Smith Chart Behaviour  - Page 1
Forum section: electronics / rf-microwave

author: trentt
author_rank: Contributor
timestamp: September 25, 2019, 07:19:32 am
content: Hi everyone,I've got access to a cheap VNA and I'm playing around with different things to understand the behaviours of components at different frequencies. With the antenna and LNA this is the result I achieve..According to this application to achieve a 'matched' network, I should add a 55nH shunt inductor. I don't have that value so I added two 100nH in parallel which should get within tolerance.After adding a shunt inductorsCan any explain or point me to some information that can explain the behaviour I'm seeing here.Thanks,Trent

author: tautech
author_rank: Super Contributor
timestamp: September 25, 2019, 07:41:26 am
content: First I would look at the antennas Log Mag to determine if its resonant frequency is where you want it to be and adjust feed point and maybe length too so to get it close to where it needs to be.In my somewhat limited VNA antenna experience I found this to be the most helpful to tuning an antenna.Then SWR and Smith chart results basically confirm if you're close or not.Some further reading that might help you on your quest.https://www.eevblog.com/forum/rf-microwave/antenna-project-log/https://www.eevblog.com/forum/rf-microwave/what-really-is-this-antenna/

author: trentt
author_rank: Contributor
timestamp: September 25, 2019, 08:04:20 am
content: Thanks Tautech,I'm just surprised that the inductors can shift the frequency by so much.Trent

author: tautech
author_rank: Super Contributor
timestamp: September 25, 2019, 09:04:17 am
content: Browsing the treads I've linked, I've added comments of what I've done with instrument settings and why as like you much of this is new to me too so I hope just to help the reader with a little understanding of processes that I found hard to find when I was starting out.Another tip, when you know the resonant frequency narrow up the span and maybe lots to get Smith chart info in particular that is not so confusing.It's good to see you're using markers and I tend to set them just on the frequency of interest so you can see how they align with the Log Mag and SWR dips and how far the might be from the magical 50 ohm center of the Smith chart.Have fun, I certainly have.

author: metrologist
author_rank: Super Contributor
timestamp: September 25, 2019, 09:14:34 am
content: According to this video, at 2:55, he says that series components move impedance along the red curves.

author: trentt
author_rank: Contributor
timestamp: September 25, 2019, 09:22:01 am
content: Yeah it doesn't help when a lot of what is theoretical doesn't translate to the practical. I've got them set on 1.561 GHz, 1.575GHz and 1.601 GHz as my target frequencies.

author: trentt
author_rank: Contributor
timestamp: September 25, 2019, 09:24:03 am
content: With the tool on their website I can use a 55nH Shunt Inductor to get to the matched frequency but I'll try out the Series Inductor and see what effect it has. 4.7nH should do it.I've also got a 33pF capacitor already in series, I'm not sure how much that effects things but I need to block the DC from that is feeding the LNA.Edit.Effects of 10nH Inductor Shunt and 4.7nH Inductor in series.

author: metrologist
author_rank: Super Contributor
timestamp: September 25, 2019, 09:35:53 am
content: Couldn't you just remove some of that series capacitance?

author: trentt
author_rank: Contributor
timestamp: September 25, 2019, 09:45:59 am
content: Are you suggestion that reduced series capacitance would be beneficial?

author: metrologist
author_rank: Super Contributor
timestamp: September 25, 2019, 10:01:24 am
content: Between your markers 2 and 3 is right on the red circle. To get to that point from a perfect match, I add series capacitance so removing some of that should do the same as adding series inductance.

author: trentt
author_rank: Contributor
timestamp: September 25, 2019, 10:16:00 am
content: I'll give it a go to see the results but the inductors have already shifted the frequency way off, I'm getting ~-1dB instead of the ~-22dB I was getting before.

author: metrologist
author_rank: Super Contributor
timestamp: September 25, 2019, 04:18:21 pm
content: BTW, I was not getting the same result as you using the same application. Did you select the correct frequency? first picAlso, when you add components, you are adding to the circuit and need to consider all effects. For example, depending on how you add inductors in parallel, you might be setting up some capacitance. second pic

author: trentt
author_rank: Contributor
timestamp: September 25, 2019, 11:22:36 pm
content: Thanks for pointing out where I went wrongWhat is the software in the second picture?

author: yl3akb
author_rank: Regular Contributor
timestamp: September 26, 2019, 05:13:14 pm
content: Quote from: trentt on September 25, 2019, 07:19:32 amDid You solve the issue?How did you calibrate your VNA? At which point of your network? Is it possible that You have a short cable (and connector+trace) between antenna+lna and VNA port, but you are adding inductor at other end of cable (pcb side)? It would not work in that case. You woudl need to de-embed at least that cable.Also, that ripply trace in second picture looks like bad calibration...

