V3 Bias Tee won't turn off on Raspbian Stretch

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Hampstershorts
Posts: 7
Joined: Thu Sep 13, 2018 4:18 am

V3 Bias Tee won't turn off on Raspbian Stretch

Post by Hampstershorts » Thu Sep 13, 2018 4:26 am

Hi there,

I just ordered a V3 dongle to use in an ADS-B setup along with the ADS-B LNA and Filter.

Long story short is I would like to test different setups, with and without the LNA, but the Bias Tee on my V3 dongle will not turn off. No matter what I do I always have 4.5VDC on the SMA port. I have blacklisted the DVB-T drivers as follows:

blacklist dvb_usb_rtl2832u
blacklist dvb_usb_rtl28xxu
blacklist rtl2832
blacklist rtl2830

I am able to run the software script to turn it on and off and it appears to run successfully. It says "Found Rafael Micro R820T tuner" when I run it in either on or off, but the 4.5v stays on the SMA connector.

I'm not sure how to test whether this is a dongle issue or a Raspbian issue.

I have also plugged the dongle into a MacBook laptop and just by plugging it in I also had 4.5v on the SMA connector. That doesn't seem right since I thought the Bias Tee was supposed to be off by default.

Any help would be appreciated.

rtlsdrblog
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Posts: 1790
Joined: Mon Nov 19, 2012 11:54 pm

Re: V3 Bias Tee won't turn off on Raspbian Stretch

Post by rtlsdrblog » Thu Sep 13, 2018 10:35 am

Hmmm strange it's definitely meant to be off by default. Can I know what SDR drivers you've installed?

And I also assume that everything else is working fine and that the LNA is powered correctly?

How are you measuring the 4.5V? If you're using a multimeter, be aware that there is a capacitor on the output which can display a residual 4.5V reading. Discharging the output by briefly shorting the SMA should zero that voltage.

Hampstershorts
Posts: 7
Joined: Thu Sep 13, 2018 4:18 am

Re: V3 Bias Tee won't turn off on Raspbian Stretch

Post by Hampstershorts » Thu Sep 13, 2018 12:58 pm

I am pretty sure the LNA is being powered off the bias tee.

If your ADS-B LNA isn’t receiving any power from the bias tee will any signal pass through it?

I’ll short the SMA connector and test it again.

As for drivers I’m using either stock Raspbian Stretch with the blacklists as you see above, or the FlightAware PiAware SD card image with the same blacklist, which is using Dump1090-fa to access the dongle. On the stock Raspbian I don’t have anything from FlightAware installed. Just your bias tee software. And I can run it without any USB access errors, so it doesn’t seem like any drivers are active that would be turning the bias tee on.

Hampstershorts
Posts: 7
Joined: Thu Sep 13, 2018 4:18 am

Re: V3 Bias Tee won't turn off on Raspbian Stretch

Post by Hampstershorts » Thu Sep 13, 2018 3:05 pm

I unplugged the V3 dongle from the RPi, shorted the SMA connector and verified it was reading 0v with a multimeter. I then plugged it into a running RPi running Raspbian Stretch. I measured the SMA connector without doing anything else and saw 4.5v across it. I then ran your bias tee software using "./rtl_biast -b 0" to try to turn it off. It printed "Found Rafael Micro R820T tuner" and then returned to the command prompt. I measured the SMA connector again and still had 4.5v across it.

Is this a dongle issue? Do you have any other troubleshooting suggestions?

rtlsdrblog
Site Admin
Posts: 1790
Joined: Mon Nov 19, 2012 11:54 pm

Re: V3 Bias Tee won't turn off on Raspbian Stretch

Post by rtlsdrblog » Fri Sep 14, 2018 1:47 am

For the drivers, I mean which rtl-sdr driver did you install? I remember that there was one driver version where they'd decided to force the bias tee on always for some reason. But I just reread your message and you said the same thing happens when you plug it into a Mac too, so it's probably not that.

The ADS-B LNA has an LED in it. If you remove one side panel, you should be able to see if it's lit or not.

It's possible that the bias tee LDO might have been damaged in some way, and now always conducts. Email me your order ID number to [email protected]. I'll send you another dongle to test with.

Hampstershorts
Posts: 7
Joined: Thu Sep 13, 2018 4:18 am

Re: V3 Bias Tee won't turn off on Raspbian Stretch

Post by Hampstershorts » Fri Sep 14, 2018 2:58 am

I will do some more investigations tonight.

I am using whatever driver is installed in the current version of the PiAware SD card image. I don’t know specifically what one that is. Is there a way for me to enumerate that from the command line?

As an aside I also tried it on a RPi that just has the current version of Raspbian on it. I hadn’t installed any RTL-SDR drivers, just blacklisted the DVB-T drivers, and as soon as I plugged the dongle into that setup the SMA connector had 4.5v on it. I then installed your bias tee software and attempted to turn it off, which was not successful.

Hampstershorts
Posts: 7
Joined: Thu Sep 13, 2018 4:18 am

Re: V3 Bias Tee won't turn off on Raspbian Stretch

Post by Hampstershorts » Fri Sep 14, 2018 4:20 am

I did some more checking tonight.

The LNA is definitely getting power. I removed an end panel and the LED on the inside is lighting up.

I also plugged the dongle into an iMac and a headless BSD system I have, and both times the bias tee came on as soon as it was plugged into the USB port.

It would seem like its a dongle problem.

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