FlightAware Discussions

RTL-SDR Blog 1090Mhz LNA

I have received the 1090MHz triple filtered LNA from rtl-sdr blog, because I want to experiment replacing my no-name dual eBay special filtered LNA.
Being prudent, I open the shell to check if it wasn’t installed backwards… it can happen.
Well, it was installed correct, but on a cursive analysis looks like it can be fitted with a voltage stabilizer to power the preamp. Right now there is nothing there, and a 0 ohm resistor passes the power directly to the chip, but… for my cable length I am worried of the voltage drop.
And I definitely wary to use the Bias-T inside my Airspy Mini to feed this (I have an external bias-t right now that I can use).
Now I am contemplating to solder the stabilizer on the board… after I check functionality as is of course. The datasheet says that max voltage is 5.5V, so a 5V stabilizer IC should be perfect. Like LT1118CST-5.
Am I overthinking this?

The board with the open pads for the voltage IC on top-right:


Looks like the SAW filters it uses are 8MHz wide? I can’t find a good cutsheet…

For reference, this is what I am using now:


My present SAW filters are 30MHz wide.

Unless you’re losing a couple of volts in the cable, I don’t think it’ll be an issue. It’ll work on 3.3V and possibly lower.

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Maybe I am overthinking…
I might use one of the Amazon Kindle or my old Asus phone power supplies, with a bias-t, because they are rated 5.2V.

Yes, I think you may be.

I’m feeding the bias-t with the 5v rail from the raspberry pi. It works fine.

I’d recommend if anything feeding the bias-t with reduced voltage instead of 5 V so it doesn’t get as hot.

I’ve used a 10 Ohm resistor and that works well to reduce the voltage.
(for the rtl-sdr LNA, the voltage reduction of the resistor is dependant on the current consumption of the LNA)

The datasheet has all the noise, OIP3, S21, gain, etc parameters shown at 5 V.

I don’t know if lowering the voltage won’t make things worse. They have already a 10 ohm resistor on board, per the application cutsheet.

I found that powering my LNA with a stable +5 supply separate from the digital SDR and Raspberry Pi did improve system performance somewhat. The improvement was slight, a few % of total count measured over a month. If you are going to the effort make sure the voltage source is low noise. Use a linear regulator at the least, one optimized for low output noise is better.

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datasheet for sf9175 saw filter cached link http://webcache.googleusercontent.com/search?q=cache:jP60FUvdA64J:bjzxsf.net/UploadFile/Files/201410181426182020.pdf+&cd=3&hl=en&ct=clnk&gl=in

How is the performance of this new filter?

I have one on my main rig and think it works fantastic for the money. Currently fed with Airspy Bias-T, but will have a dedicated line on it soon. Super glad that SAW is after the 1st (and 2nd) amplifier stage with that 3db center loss. I’ll be picking another one up for sure. I think they knocked it out of the park on this one given the size and cost.

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I currently use bias tee to feed my triple filter (not using airspy bias tee),
was hoping to hard wire, to do away with bias tee and power direct
watching this with interest in how you hard wire
Screenshot_19 Screenshot_42
i currently have 2 bias tees
and 2 triple filters brill little things

I was going to use the pads on the board itself, move the 0 ohm resistor in the empty position… I can draw a diagram on the picture.
But now I am not so sure. Maybe is more trouble that it’s worth.

It will be a while till I get this up on the antenna, it’s too cold for now (and have other indoor projects).

Thanks there is no rush :slightly_smiling_face:

I was acclimatized to max temps around 18-20C and being in short sleeve outside, and in two days dropped to a max of 4C. Canadian/polar air pushed down by jetstream… brought the winter really quick.

Now is up a little to 9-12C. I know it’s won’t get better, just need to get used to it :slight_smile:

This is a timely thread since I’m about to order the same LNA. Would appreciate advice on how to power the bias tee and how to put it in some sort of protective housing. My cats tend to unplug and disassemble stuff.

You can get a bias tee that is already in a housing:

Thanks. Looks cat proof to me. How does this thing get power?

It is powered via the feed through capacitor. Just solder your 5v feed cable to it. +ve to the feed through itself and -ve to the earth tag.
On mine I put a short cable with a female coaxial socket on it. That way I can plug one of the many 5v wall warts I have into it. I used something like this:

Don’t think any DC cable is cat proof.
Wouldn’t expect any difference to the other device.