author: metrologist
author_rank: Super Contributor
timestamp: September 26, 2019, 05:22:26 pm
content: Quote from: trentt on September 25, 2019, 11:22:36 pmhttps://www.will-kelsey.com/smith_chart/

author: trentt
author_rank: Contributor
timestamp: September 28, 2019, 04:13:51 am
content: Quote from: yl3akb on September 26, 2019, 05:13:14 pmHi Yl3akb,I'm still working slowly through everything and trying to get a grasp of how these signals like to operate at these frequencies. VNA was calibrated as per documentation provided. I'll have to do some research and work out how I would de-embed the cable. Why do you suggest that the inductor would not work on the pcb side? The inductor is on an adaptor board that goes from the Antenna to the VNA Port.Trent

author: yl3akb
author_rank: Regular Contributor
timestamp: September 28, 2019, 07:10:36 am
content: Its hard to understand how everything is connected. Can You make some picture or drawing with point where is Your calibration plane - point of the network where you did OSL calibration. I assume You did the OSL? And point where inducator is soldered and staff which is between.My suspicion is that You have a transmission line between Your calibration plane and inductor position. In that case You need to at least the shift calibration plane by the delay of transmission line. Its usually called 'port extension'. For example:http://zone.ni.com/reference/en-XX/help/373153D-01/vnahelp/reference_plane/

author: trentt
author_rank: Contributor
timestamp: September 28, 2019, 07:40:48 am
content: Here is a quick schematic I made up. OSL Calibration was done at the VNA Port. Is this acceptable?The SMA connector is only about 30mm. At what point does the length of the connector begin to matter?

author: yl3akb
author_rank: Regular Contributor
timestamp: September 28, 2019, 07:59:56 am
content: And no cable at all between vna port and 'perf board'? And what about trace length on 'perf board' between SMA connector at VNA side and C7?Anyway, 30mm is already ~80 degrees phase shift at 1.57 GHz if we assume velocity factor of 0.7. On FR4 velocity factor would be even smaller, like 1/sqrt(4.2)~0.5 and phase shift larger accordingly.P.S. L3 would short out the LNA power supply on bias-T...

author: trentt
author_rank: Contributor
timestamp: September 28, 2019, 08:37:06 am
content: Is it likely that the frequency will still remain the same with the phase shift? Or does the act of phase shifting move the antenna "off frequency"?L3 is meant to be C3

author: yl3akb
author_rank: Regular Contributor
timestamp: September 28, 2019, 09:19:02 am
content: If that phase shift is between your VNA and your DUT+matching circuit, then it would not change the measured abs value return loss (or VSWR), but that is not an issue here.In Your case You need to know correct impedance at point where You are modifying Your network, not just at VNA reference plane. Right now You are measuring impedance of this cascade:LNA+C7+delay in PCB trace+delay in connector+other delays maybe <---But You need to measure impedance of this cascade:LNA+C7 <---Then you can determine matching element values and add them near C7.To compensate the delay (or phase shift) or extend the port you would need to add negative delay to Your measured S11 value. Mathematically You would multiply the S11 with exp(+2*phase_shift*j). Factor '2' is there because we deal with reflection coefficient. 'phase_shift'  is composite phase shift between C7 and VNA ref plane, which depends on physical lengths and velocities in material and frequency.

author: virtualparticles
author_rank: Regular Contributor
timestamp: September 30, 2019, 01:43:14 pm
content: Greetings! When you make a VNA measurement to match an antenna you have to insure that you have calibrated right up to the point where the matching components will be installed. For instance if you calibrate on the end of your test cable and then have a short pigtail leading to the spot where your matching components will go near the antenna then the measured impedance will be "rotated" around the center of the Smith chart by twice the delay (or phase) of the pigtail cable. You can either calibrate at the end of the pigtail cable somehow or you can use port extensions to move the reference plane to the end of the pigtail.In any case you'll find the matching process to be iterative. It's almost impossible to get it right the first time. Particularly since one usually assumes that the matching components actually have the value they are supposed to have. Real inductors have a parallel resonance and capacitors have a series resonance.Good Luck!

author: trentt
author_rank: Contributor
timestamp: October 17, 2019, 05:51:57 am
content: Thank you both for your replies. I've been able to calibrate the device and having been playing around for a while.Result of the Antenna with no DC Voltage AppliedVs with DC Voltage Applied / LNA OnAs you can see, with the LNA on, the noise is greatly reduced but my return loss takes a hit. Is this to be expected? Can this be primarily attributed to the LNA or the DC Voltage?Antenna Schematic

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 11, 2019, 07:00:50 pm
content: It's difficult to say where the return loss problem is. Are you looking at the return loss of the output of the LNA or the Return loss looking into the SAW filter with the antenna removed? You'd have to measure the filter and the LNA input separately to determine the culprit there. The output return loss of the LNA usually doesn't matter too much as long as it's reasonably close to 50 ohms. The mismatch will give you a little extra insertion loss but with enough gain from the LNA it usually doesn't matter much.