, this wouldn't be possible so the aim is to get the best S21 shape across the passband with minimum passband ripple.If this is done correctly, it should also coincide with the best overall s11 shape across the passband even though it can't (yet) be measured.QuoteYes, it lets me know Zopt. If you compare 361 - j487 and 1010R || 23pF at 9MHz they are essentially equivalent.QuoteI'm not sure what you mean here. I quickly added an L match to the input and output as in the image below and you can see that the passband is still essentially identical with 2dB loss. The traces overlay almost perfectly. I've also added s55 which is the input of the L match. This is as good as it gets with this crystal filter. It isn't possible to get better than 11dB return loss across the whole passband and this is because this filter was designed for this return loss. In other words, no matter what I do with the L match values, I can't get a nicer return loss plot that what you see in the plot below.QuoteYes, that's it. Obviously, this technique isn't as elegant or as powerful as using a VNA, but it does usually give accurate results for Zopt

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 10, 2024, 01:02:04 pm
content: Quote from: G0HZU on July 09, 2024, 10:07:39 pmRight, this is what I meant by "impedance transformers to transform whatever impedance(s) are facing the DUT (50ohms or whatever) into Zopt". Produces the same response as S56 in your video (around the passband, anyways).QuoteI don't see the S55 trace in your screenshot, but I think I get what you're saying. 11dB isn't that bad, actually. One could probably get it much lower at specific frequencies, but not across the entire passband. And trying to do so would make the S21 ripple terrible.QuoteTo be clear, I'm not saying the method isn't useful (especially after having been walked through it). My issue the way the results are communicated in the datasheet. They could just say "present the ports with this Zopt". But instead they say "the input impedance is Zopt*" which is simply untrue and will lead to confusion for engineers who notice that.

author: G0HZU
author_rank: Super Contributor
timestamp: July 12, 2024, 07:55:30 pm
content: QuoteWhoops, sorry, the S55 ident is missing off the legends at the bottom. This sometimes happens if I set the fonts too large and it pops off the graph area. The S55 trace is the pale blue trace (trace d) in the plot and it has the 1d marker pip on it at -11.06dB.If I try and retune anything, this S55 trace loses symmetry and so it is tuned to the best result I can get.Note that the scale for S55 is the light blue scale on the right side of the graph at 5dB/div.