FlightAware Discussions

Power Bias Tee from USB port

I need to power a bias tee to run the RTL-SDR ADS-B LNA. I found one from GPIO Labs that has a micro USB power input. Is there any reason I couldn’t run a cable from one of the USB ports on the Raspberry Pi 3 B+ to power the bias tee?

I need to use an external bias tee because I’m going to be using an Airspy Mini.

Thanks.

It will work, but it will be noisy.
To a greater or lesser extent, the LNA gain varies with voltage, so noise on the supply line manifests as gain fluctuations.
It’ll work, but you won’t be getting the best out of your amp.

I bought this guy


Not cheap but should do the job.

I see some people are using the 5V rail on the GPIO. Is that also noisy?

I also see some people just using the bias tee built in to the Airspy mini. I’m probably going to just run it in 12MSPS mode. Is using the built in bias tee a bad idea?

I use the airspy built in bias-t.
I have seen complaints that it doesn’t provide enough power but it seems to work for me.

2019 - My current Bias-T

 

 

2013 - My old DIY Bias-T, made from a recycled TV Splitter

DIY_BiasT_2

 

CLICK ON IMAGE TO SEE LARGER SIZE


 

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What LNA are you using?

Same, I’ve been feeding the rtl-sdr triple-filtered LNA for the last 7-8 months or so through the Airspy bias-T without issue on my main rig. That said, my recommendation would be to use an external power source if funds are available but my situation came out of sheer laziness.

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Well its good to know that it works without an external bias-tee.

I’m doing that, and haven’t noticed it being particularly noisy - no more than powering via a dongle supplied from the USB port anyway. The quality of power supply probably has more of an impact.

So maybe powering it using a cable from another USB port on my RasPi isn’t such a bad idea?

I don’t see that it would matter either way - They are connected to the same 5v rail, so use whichever connection is most convenient. There is a maximum current draw of 1.2A shared between all ports on the Pis since the original B+ and that can all be used by a single port. The B+ and Pi 2 had a default maximum set to 600mA, but could be software switched to 1.2A with a boot config flag but this isn’t needed since the Pi 3.

Ya the easiest thing for me is to use a cable from another port since there seems to be a hesitation in using the bias tee built in to the Airspy Mini to power the RTL-SDR LNA. I am already using the POE hat because of where my Pi is installed and I’d rather not have to go back and run separate power.

This is what I’m doing, I’ve soldered two wires to the bottom of my Pi and they go into the bias-tee.

When I added an additional fan, I just soldered the power cable onto the input of the bias-tee.

It works very well and I’m not sure whether it’s noisy or not, I’ve never noticed any issues.

Well if it works for the amount of traffic you see it’ll be no problem for me.