VSWR Measurement - Page 1
Forum section: electronics / testgear

author: yanir
author_rank: Regular Contributor
timestamp: November 02, 2014, 07:40:24 pm
content: Hi, I'm designing some boards with chip antennas, and I would like a test setup to determine basic system performance and match transmission lines to the antenna (using pi network and the like).After doing some reading I have found that a spectrum analyzer and a few accessories can be used to accomplish this (VSWR measurment). Can anyone comment on the what exactly what I would need and how to perform the test (or point me to a good resource)? It would be greatly appreciated.If you think there are other more appropriate tests I'd like know that too. I'm an RF newbie.Thanks!

author: alex.forencich
author_rank: Frequent Contributor
timestamp: November 02, 2014, 09:10:00 pm
content: Well, there are probably several different methods for doing the same thing, but basically the idea is to measure the reflected signal coming back from the antenna and then adjusting the matching network to minimize this. A VNA is the correct tool as it will tell you the magnitude and phase of the reflection, but you can get some insight with a spectrum analyzer with tracking generator (ideal) or external RF source (works, but it is tedious) and a good directional coupler. The basic idea is to attach a coax to your board where the RF signal would be driven from (e.g. power amp output or LNA input) and connect this cable to the directional coupler. Then the RF source and spectrum analyzer are connected such that the RF source signal passes through the coupler to the board with minimum attenuation, and any signals coming back from the board get coupled back to the spectrum analyzer. Then you can sweep analyzer and source to see what the frequency response looks like and start playing around with the matching network. The idea is to get the deepest dip you can at the transmit frequency. A VNA has the source and directional coupler built in, so it makes the measurement a lot easier. However, a VNA can only be used for single frequency measurements and it cannot be used to receive an external signal, so it serves a rather different purpose. The nice thing about a VNA is if you set it up correctly, it can simply measure the impedance of the antenna input and then you can just calculate the required matching network components directly and put it together without much screwing around.

author: Howardlong
author_rank: Super Contributor
timestamp: November 02, 2014, 09:49:37 pm
content: Quote from: alex.forencich on November 02, 2014, 09:10:00 pmNot sure if you meant this?You can also get away with an RF signal generator plus directional coupler in conjunction with either an RF power meter or a crystal detector with AC voltmeter (or scope with measurement capability).The RF signal generator, modulated with AM, directional coupler, crsytal detector and AC voltmeter was the way I started. It takes patience, but you can achieve reasonable results to measure return loss.If you use the crystal detector, you should precede it with a 20dB pad or so so as not to upset the directionality and accuracy of the directional coupler.

author: yanir
author_rank: Regular Contributor
timestamp: November 02, 2014, 09:50:42 pm
content: Thanks Alex, I don't have a large budget so I'm looking to get the most versatile tool. A spectrum analyzer seems like the right way to go right now, since it has other uses.Thanks for the detailed explanation. I need to do some more reading. I'll probably have more questions soon.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: November 02, 2014, 10:35:14 pm
content: Quote from: Howardlong on November 02, 2014, 09:49:37 pmYes - a VNA can only measure one frequency at a time. i.e., you cannot use a VNA to look at mixing products, distortion, intermodulation, etc. A VNA consists of a swept RF source and a multichannel downconverter. The multichannel downconverter uses the same local oscillator for all of the inputs, so it is not possible to look at different frequencies on different ports. Since the source is received through the reference port, it is also not possible to source one frequency and analyze a different frequency. Hence single-frequency. It can sweep that frequency, but you only get one at a time. A spectrum analyzer does not have this constraint, but then you lose the insight into phase. A spectrum analyzer with tracking generator can be used as a scalar network analyzer since it can give you amplitude information but not phase. Also, looking at phase only makes sense with mathematically interrelated frequencies. The phase difference between 2.4 GHz and 2.5 GHz is not really meaningful as it varies with time due to the frequency difference anyway. Like I said, it is a different instrument with different uses. In this case, it is the right tool for the job. But there are probably instances where a spectrum analyzer is required, so if you need to chose one over the other, a spectrum analyzer may be a better general-purpose instrument.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: November 02, 2014, 10:53:22 pm
content: I bought one of these when they were on special for about $50.https://estore.ti.com/MSP-SA430-SUB1GHZ-Sub-1-GHZ-RF-Spectrum-Analyzer-Tool-P2675.aspxIt's little more than a toy, but it actually does give useful information for the < 1 ghz bands.What frequency? I guess you are higher than this if you are using chip antennas.What about a wireless router with some flavour of openWRT, they probably have firmware that could provide you with a similar uncalibrated Spectrum Analyser, which will be fine for relative measurements.With my SA it I could see my pcb antennas were only a few db down on bought ones.

