Test points/connectors for matching - Page 1
Forum section: electronics / rf-microwave

author: okw
author_rank: Frequent Contributor
timestamp: May 15, 2023, 12:46:03 pm
content: I am reading and reading about matching circuits between equipment and antenna, but can't say I become more confident. Maybe some of you gurus here can help me?I want to match a GPS module with an active antenna, with a SAW filter (SAFFB1G56KB0F0A) in-between them. I already had a few prototype boards made, and have a big service kit with both capacitors (0402) and inductors (0402), so I'm ready to experiment. In my prototype design, I put a pi-filter between the SAW filter and receiver, and a combo pi/T filter between the SAW and antenna (ANT1 IPEX connector). Don't mind the L/C choice and values, they are just all 0402 placeholders (I didn't know if pi or T makes most sense for a GPS, and with which base/starting values).My question is, where do you guys place test points, connectors, etc. for matching? Both for prototype and production versions?And with RF switches, which orientation (to break the circuit) is correct at which placement?I placed two RF test switches (MM8030-2610) along with an IPEX connector (ANT2) near the receiver RF input. It's the first time I've used RF test switches, and they seem difficult to use without the appropriate probes ($$$).I tried with my PS100 Antenna analyzer in the ANT2 connector (between receiver and SAW filter), and measured S11 with a really nice dip. -23.3dB @ 1512MHz, sadly not at the GPS frequency (1575MHz) where I read only -1.8dB. Only C1 (33pF) was in in place. I added L1/L2 = 2.2nH, and this moved the center a bit, now it's -40dB @ 1512MHz, -1.89@1575MHz.First, would the PS100 be appropriate to measure like this (not antenna as end-point, but rather in the middle of to devices)? And secondly, is this point good for measuring between the saw and receiver? What about between the SAW and antenna?Should I also be worried about the output level of the PS100, considering the GPS receiver is probably very sensitive (not blowing the input stage or something)?I'm considering buying a NanoVNA, or even an old lab analyzer, e.g. HP 8753.Any help is greatly appreciated.

author: yl3akb
author_rank: Regular Contributor
timestamp: May 16, 2023, 05:26:27 pm
content: You should reduce the length of Your RF cascade to find where the offset in resonance frequency originates.1. Measure only the antenna + bias-T. Did You power up the active antenna when measuring?2. Measure only the SAW filter, but with 47..50Ohm resistor at its other side (Resistor at antenna side for example in place of L3 and with C2 open).That SAW filter should not need additional matching to 50 Ohms.That ANT2 connector and 'GPS MODULE RF IN' label is confusing. Is there receiver connected in parallell with ANT2 connector?That antenna analyzer should be okay for reflection coefficient measurement like this.

author: okw
author_rank: Frequent Contributor
timestamp: May 16, 2023, 06:49:39 pm
content: Thanks for your reply.Quote from: yl3akb on May 16, 2023, 05:26:27 pmKeep in mind I'm completely new to thisHow do I reduce it?Quote from: yl3akb on May 16, 2023, 05:26:27 pmBias T, would that be to remove everything except C2/C3/L4 (and L5/C4 to power the antenna of course)?Quote from: yl3akb on May 16, 2023, 05:26:27 pmI didn't. I'll do new measurements with power.Should I change values, remove, or replace any components first?I have 10 fresh boards with C1/C2/C3 (33pF), L5 (68nH - not 47 like the image) and saw filter assembled. Everything else, including the gps module, isn't assembled yet (I have full RCL 0402 service kits).Quote from: yl3akb on May 16, 2023, 05:26:27 pmThis would be to measure in the ANT2 (left most) connector? Or solder into any other point?Quote from: yl3akb on May 16, 2023, 05:26:27 pmThe datasheet says it's unbalanced. Doesn't that mean I need to match it to 50ohms? On both sides? Or isn't a saw filter looked at like a device (like an LNA, which needs matching on both sides(?).Quote from: yl3akb on May 16, 2023, 05:26:27 pmYes, I know. GPS MODULE RF IN indicates the receiver input (arrow should be oriented the other way). ANT2 connector was only placed there to have a snap-in connector for the antenna analyzer at that point.

