Cable length measurements with "cable_length.py"

[Drachenfrucht]

Hi all,

I was impressed by Cable length measurements & Reflectometry and made the same setup.

At RedPitayas' IN2 and OUT2 port I connected a T-adapter.

On the T-adapter at IN2 port I connected: the first end of this T-adapter is connected with a 50Ohm resistor. On the other end of this T-adapter I connected a 50 Ohm coaxial cable (20cm length).

On the T-adapter at OUT2 port I connected: the first end of this T-adapter is connected with the previous mentioned 20cm coaxial cable. The other end of this T-adapter is connected with an adapter SMA<-->Coaxial<-->SO232 so I can connect a standard coaxial cable with SO232.

I hope my connection schema is correct, isn't it? I did et exactly as shown on the presentation video on 05:08min

Then I ssh into RP and run

Code: Select all

systemctl start redpitaya_scpi

to start the SCPI server which is needed for the python script to run successfully.

Then I run "python Cable_length.py". I press 'm' for measuring cable length and on the next question "Is your cable longer than 10m?" I press "n" for NO. On the first run is asks me to create a .CSV file and to disconnect my cable first. I do so: I disconnect the coaxial cable that I want measure and press ENTER. The program then creates a new file called "no_cable_response.csv". Then is asks me to reconnect the coaxial cable. I have tried connecting various coaxial cables (RG58, RG213) in different lengths <10m. My tested cables are open circuit, the other end is OPEN. But whatever cable I measure, I get the answer ==> IMPEDANCE MATCHED TERMINATION. which looks like shown in following screenshot:

16-04-04-165611.png

I do not understand that. Even when I unplug the coaxial cable I want to measure from the T-adapter at OUT2 the python script "Cable_length.py" return the same output.

Why is the impedance matched whereas the end is OPEN in real?? Any help really appreciated, I'm kinda clueless

Thanks to @ll

Even when I disconnect

[Nils Roos]

I haven't tried the cable length measurements myself, but in the documentation there is a mention of "With Red Pitaya's sample rate, we can estimate cable's length with resolution of 80cm." Your 20cm cable is indistinguishable from an open circuit for the time resolution possible with the Red Pitaya.

[Drachenfrucht]

Hi Nils,

I have tried measuring:

- a 20cm coaxial cable
- a 300cm coaxial cable
-a 500cm coaxiabl cable
- no cable at all

at all cirumstances I get "MATCHED IMPEDANCE" and this is weird

[redpitaya]

Hi guys,

If you are using OUT1 and IN1 instead OUT2 and IN2 you should change that in the code.
But think that issue is in this line in Cable_length.py

Code: Select all

 rp_s.rp_write('ACQ:TRIG:LEV 0.5')  # trigger level set to 500 mV

In original version it is "500" instead of "0.5".
Our developers have changed trigger level settings from mV to V in the latest SCPI server release.

So try to change that and run the code.
- Use OUT2 and IN2
- Make connections as is described in video
- Delete your old "No cable response.csv" and start the code.

Minimum length of the measured cable is 80cm.
Resolution should be around 8cm.

All Best, RP

[Drachenfrucht]

Thanks a lot for this helpful information. You guys should change the code and upload it to your webserver, so the latest code is downloadable for the users

Cheers

[Drachenfrucht]

Unfortunately I realized you still are offering the faulty version on your web page Cable length measurements & Reflectometry.

The download link in line 105 says:

Code: Select all

   rp_s.rp_write('ACQ:TRIG:LEV 500')  # trigger level set to 500 mV

which is WRONG!

The program code in line 105 should look like that:

Code: Select all

   rp_s.rp_write('ACQ:TRIG:LEV 0.5')  # trigger level set to 500 mV

What do you think about fixing the download link with a correct working program version? Thanks a lot in advance for your help and support.

Best regards