author: Howardlong
author_rank: Super Contributor
timestamp: November 02, 2014, 11:06:14 pm
content: Quote from: alex.forencich on November 02, 2014, 10:35:14 pmOK, understood, the way I read it in relation to the OP was that you were suggesting that the VNA didn't sweep. I'm not sure given the OP's question that they are interested in IM distortion or products, but it's still worthy of comment.

author: VK5RC
author_rank: Unknown
timestamp: November 03, 2014, 12:04:07 am
content: A simple trick if using SA and tracking generator to get the sign of the reactance, is to look at how the impedance changes with frequency, if impedance goes up, with frequency the impedance has an inductive (+) reactance component , down is capacitive.

author: yanir
author_rank: Regular Contributor
timestamp: November 03, 2014, 01:44:50 am
content: Quote from: HackedFridgeMagnet on November 02, 2014, 10:53:22 pmYeah, 2.4Ghz (Wifi, Bluetooth). Also 1575.42 (GPS)Quote from: HackedFridgeMagnet on November 02, 2014, 10:53:22 pmI'm certainly open to that, I did some searching and found some things but nothing like a polished project ready to use. It also woudn't help me with GPS or other other frequencies.

author: Bud
author_rank: Super Contributor
timestamp: November 03, 2014, 01:55:39 am
content: Quote from: alex.forencich on November 02, 2014, 10:35:14 pmIt is irrelevant for what OP is trying to do, since antennas are linear devices.And that is not 100% true anyway, many VNAs including home brewed ones can do non-linear measurements, too.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: November 03, 2014, 02:11:33 am
content: Quote from: VK5RC on November 03, 2014, 12:04:07 amSo basically the sign of the reactance can be determined by the slope of the magnitude of the reflection wrt. frequency. That's good trick to know. I don't have a VNA at the moment, but I do have a spectrum analyzer with a built-in tracking gen and a wideband external tracking source (8593E + 85645A) and I was wondering if there was any way to get any sort of phase information out of that setup. It's not so straightforward as positive slope = inductive, though, as positive slope does not imply increasing impedance, only increasing mismatch. Minimum reflection occurs when the impedance is matched, and the reflected power increases as the match worsens. So positive slope would imply inductance if the load impedance is larger and capacitance if the load impedance is smaller. So now the question becomes, how do you determine if the load impedance is larger or smaller than the source impedance (iow, the sign of the reflection coefficient).

author: alex.forencich
author_rank: Frequent Contributor
timestamp: November 03, 2014, 02:23:31 am
content: Quote from: Bud on November 03, 2014, 01:55:39 amIt may be irrelevant for this particular use case, but if OP is looking to possibly procure a new piece of test equipment, then it helps to know what different pieces of equipment can do.Modern VNAs are starting to look like multi-port vector signal analyzers with built in signal sources with less internal coupling between the ports (i.e. more generic, more complex, and more capable RF front ends). These tools when put in the same box with a switch network and directional couplers can be used either as a VNA or as a spectrum analyzer with possibly coupled RF sources, so they can perform these types of measurements. But older VNAs cannot because the architecture does not support this. If OP is on a budget and can only afford old, used HP equipment, then a VNA from that era will most likely not support this. If money is no object, then buy a new, top-of-the-line VNA that does support this and you're good to go.

