FlightAware Discussions

Wideband LNA to Get 1090+978?

Current Setup: FA 1090 Antenna <–> Pigtail <–> RTL-SDR Blog 1090 LNA <–> SMR-400 Cable <–> Nooelec Smartee <–> RaspPi running dump1090-fa

Proposed Setup: ADSBExchange dual 1090/978 antenna <–>Pigtail<–>RTL-SDR Blog Wideband LNA<–>SMR-400 Cable<–>Y-Splitter<–>1090 Bandpass Filter and 978 Bandpass Filter<–>Nooelec Smartee and an FA Orange Stick, the former doing 1090 and Bias-T injection, the latter doing 978 decoding <–>RaspPi running dump1090-fa and dump978

The question is, anyone running this? Or think using a Wideband LNA to boost signal and then relying on the bandpass filters and so forth to filter out what I need is a good idea? (saves running another cable ~70’ to the basement, but…)

Or am I going to boost a ton of noise and just make it a headache? Thoughts?

Yes, sure if there is load of other strong RF signals in the area. In such noisy areas, filtering before amplifying is better, as it minimizes interaction between load of frequencies during amplification.

Conduct a wide band scan (24 Mhz ~ 1800 Mhz) to see how bad is noise in your area. If it is too much, then add following Dual-band filter between antenna pigtail and input of wide-band LNA

ADS-B Dual 978 MHz + 1090 MHz Band-Pass SMA Filter

41UUDuhH+CL.AC_SY580

 

Please compare Scans 2 & 3 below. The comparison makes obvious the need for filter before the built-in LNA of Pro Stick Plus, whose built in filter is after the built in LNA. The scans were done at my apartment in a high-rise buildingin an urban area, surrounded by many other high rise buildings, and having lot of Cell phone antennas on top of my building and other surrounding buildings.

A pretty realistic, wide-band scan covering the entire bandwidth of RTL based dongles.

Spektrum - How-to Speedily Scan RF Noise in band 24MHz ~ 1800MHz

 

The results of scans are below:

The scan 1 shows RF noise picked by antenna and processed without any filter , The scans 2 & 3 were done with filters, and show how filters remove this noise.

Scan 1 of 3 - FA Antenna + Generic DVB-T (no internal or external filter)

Thumb-Generic DVB-T

CLICK ON IMAGE TO SEE LARGER SIZE

Scan 2 of 3 - FA Antenna + ProStick Plus (Only Internal filter of ProStick Plus. No External filter)

image
CLICK ON IMAGE TO SEE LARGER SIZE

Scan 3 of 3 - FA Antenna + ProStick Plus (with internal filter) + External Filter (FA Light Blue)

imageimage
CLICK ON IMAGE TO SEE LARGER SIZE

I’m afraid that will be the most likely outcome.
Filtering at the output of a wideband amplifier will very rarely give decent performance as intermodulation within the amplifier will be the limiting factor. You really need to filter before the amplifier to ensure you limit the frequencies entering the amplifier.

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You can filter before and after :slight_smile:

You can, but filtering after can never remove the spurious signals generated in the wideband LNA by not filtering before :wink:

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Oh no i meant having a filter before and having a filter after at the same time :slight_smile:

Won’t some types of filter block the bias-T DC voltage that powers the LNA? I haven’t looked at the schematic of SAW (surface acoustic wave) filters but the name suggests that they work similarly to this one.
filter
As you can see there’s no DC path, there are actually air gaps. Putting such a filter between the LNA and the dongle would block power to the LNA causing it to not work.

abcd567 tnx for the the info that the filter on the Pro Stick Plus is after the LNA, I was wondering what the RF chain was like there.
With the understanding that there can be exceptions, I tend to always want some sort of BPF where the antenna comes into any system (receiver, transmitter, repeater cavities). Loose 1.5 dB or so, live a happy life.
Since very often overloaded amplifiers turn into mixers, it’s just a good idea. And make sure it’s a BPF or cavity, have seen people use crystal filters which are also turn into mixers in the present of a strong out of band signal.

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The FA Light Blue ADS-B Dual 978 MHz + 1090 MHz Band-Pass SMA Filter is NOT SAW or Ceramic, or Cavity type. It is a LC ladder circuit.


.

,

LC LADDER - UNBALANCED

LC%20Ladder%20-Unbalanced

.

LC LADDER - BALANCED

LC%20Ladder%20-Balanced

 

 

Yeah so you have the 2nd filter after a dedicated biast-t to inject the power.
Easy issue to solve.

Did you actually TEST that or you just assume?
I personally use a LNA that has no filter in front of the first stage and it’s fine. Actually you can’t buy a LNA that has a filter before the first stage.
That is the most critical place, where loses of signal count a lot for the whole chain S/N ratio.

Actually all the cellphones have their first stages of reception not filtered. It would not be enough space in a phone to fit 10’s of filters for all the bands a modern LTE/5G phone has.

The modern LNA chips are way better than you give them credit for, those are not the Si transistors from 1990’s.

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Yes, I have tested.
Here, without any filtering in front of the wideband preamps I have tried, I see intermod products being generated. They mostly seem to be caused by mixes with nearby broadcast FM (87-107MHz) and DAB (175-230MHz) signals.

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Thanks for the tear down on the BPF. When I swept it, it was pretty clear it was a discrete component design, and you’ve provided the proof.
I should have been clearer, by BPF I meant a discrete design, like the Flight Aware module, or a SAW filter or ceramic. Which one to use typically comes down to the power levels the filter will subjected to, and how sharp the filter needs to be. While you can do a PCB based filter, the board material becomes critical at higher frequencies. The joys of a generic term like BPF where the details of the requirements are numerous. Thanks for the clarification and pictures !

Maybe you are a candidate for the FM trap inside the only LNA that has one. FM traps have way less insertion loss than a SAW filter (probably close to zero dB at 1.1GHz).


In my case it didn’t make any difference in decoding.

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It’s a high pass not an FM notch.

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Actually, that’s the LNA I use but I also have a cavity filter in front of it :slight_smile:

That’s what I said. In my mind :laughing:
You are right it is a highpass Tee network, N=5.

High-pass filters are tricky. Because of parasitics your high-pass filter will actually end up more like a band-pass filter, its frequency response will eventually die out. But you can certainly design a filter that will reject low frequencies using lumped elements.

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In my designs, quite often I’d design the bias circuitry as a proper HP filter (coupling cap > bias feed L, > gate), then series resonate the bias inductor with a specific bypass cap at a band of interest.

Granted, won’t provide a lot of attenuation, but if the bias circuitry is needed…might as well make the most of it since the parts are needed anyways. Plus, if you are careful, you pretty much have minimal impact on NF. But in the end, there’s only so much you can do with 3 parts. The series resonate circuit does come in handy since you can get a pretty decent amount of rejection (depending on the freq and Q of the circuit).

Capture

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