Gear for testing HF pcb impedance and so on? - Page 1
Forum section: electronics / testgear

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 05, 2021, 07:50:15 pm
content: I would like to begin to play with HF PCB design and be able to detect what différance vias, tracks placements and all that means for a signal.The frequency I hope to work with is in the range you think is easiest to learn by and manage. i do not have any particular projects in mind except projects that are great for one trying his first steps out in hf.But what gear do I need to measure, test and learn from?FFT, VSN, VNA, RNA or DNA? :-)

author: bob91343
author_rank: Super Contributor
timestamp: June 05, 2021, 08:11:08 pm
content: I think VNA is a good instrument for this. You can use the Smith chart to see the electrical length and characteristic impedance and losses in microstrip.

author: nctnico
author_rank: Super Contributor
timestamp: June 05, 2021, 08:22:40 pm
content: Start with the Nano VNA (the model that goes to 3GHz). Even with a decent amount of HF gear it is a welcome addition to make some measurements quickly.

author: radiolistener
author_rank: Super Contributor
timestamp: June 05, 2021, 08:38:09 pm
content: Quote from: FriedMule on June 05, 2021, 07:50:15 pmyou're needs TDR impedance measurement. VNA can do that, also some oscilloscope with TDR can do.Here is example on how to transform VNA output into TDR impedance trace in MATLAB:https://github.com/qrp73/NanoVNA-MATLABYou can see measurement example which represents a short piece of 50 Ω coax cable loaded with 25 Ω load. It was captured with NanoVNA. But for better resolution I recommend to use VNA which can sweep up to 3-6 GHz.

author: bdunham7
author_rank: Super Contributor
timestamp: June 05, 2021, 08:42:28 pm
content: https://www.ebay.com/itm/133670457786?epid=10041818111https://www.ebay.com/itm/154407710275Maybe some unclad FR4 board and a silver ink CircuitWriter pen?And do some reading and try to learn the math, or at least the rules. There's a lot to learn.https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/14/High-speed-layout-techniques-slyp173-1_5F00_18_5F00_16.pdf

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 05, 2021, 08:53:48 pm
content: A TDR or VNA, do I understand correctly that the VNA is separately showing capacitance and inductance combined with resistance where the TDR is combining it to one result?nanoVNA looks great, I understand that it's a vector network analyzer, (the important here is network)? How does that differ from vector signal analyzer (signal important her)?My math skills are severely below average my math goes to somewhat beginning to grasp algebra, but everything beyond that is a black hole!

author: radiolistener
author_rank: Super Contributor
timestamp: June 05, 2021, 10:45:21 pm
content: Quote from: FriedMule on June 05, 2021, 08:53:48 pmVNA shows impedance (active R and reactive X components) in frequency domain, where X axis means frequency.TDR is a time domain, where X axis means time.If you transfer VNA output from frequency domain into time domain, you will get impedance vs time chart and can see how impedance is changed across transmission line. You will see every impedance mismatch on transmission line like pulse on oscilloscope.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 05, 2021, 11:15:31 pm
content: Thank you, I did fast read some forum post about TDR and that person does not sound like he knew what it was about.-)Mr. EEVblog has talked about the small "pocket" scopes being a toy, compared to a real scope, do I assume the same about the nanoVNA? Or is it really working fine?

author: bob91343
author_rank: Super Contributor
timestamp: June 05, 2021, 11:23:20 pm
content: The nanoVNA is the most amazing piece of gear to hit the market for many years. For about $50 it can measure all sorts of things easily. It is NOT a toy! It has given us average hobbyists a powerful tool formerly only reserved for the wealthy companies.It can be a TDR as well, even some spectrum analysis, component test, and much more, over a wide frequency range. Any serious engineer, technician, or hobbyist should have one.

author: radiolistener
author_rank: Super Contributor
timestamp: June 06, 2021, 10:53:28 am
content: Quote from: FriedMule on June 05, 2021, 11:15:31 pmNanoVNA allows to see TDR impulse response. Basically it's just inverse FFT transform of a complex reflection coefficient taken in a frequency domain (this is what the VNA actually measures). In such way you can transfer reflection coefficient measurements taken in frequency domain into time domain and can see how reflection coefficient was changed over time. If you want to see TDR step response you can use MATLAB script that I posted above.The only con of NanoVNA is too small bandwidth, in time domain it leads to small resolution. It good enough for cable and long transmission lines, but too small to see all details of transmission line on PCB, because it's too short. This is why I recommend to use VNA with 3-6 GHz bandwidth, it allows you to see more details.For example here is TDR impulse response of two coax cables connected in a series with open end. It is taken with NanoVNA. The first cable is 0.5 meters RG316 and second cable is 1 meter RG58. RG316 has SMA connectors. RG58 has BNC connectors. There is also SMA-BNC adapter between cables.You can see a small impedance mismatch at about 4.8 ns, this is impedance mismatch on SMA-BNC adapter. Also you can see a high impedance mismatch at 15.08 ns, this is impedance mismatch at open end of RG58 cable.As you can see NanoVNA is good enough to start. And if you will need better resolution then you can select more expensive VNA.

author: Noy
author_rank: Frequent Contributor
timestamp: June 06, 2021, 11:36:19 am
content: Great script above.May it be possible to edit it to be used with the Siglent SVA export and octave or another free tool?

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2021, 12:01:51 pm
content: Fantastic!! It's amazing what you can get for the price of a low quality DMM! :-)May I please ask what the différance is between VNA and VSA? (lol please explain in idiot language) :-)

author: radiolistener
author_rank: Super Contributor
timestamp: June 06, 2021, 12:59:11 pm
content: Quote from: FriedMule on June 06, 2021, 12:01:51 pmVSA is used for signal analysis. It get existing external signal on the input connector and performs its measurements or processing.VNA is used for network analysis. It has it's own signal generator which produce different signal and analyse network response on applied signal.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 06, 2021, 10:23:18 pm
content: So that perform about the same job but the VNA has all it's "tools" build in?

author: tautech
author_rank: Super Contributor
timestamp: June 06, 2021, 10:44:43 pm
content: Quote from: FriedMule on June 06, 2021, 10:23:18 pmHow many tools do you want in just one box ?SA, VNA, TDR......SVA1000X series.

author: Noy
author_rank: Frequent Contributor
timestamp: June 07, 2021, 06:28:34 am
content: @Tautech: Where is the TDR function (to see impedance mismatches like in the script above) in my SVA device? I think i missed this button..Only found the DTF which is quite useless for me (i think) don't know what i can do with this mode except i have to handle long cables or so ??

author: tautech
author_rank: Super Contributor
timestamp: June 07, 2021, 08:10:22 am
content: Quote from: Noy on June 07, 2021, 06:28:34 amDTF uses TDR.Experiment with DTF settings.I haven't played with it for a while but some coax cable analysis I did a while back:https://www.eevblog.com/forum/rf-microwave/what-really-is-this-antenna/msg2680419/#msg2680419

author: busaboy
author_rank: Contributor
timestamp: June 07, 2021, 06:07:42 pm
content: Tektronix CSA803C with a differential TDR plugins. Cheapest high speed TDR you can buy. Massive beast.I used it to test HDMI/USB cables and PCBs. Compare differential traces on PCB. You can locate problems with vias and find trace discontinuities and measure crosstalk.