author: Bud
author_rank: Super Contributor
timestamp: November 03, 2014, 05:41:11 am
content: I am sorry but this is incorrect. 25 years old HP 8753 series VNA can measure gain compression, harmonics, IMD, mixers,

author: es
author_rank: Contributor
timestamp: November 03, 2014, 06:00:31 am
content: What about a vector reflectometer such as the Planar R54? I was wondering if anybody has an opinion on theses.I don’t know a lot about RF, but it seems to me a better tool for the task than a spectrum analyzer + directional coupler since it measures phase too.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: November 03, 2014, 07:01:19 am
content: Quote from: Bud on November 03, 2014, 05:41:11 amInteresting. From looking at the block diagram, it seems like that this would not be supported. But it seems like it is possible to decouple the source from the receiver in these units to look at a different frequency. I'm not sure if this is possible for the older 8510, though.

author: Howardlong
author_rank: Super Contributor
timestamp: November 03, 2014, 02:52:31 pm
content: Quote from: alex.forencich on November 03, 2014, 07:01:19 amIt's been a while since I last investigated this, but ISTR the 8753B and later allowed this, my old 8753A won't, the sweep and RX frequencies are always the same, so testing up- and down-converters directly isn't possible on the 8753A. Unless someone can educate me differently, that is.

author: yanir
author_rank: Regular Contributor
timestamp: November 03, 2014, 03:13:25 pm
content: Can a VNA be used as a basic spectrum analyzer? It's seems possible, but it is practical? It's starting to look like the right tool is a VNA, and I thought these were totally out of my budget, but some can be found for under 3K that match my needs (I think...). As an SA I want to measure the power output of my wifi and ble transmitters and see the center frequency. Seems like I'm not asking for much.The HP 8753D seems like a good tool for me that covers everything.Is seems I also get an RF signal generator out it too. Can this feature be used separately?

author: alex.forencich
author_rank: Frequent Contributor
timestamp: November 03, 2014, 07:14:33 pm
content: Quote from: yanir on November 03, 2014, 03:13:25 pmIt will work as an SA, but I think the dynamic range might be limited due to a high noise floor. Seehttp://www.keysight.com/owc_discussions/thread.jspa?messageID=101484&tstart=0. VNAs are just not designed to receive a signal from an external source, so the design will be traded off differently. The ability of the RF signal generator will depend on the specific unit. It seems like in the 8753 series, the generator can be offset from the receiver by an adjustable offset. I'm not sure if it can be used in CW mode while the receiver sweeps, this may not be possible.

author: Bud
author_rank: Super Contributor
timestamp: November 04, 2014, 06:14:56 am
content: I remember trying it on my 8753C but it was not worth it. It was slow, normalizing the signal to measure absolute power was difficult, and due to the nature of sampling receivers the trace showed pronounced Sin(x)/x artifacts. For spectrum analysis one should use a spectrum analyzer.

author: yanir
author_rank: Regular Contributor
timestamp: November 04, 2014, 04:44:28 pm
content: Quote from: Bud on November 04, 2014, 06:14:56 amMakes sense, I think I can squeeze both into the budget. Thanks for everyones assistance. I'll be sure to post some results.