author: yl3akb
author_rank: Regular Contributor
timestamp: May 17, 2023, 02:20:23 pm
content: Quote from: okw on May 16, 2023, 06:49:39 pmBy reducing I mean first verify each sub-block separately and then measure the whole cascadeQuote from: okw on May 16, 2023, 06:49:39 pmYes, leave C2, C3 and L5,C4, but leave L4 openQuote from: okw on May 16, 2023, 06:49:39 pmI think first measure just the antenna with power applied through bias-T. Do not connect shunt inductors.Quote from: okw on May 16, 2023, 06:49:39 pmYes, measure at ANT2, but without GPS module connection of course.Quote from: okw on May 16, 2023, 06:49:39 pmThe unbalanced is what You need here, it doest mean it needs matching. See 'measurement circuit' in datasheet, it says 50 Ohms. Also VSWR is stated ~1.4 which corresponds to good enough match.Quote from: okw on May 16, 2023, 06:49:39 pmIn general I would not connect RF connector parallel to transmission line like this, because it can act as open stub and cause mismatch. Probably at this frequency it does not matter, but depends how long is the stub.

author: okw
author_rank: Frequent Contributor
timestamp: May 30, 2023, 06:20:23 pm
content: Thanks!Are the RF test switches (MM8030-2610) necessary for testing in mass production?And where should I put them?

author: okw
author_rank: Frequent Contributor
timestamp: June 03, 2023, 10:52:32 am
content: Bump...

author: mayor
author_rank: Regular Contributor
timestamp: June 15, 2023, 10:52:04 am
content: Posting this link for posterity. I've found it really helpful to match antennas:https://www.ti.com/lit/an/swra726/swra726.pdf

author: Kean
author_rank: Unknown
timestamp: June 15, 2023, 11:12:48 am
content: Quote from: okw on May 30, 2023, 06:20:23 pmThis will depend on whether you need to do radio calibration as part of the manufacturing process, and if your CM can support that.I don't believe I've ever seen them used on GPS or Bluetooth designs which are receive only and low power respectively. They are more commonly used on higher power radios like WiFi to ensure they are calibrated to meet regulations.

author: okw
author_rank: Frequent Contributor
timestamp: June 15, 2023, 01:12:24 pm
content: Quote from: Kean on June 15, 2023, 11:12:48 amOh, I see.Maybe I should put one in the LTE path? We have band selection depending on where in the world you are. So:LTE radio <-> RF switch <-> 6 different matching networks <-> RF switch <-> matching for antenna <-> antenna.Where would I put the test switch?

author: Kean
author_rank: Unknown
timestamp: June 15, 2023, 08:01:51 pm
content: Quote from: okw on June 15, 2023, 01:12:24 pmThat depends on what you want to test.I'd assume the LTE radio is a module and thus has already been factory tested and calibrated. So, if you want to verify the antenna and matching circuits, you would add a test switch to allow probing and measurement of the circuity you have added after the LTE radio. You would use a VNA and via software select the various matching networks to verify each position of the RF switch measures as expected.The next question is what is the effort involved in doing this, and what are the testing requirements? In other words, if implementing this and running the test adds $100 cost to each device (sum of NRE and CM time), is there a positive value to doing this? What is the disadvantage to having the occasional faulty device slip through to the end user? Is there another way to test it without probing?So far I've avoided adding 3G/4G/LTE to any of my PCB designs, and have just used separate radios connected via UART or USB. Maybe someone here has a different experience to me, but I've only seen the RF test switches on PCBs using bare radio silicon, and not when there is a module. I can see though that there might be a case like you describe where there is a fair amount of additional RF circuitry being added and a high testing requirement.

author: tszaboo
author_rank: Super Contributor
timestamp: June 15, 2023, 08:18:59 pm
content: Hey. Your schematic looks OK for first try. Maybe you will need an actual resistor on the power partinstead of 0R.https://www.megiq.com/rf-measurements/rf-measuring-toolsRead through the guides on this link, I think it answers a lot of your questions. Depending on your antenna, you could get free matching service from your supplier. I know at least 3 supplier who does this, contact them. You send them a board, and they send you a BOM of components. Matching LTE is not trivial, the frequency bands are wide, far apart. There is no shame in getting help for that.I don't think you should even consider production testing every board, if you never matched a single antenna. It can be a functional test, with RSSI values read from the RF parts. The circuit is guaranteed by design, and purchase, you are only looking for soldering issues here.

author: Kean
author_rank: Unknown
timestamp: June 16, 2023, 01:24:30 am
content: Quote from: tszaboo on June 15, 2023, 08:18:59 pmA much more concise answer than mine.