FlightAware Discussions

1090Mhz, Double LNB, Double Filtered Alternative

Here is a viable alternative to the RTL-SDR Blog Triple-Filtered LNB:

I grabbed one a couple years ago and had really nice results until I left it out on my test rig on a stormy night (stupid monsoon!) and water-logged it. I just grabbed another from the seller I linked which only took two weeks to get to me this time and took them apart since I’m the curious type. Luckily they forgot to shave one of the LNA chips on my old one.

S20200921_0002
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From what I can eyeball, stage 1 goes through a Qorvo 3M9008 LNA, then through a Tai-Saw TA0970A Filter, rinse and repeat for the second stage. (The blue links will get you to the datasheets of each).

While the components aren’t top-dollar, they have respectable specs and the final result parallels that of the RTL-SDR triple-filtered LNA for my location.

The pitfall to this design is that it doesn’t work with a bias-tee, so you’ll need to supply ~5v (Max = 5.25v) external power. Also, since amplification comes first, it could get overloaded in a very high noise environment, but I’m in the middle of a large city (5M people or so?) and haven’t seen any issues, actually less than with the ProStick Plus.

It comes as pictured in the linked ad and will need to be taken apart and waterproofed if it’s going to live outdoors since the case leaks like crazy as-is.

I have zero affiliation with this device or any of it’s supply chain, I’m just an average dude that likes to test and am sharing what I ̶f̶o̶u̶n̶d̶ purchased with my hard-earned money.

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That’s the one I reported a long time ago. Works pretty good, but the noise factor of that first LNA is 1.3dB.
On mine I have replaced the first LNA with a SPF5189Z, noise factor 0.8dB (at 1GHz). And I made it Bias-T fed, with a 5V linear stabilizer source.
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New:

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Wow, I just got up a bit ago with that epiphany, actually logged on to pose the question on whether or not others here would think it would be a better idea to replace the first stage with the SPF5189Z given the lower NF and gain. NF is obvious, but also figured having less gain would clean up entry into the first filter and second stage amplification. Did you notice improvement?

Would you be able to detail the BOM/schem for the Bias-T conversion? I’d be interested in the SMT counterparts because I’m a glutton for punishment.

Apologies if you already had posted about this unit in the past, I didn’t really sit and search prior to opening this thread.

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There are no pads for SMDs, that’s why I build it like that. I used a generic 7805A lin reg chip, two 0.1uF capacitors that I had around and an inductance coil that I had around. That coil is almost too big… I use it with a straight line at the coupler to block RF, that should be enough impedance for 1GHz.

Dangit, that was the portion i was most curious about, the rest I could eyeball. I’m not a guru in this respect, only know enough to get me into trouble and to know what I don’t know.

The inductance that had was something that I had around. For 1GHz, you certainly don’t need the core.

These will do:

Use 2.2 uH/1W.

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:smiling_face_with_three_hearts: Thanks @Dxista. SoNic67’s messing with me being clear as mud haha. Probably karma from me messing with him somewhere along the line I suppose. I’ve got boxes of components, but wasn’t sure on the inductance or how to calculate.

In 1090 Mhz Bias-T neither capacitor value, nor inductor value is critical. Here are two examples of home brew Power Inserters (Bias-T)

(1) By @abcd567

CLICK ON IMAGE TO SEE LARGER SIZE
See bottom of image for range of values

20200922_211439

(2) By @jepolch

7 turns wound about 1.5 mm inner dia

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I’m not sure how much of a problem it is, but SAW filters commonly say not to put DC on the I/O ports.
There are no blocking caps on the PCB (except the input, where there is no bias)

image

Compare this with the RTL-Blog filter /amp

image

Murata say Max 3V
TriQuint don’t specify
TST say 0VDC

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Yeah, correct, they all say so. It bothered me when I added the Bias-T to the output, but then all the other points had voltages on them. Even the input of the LNA chips have some voltage bias.
Many LNA apply some kind of DC voltage on across those. I am not saying that’s correct or not, but it seems that up to 5-7V they are fine.

And yes, I have also switched to the RTL-Blog LNA because I wanted to see if it makes a difference. It is built nicely, has a FM/TV trap. Sadly not too much of a difference in numbers of planes and signals.
The chip they used has pretty nifty noise factors:

PS: I have added a voltage stabilizer inside the RTL-Blog LNA too.