author: yanir
author_rank: Regular Contributor
timestamp: November 11, 2014, 04:44:50 pm
content: So, I've decided to get an 8753D VNA and 8595E SA.After more continued research I see I need to perform a calibration before my test. The test I'm most interested in right now is S11 for antenna tuning on a 50Ohm transmission line (pcb). What specific items do I need to use for calibration? So far I think I need 1: the cable used for the test, and a calibration load. Is this right? What kind of load do I need?I plan on retrofitting an SMA connector on my DUT pcb.Thanks

author: XFDDesign
author_rank: Frequent Contributor
timestamp: November 11, 2014, 05:46:59 pm
content: If it's low-cost, and you're looking to figure out where the antenna rests, I would opt for a 6-db return loss bridge mated to an AD8302 Gain-phase detector (i.e. a VNA-on-a-chip).You can get a freebie sample from Analog, the resistors can be any old 1% 49.9 ohm part from mouser or digikey, and the device itself is a TSSOP so you can etch something at home.With off-the-shelf FR-4, just design the traces to about 130 mils width. Then it's just figuring out the RF source.Also, if you're a newbie, I strongly recommend getting a copy of this:http://tinyurl.com/keboa38and a copy of the ARRL Handbook if you don't already have one:http://tinyurl.com/mmqx8ue

author: jolshefsky
author_rank: Regular Contributor
timestamp: November 11, 2014, 05:51:44 pm
content: Pardon me for skimming thread since I just wanted to make sure I wasn't duplicating a post. w2aew posted a video on directional couplers and using it to measure reflections (and SWR) with a scope on YouTube:.

author: KJDS
author_rank: Super Contributor
timestamp: November 11, 2014, 05:55:55 pm
content: To calibrate a VNA for S11 measurements you need an open. short and load.The more accurate the cal standards the better your measurements will be. Known imperfections can be loaded into the analyzer to be taken account of during calibration. Adequate quality cal kits are readily available via ebay, but for professional use then you're looking at expensive used Agilent kits.

author: yanir
author_rank: Regular Contributor
timestamp: November 11, 2014, 06:01:05 pm
content: I've seen some cheap ones listed on eBay from hong kong for 40 each. Are those worth getting for my needs?

author: yanir
author_rank: Regular Contributor
timestamp: November 11, 2014, 06:48:41 pm
content: Quote from: XFDDesign on November 11, 2014, 05:46:59 pmVery interesting. For home experiments that sounds like fun. I usually prefer to buy my test gear vs build. I have enough trouble validating my own designs.Quote from: XFDDesign on November 11, 2014, 05:46:59 pmI definitely will. Thanks!

author: KJDS
author_rank: Super Contributor
timestamp: November 11, 2014, 07:08:31 pm
content: Quote from: yanir on November 11, 2014, 06:01:05 pmThat sounds a bit too cheapI'd be tempted withhttp://www.ebay.co.uk/itm/Low-Cost-SMA-Connector-Calibration-Kit-for-VNAs-/281360576053?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item418265ae35

author: yanir
author_rank: Regular Contributor
timestamp: November 11, 2014, 08:51:09 pm
content: Quote from: jolshefsky on November 11, 2014, 05:51:44 pmThanks for posting. Very interesting.I bit the bullet and got a VNA and SA today.

author: yanir
author_rank: Regular Contributor
timestamp: November 20, 2014, 05:23:34 pm
content: So, I got the VNA in a few days ago and have been attempting to characterize the return loss of a GPS chip antenna on a pcb I designed.Read a decent article I found on how to test such a setup, but they don't go into much detail on VNA setup, error correction for the cabling etc. I've attached a picture of the DUT PCB that shows the coax connected to the feed line on the antenna (trace is cut before the coax to eliminate reflection). I've also attached a pic of the VNA in log mag view.The results show the antenna is resonant at 1665.17Mhz. This is a 1575.42Mhz antenna. I started tuning the antenna with a 2.2pF shunt and settled on 2.7pF to achieve -34db shown in the pic.During the tuning process the frequency seemed to be stuck in the ~1665Mhz +/- 5Mhz. This leads me think that I have something wrong with my setup.Questions:1) Do I need to use port extension to eliminate cable delay? How to I measure the delay of the cable terminated on the pcb? Cable setup is 7mm to N, N to SMA (M) cable 24", SMA F cable (I made) cut to the board (~6", I haven't measured). I played a bit with the feature I didn't see any change in log mag view, but I did in the smith view. Seems to affect gain, but not impedance.2) I am still waiting on my Open, Short, Load kit. I ordered it from RF Shop (Thanks KJDS for pointing that out). Would not doing an OSL calibration produce siginificant enough error to produce the results I'm seeing?The VNA has be recently lab calibrated.

