Author Topic: VNA for beginners  (Read 815 times)

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Offline okwTopic starter

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VNA for beginners
« on: July 09, 2024, 08:24:59 pm »

Hi,
I have a custom PCB with GPS (9x11mm generic module - external active antenna, plus the option to route it to LTE/4G on-board antenna in case external is missing), 433MHz tranceiver (si4455, PCB trace antenna), Bluetooth (nRF52840, chip antenna) and LTE/4G (nRF9160, PCB mounted antenna, plus IPEX connector to test off board antennas), which I'm trying to tune to achieve better RF performance. But I'm not sure where to start. I've googled a a lot, but can't seem to find the "beginners guide".
Where do I tap in, and which readings am I looking for? And do I use a handheld probe or solder in to not interfere with my hands? And which components do I leave in, and which do I remove for measurements? And which components would move the curve in either direction?
I have a PS100 RF Vector Antenna Analyzer (resistance/reactance/SWR/S11), a Tek oscilloscope (up to 1GHz) and a R&S signal generator (up to 3GHz).
I've attached the schematics for each of the macth/filter circuits. Some might be overkill(?), I'm still quite new in RF designs.

Any help would be greatly appreciated.
 

Online tszaboo

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Re: VNA for beginners
« Reply #1 on: July 09, 2024, 09:58:34 pm »
Read this guide:
https://colinkarpfinger.com/blog/2010/the-dropouts-guide-to-antenna-design/

It's good, and while it might tell you more than you need, it tells you also what you need to start.
For LTE, what's that schematic? You have a bunch of matching circuits for each band?
« Last Edit: July 09, 2024, 10:08:27 pm by tszaboo »
 
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Offline okwTopic starter

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Re: VNA for beginners
« Reply #2 on: July 10, 2024, 01:02:46 am »
For LTE, what's that schematic? You have a bunch of matching circuits for each band?

Thanks. Yes, it's from the Thingy:91. Nordic told me they needed it to match various bands due to very small sized PCB. I guess in my case, it might not be needed.
 

Online tszaboo

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Re: VNA for beginners
« Reply #3 on: July 10, 2024, 11:38:43 am »
For LTE, what's that schematic? You have a bunch of matching circuits for each band?

Thanks. Yes, it's from the Thingy:91. Nordic told me they needed it to match various bands due to very small sized PCB. I guess in my case, it might not be needed.
I think I've seen that previously on a Nordic evaluation board. They are right, different bands for 4G make it impossible to match antennas to 50 Ohm on all bands. TBH I never implemented this myself, and sent my boards to match my antennas to the antenna manufacturer for their recommended matching. They have experts who do this, and provide with the recommended part numbers for the BOM, and test reports. Some bands are not 50 Ohm, but the results are often good enough, plus 4G compensates for this mismatch with more power.
 

Offline okwTopic starter

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Re: VNA for beginners
« Reply #4 on: July 14, 2024, 06:19:24 pm »
I see. I'll keep that in mind.
I read the article (very good), but I still have some questions. So I put my connector before the SAW filter at L13/C136 and then after at L27/C163? And since the SAW filter is unbalanced, I need to have a matching circuit on both sides?
And my design is very tight, I can't fit an SMA connector like the one in the article. Can I use an IPEX to SMA cable instead, and just remove the IPEX connector and solder in the wire and shielding? Or could this introduce too much noise?
 

Offline mayor

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Re: VNA for beginners
« Reply #5 on: July 15, 2024, 10:15:11 am »
Take a look here, this is a good resource, and should work even if you don't have much room.

https://www.ti.com/lit/an/swra726/swra726.pdf
 

Online tszaboo

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Re: VNA for beginners
« Reply #6 on: July 15, 2024, 04:30:13 pm »
I see. I'll keep that in mind.
I read the article (very good), but I still have some questions. So I put my connector before the SAW filter at L13/C136 and then after at L27/C163? And since the SAW filter is unbalanced, I need to have a matching circuit on both sides?
And my design is very tight, I can't fit an SMA connector like the one in the article. Can I use an IPEX to SMA cable instead, and just remove the IPEX connector and solder in the wire and shielding? Or could this introduce too much noise?
SAW filters and LNAs for GPS are going to be already 50 Ohms, if they are not, they will tell you in the application circuit.
For VNA measurements up to 4 GHz, IPEX connectors and cables are fine. You can also remove the connector, but I usually try to make it with an IPEX, see later. It's not noise that you are concerned with, it's measurement repeatability, and physical layout. You want to calibrate your VNA on the PCB. Sounds scary but all you need is copper tape, scalpel and 0402 100 Ohm resistors, or 50 Ohm RF resistor. Calibration is very important, if it's not done correctly, the measurements and the matching you are trying to apply will not make sense.
And for the matching, you need typically a capacitor and an inductor sample kit.

So don't try to match circuits that are already 50 Ohm. Or transmission lines. You want to match parts that are variable, antennas in this case.
Don't forget the housing. Drill a hole on it for the cable. This is why I always try to make it with connector.
I usually ask for un-populated PCBs to make the calibration kit, and to test the antennas. Then sample it from production to verify.
 


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