FlightAware Discussions

Airspy and bias tee

is anyone using the bias tee on the airspy mini to power an rtl-sdr 1090 lna? i note that the spec for the airspy mini says 4.5 vdc but no reference to current draw (that i can find, sort of…see further on). i am currently using a mini circuits bias tee and draw slightly more than 150mA. i found a reference to 50mA for the mini but can’t tell if this is a max or not…any suggestions?

The Airspy is rated at 50mA output which I think is right on the edge for powering the RTL-SDR LNA. I believe @wiedehopf powered one from an Airspy for a while but switched to a separate bias-tee adapter.

I use an Airspy mini and an RTL-SDR LNA but I’m not powering the LNA from the Airspy, I don’t want to risk it.

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thanks keithma, that’s why i originally used the external minicircuits bias tee. too bad the airspy can’t do 150mA (it seems), it would mean one less block of insertion loss in the signal chain.

The 50 mA limit is mainly to cater for the 500 mA USB current limit i believe.
That limit isn’t really strict so there might not be a problem to use the internal bias-t for the rtl-sdr LNA.

Maybe it’s also a heat thing and having a fan blow onto the Airspy and whole RPi might help with that.

I used the internal bias-t for several month with the rtl-sdr LNA and combined it with 20 MHz which probably makes the Airspy draw more current.
It worked fine.

But the extra bias-t in the signal chain is unlikely to be a problem after amplifying the signal.

I use the bias tee on the airspy mini to power the RTL-SDR LNA. Have added a discarded heatsink from an old graphics card to the airspy mini to get a bit better heat transfer.
But since I have not tried an external bias tee module I have no reference to compare with.
Anyone that have any recommendations on a module with good price/performance ratio?

This is the one I used and it’s working perfectly well. There are many available on eBay and I could probably have saved a few quid by buying an uncased one.

Yes, you’re adding some additional insertion loss by using a bias-tee injector but those LNAs are pretty high gain and unless you’re running it at maximum gain then adding a little extra loss isn’t really going to make any difference in the grand scheme of things as you can just raise the gain by one level to compensate (if needed).

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How is this powered?

I’ve soldered wires from the +5V and ground rails on the underside of the GPIO connector on the Pi out to the injector.

I’d actually not ground it.
Should already be grounded via the dongle.

Ground loop noise is unlikely and i’m not sure if it’s even applicable.
But it saves you one connection and the ground via dongle and USB is just as good :wink:

That was the side I was interested in :slight_smile:

image

Is there a name for this type of connector? If you look at bias-t PCBs they have a clamp or solder pads fro 5V and ground, and just soldering something to that pin is enough to power it? Are there other ways to connect something to it?

Why would it not be? We are talking 150 mA here.
Put the stranded copper wire from your cable parallel to that piece of wire and solder it.

The solder joint won’t be the problem in regards to current it can carry.
I’d say if you solder an equal sized copper wire to it, it’s probably good for 3A.
(the wire and connection only, not the bias-t)
A solder joint isn’t too mechanically stable against repeated movements. But that only becomes a problem if you were to move the wire multiple times daily.

But you can just as well get a (googling Lüsterklemme) terminal strip and screw down on it.
Any type of pinching terminal should also work no problem.
Hell for 150 mA a crocodile clip will be fine. (though i wouldn’t do that they short to ground too easily)

It is probably my limited understanding of electricity, shouldn’t you need a ground connection as well?

I just think now you have a nice case to protect it, but to power it you just have a short metal pin. Maybe there are areas where this is common and has it advantages?

That connector looks like it is a waterproof isolated passthrough.
But the box itself is not waterproof i suspect, but you could make it waterproof without too much trouble.

The ground isn’t needed if your power supply is the same as the power supply of the RPi.
The RPi ground is via USB connected to the dongle ground, which in turn is connected via coax shield to the bias-t.
The bias-t housing is the ground as obvious that small latch just servers to connect the housing of the bias-t.

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So if I get one, it should be enough to power it from the USB port of the NUC via the +5V lead of a USB cable? Everything is connected to the same NUC and should have a common ground?

That should work very nicely.

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