author: KJDS
author_rank: Super Contributor
timestamp: November 20, 2014, 06:13:36 pm
content: I'd do a crude OSL cal by removing R26 and any shunt before it for the open, then shorting to ground where the shunt is for a short, then soldering in a 51R resistor in shunt for the load. It won't be brilliant for a lot of reasons, but it will eliminate some of the effects that you're currently seeing.It's probably worthwhile improving the coax to microstrip transition as well. Get hold of the thinnest coax you can find to attach to the board and connect it so that there is the neatest connection you can manage. It will never be great with your current layout but at the moment it's a cause for concern.

author: Bud
author_rank: Super Contributor
timestamp: November 20, 2014, 06:14:09 pm
content: I do not think anyone is using a vna without calibrating the ports first. Factory calibration has little to do with port calibration. Factory calibration is all about internals of the VNA and lasts long term, whereas port calibration is setup specific and may be performed dozens times a day.I would recommend a couple things:-get yourself a APC to SMA adapter(s), you do not want another N adapter in the middle-do not use commodity coax cables with polyethylene dielectric, get yourself semi-rigid 0.08 SMA terminated cables, easy to work with, not bulky and is much more stable when bent.For this setup you can use port extensions feature. First calibrate the VNA with OSL at the port. Then connect your setup. Solder a small short jumper at the far end of the cable where it connects to the antenna. Switch VNA to display Phase. Turn on port extension for Test Port. You will see the slopped phase line. Now rotate the dial until the phase line becomes horizontal as best as possible. This means you moved your calibration plane to the shorted far end of the cable. Now remove the jumper and make your measurements - switch the display to whatever mode you need.

author: Co6aka
author_rank: Unknown
timestamp: November 20, 2014, 06:43:59 pm
content: Quote from: XFDDesign on November 11, 2014, 05:46:59 pmFWIW, an old-school trick is using two 100 Ohm resistors in parallel. Another old-school trick is using three 150 Ohm resistors, and making one switchable for 50/75 Ohm terminations.49.9 Ohms is pretty darn close though, and your margin of error will most likely be more than 0.1 Ohm!Just to be a purist ("an RF-psychopath") I've got reels of 100 and 150 Ohm 0.1% resistors for making precise loads; I bin and pair up two that give me 50.000 or 75.000 Ohms! O0And to redeem myself, I've got reels of 49.9 and 75 Ohm 1% resistors too. But then, I've also got several other values for making 1.25:1, 1.5:1, 1.75:1, 2:1, 3:1, etc terminations.(I also have resistors for making 0:1 and infinity:1 terminations, so maybe I really am a RF-psycho...?)

author: yanir
author_rank: Regular Contributor
timestamp: November 20, 2014, 06:44:29 pm
content: Quote from: Bud on November 20, 2014, 06:14:09 pmI've been buying these adapters and cables from pasternack. Here is a link to what you recommend:http://www.pasternack.com/sma-female-7mm-sexless-straight-adapter-pe9214-p.aspxI assume I should stick with precision adapters to eliminate as much error as possible, but it's pricey. Do you know of any better sources?Quote from: Bud on November 20, 2014, 06:14:09 pmWill do.Quote from: Bud on November 20, 2014, 06:14:09 pmThanks for this explanation. I'm a little confused about the jumper. Short to ground?

author: yanir
author_rank: Regular Contributor
timestamp: November 20, 2014, 06:52:14 pm
content: Quote from: KJDS on November 20, 2014, 06:13:36 pmNice idea, I'll see if I have the components.Quote from: KJDS on November 20, 2014, 06:13:36 pmI got this layout from here:http://www.lsr.com/news/antenna-matching-within-an-enclosure-part-2Pic also attached.Upon review the LSR pic is much neater. It is a bit difficult attaching the shield to ground due to the layout. What do you think is biggest issue?

author: Bud
author_rank: Super Contributor
timestamp: November 20, 2014, 07:49:05 pm
content: Quote from: yanir on November 20, 2014, 06:44:29 pmYes from the coax center conductor to coax shield where it is soldered to ground. You want a short and flat shunt to minimize inductance, i use short strips of copper foil.

author: yanir
author_rank: Regular Contributor
timestamp: November 20, 2014, 08:12:37 pm
content: Cool, I'll give that a try.

author: cncjerry
author_rank: Unknown
timestamp: November 20, 2014, 09:07:24 pm
content: I don't know how far you are with this project, but if you haven't, take a look at the Analog Devices ADL5519. I'm building around that chip, up to 10Ghz. From my testing I get really flat performance so far up to 500Mhz and haven't tested it higher.Quote from their site, no conflict of interest as I've heard from LInear Tehcnology's VP of engineering that LT has a comparable product which I haven't explored:The ADL5519 is a dual-demodulating logarithmic amplifier that incorporates two AD8317s. It can accurately convert an RF input signal into a corresponding decibel-scaled output. The ADL5519 provides accurately scaled, independent, logarithmic output voltages for both RF measurement channels. The device has two additional output ports, OUTP and OUTN, that provide the measured differences between the OUTA and OUTB channels. The on-chip channel matching makes the log amp outputs insensitive to temperature and process variations.The interesting piece that I overlooked initially is that it is db scaled already. Using two directional couplers, and there I use Minicircuits' SMD transformers, you get forward and reflected power into the two channels. i think it handles up to 60Db of gain. So with minimal parts, using for instance an STM micro with ADC, you can calculate forward/reflected and SWR really easily. Their datasheet is very comprehensive as well.If you are scanning with a real signal and don't have I/Q, someone mentioned in the one VNA thread that phase increase/decrease by frequency determines phase. Excuse me for teaching here as I had never thought about it, but if you get an increase in impedance with an increase in frequency then the phase is clearly positive/inductive and conversely the same with negative/capacitance. I haven't thought about the actual phase angle calculation using this method but I'm sure someone smarter could chime in.Anyway, I spent a ton of time on scalar analysis and didn't make the frequency/phase calculation jump until the poster on the VNA thread threw it in there. In my application for power management, I initially didn't care about phase angle because if it is out of whack it is broken, though some power amplifiers can handle more of one load than the other (capacitive v inductive). So having the phase sign will allow the transmitter to keep working under duress where it might have folded back unnecessarily. My objective in power management/SWR monitoring is to ensure no damage while maintaining the highest level of output power for emergency communications. Clearly a number of other factors play into the control system but it is much better than, "...if SWR exceeds 2:1 turn off the transmitter."

author: yanir
author_rank: Regular Contributor
timestamp: November 20, 2014, 11:24:54 pm
content: Quote from: Bud on November 20, 2014, 06:14:09 pmOn second thought, if I calibrate at the N while APC to N connector is in place doesn't this remove any error due to that adapter? I bought an N cal kit (OSL)

author: Bud
author_rank: Super Contributor
timestamp: November 21, 2014, 08:09:59 am
content: Yes but N is bulky, inconvenient to use N-s on the bench. So most likely you again may need another adapter N-to-SMA, and that may be hit and miss in terms of manufacturing quality. But forget about nitpicking for now, just get yourself going and learn VNA techniques, you are not running a metrology Lab.

author: yanir
author_rank: Regular Contributor
timestamp: November 21, 2014, 10:06:31 pm
content: Quote from: Bud on November 21, 2014, 08:09:59 amYes, I agree. This device seems to be so sensitive I'm not sure where the line is.