Simple ADSB hardware question

[9a4qv]

Hello,

this is nothing unusual (losing the traffic after inserting the LNA before the DVB-T dongle).
This is why:

I hope that there is some basic RF knowledge which can help understanding and most important resolving the problem.

The main reasons why the dvb-t stick is poor performer is no selectivity (means no filters at all at the input) and bad dynamic range.

The selectivity can be solved with the filters, but the dynamic range is limited by the A/D converter used in the stick, presently 8bits.

The same stick have the problem that saturate very fast.
If there is no filtering, it will saturate because all the strong signals from the gsm base stations, broadcast radio, TV transmitters etc. are saturating the E4000 even not tuned to this frequency, just because there is no filter at the input.

Adding the LNA you saturate the DVB-T even more because the strong signals are amplified even more.

Hot to resolve the  problem, without the filter?
First, uncheck the both AGC options.
Decrease the Gain (manually) with the slider to the lowest number.
Start to increase the gain with the slider, of course with the LNA in line.
You will notice that the number of packets increasing.
Proceed and monitor the number of packets increasing.
At one point, approx 3/4 of max slider gain you will notice the drop of the signal!
This is the point where the DVB-T dongle receiver is saturated and the reception is bad.
Decrease the gain just a little bit lower to have good reception.
With the LNA inserted, you shoud have at least 10% better range than without the LNA, so more planes and packets.

If you insert the proper filter in line with the LNA and DVB-T then you can have the maximum gain set on the DVB-T controll, or both AGC checked and there will be no saturation of the DVB-T dongle at all.

This is the setup I have where 400km radius is covered just with the simple ground plane antenna.

Try to do the procedure i explain and give us the feedback, how does it work for you.

Thanks
Adam

[thekubiaks]

Adam,
    Thanks for the reply.  I went out and ran the LNA with my E4000 R820T as you described.   I could see the "choke" point as I moved the gain setting up.  I was only able to get about 12dB of gain before my E4000 was saturated.   The maximum framerate was about 1500 frames/minute.  I took the LNA back out of the system and immediately hooked my antenna up directly as before and framerate shot up to > 6000 frames/minute.

Perhaps I am in a noisy RF area and the LNA is simply amplifying all RF around 1090mhz??  I think the secret might be in the filtering as you mentioned previously.  What filter do you recommend?  Thanks

***  UPDATE  ***   I decided to make a 1090mhz band pass filter, I'll report how it works ASAP....   in the meantime, have a look at the first few posts on this thread 

http://www.rcgroups.com/forums/showthread.php?t=1758166

[9a4qv]

Hello,

The notch filter from the link is a good thing, but it does not covering all RF strong signals.
The better approach is to use the bandpass filter as you said.
I do have a very (excellent) quality professional bandpass filter with a low insertion loss and high out of the band attenuation.
I can say that the signals just 100MHz of the 1090 are attenuated for more than 90 dB with sharp skirt so I do not have any problems with the cellular towers running on 960Mhz, and no problems at all from the nearby AN-FPS117 radar running on two channels on 1260Mhz plus two sub radar channels.

The simple filter to build may be this one:
http://www.w6pql.com/microwave_filters.htm

I will make some photos of my setup and will post it here this days.
Adam

[thekubiaks]

SUCCESS !!      I installed a filter that eliminates most everything below 1090mhz and above about 1500mhz.  I looked at it on my spectrum analyzer to verify.  My framerate shot to over 13,000 using the filter with the LNA4ALL, WOW, I never had a framerate over 8800.  My ranges are farther too, I am getting at least 10 -20% further contacts.  So, bottom line is, even if you don't have a LNA4ALL, I think filtering out signals < 1090mhz > is the key.

Look at the big jump in framerate on the second picture.

By the way, the band pass filter from the previous post (RCgroups) wasn't good.


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[9a4qv]

I am glad that we sort out the problem.
See how easy was to do that 

I agree, more important is the good filter than the LNA in case where the cheap DVB-T dongles should be used.

Why is good to use the LNA with the filter and DVB-T dongles?
Well, if you go through the DVB-T tuners datasheet you can notice the quite high noise figure number (from 4-6dB).
This is quite high number.
Adding the losses in the filter before the tuner, this number is even bigger, depending on the losses in the filter.

If you add the LNA before the receiver and the filter, just after the antenna your overall noise figure of the system will be much lower.
You can download free ACAD Agilent RF software to simulate and calculate the gain and the NF of the system.

Adding the LNA will result lower NF and the signals that are buried in the noise will be received with the lower noise figure, means more packets and more plains. This weak signals will not be masked with the higher noise figure (without the LNA) and can be decoded.

So the main purpose of the LNA in the DVB-T setup is not to amplify the signal but to improve the overall nose figure of the system.
Anyway the gain and the sensitivity of the DVB-T dongle is quite good.

JUST USE THE FILTER AND YOU WILL BE FINE 

Adam

Just to add, I have the latest lab measurement of the LNA that you are using, reporting NF 0.7dB and gain 17.1dB @ 1296Mhz, so you should have even better figures on 1090MHz

[thekubiaks]

Adam,
     Thanks again.  I am running Antenna > Hi/Lo pass filters > LNA4ALL > DVB-T > ADSBSharp > ADSBScope.  I peaked out today at 17348 frames/min (Double my non-LNA/filter rates and at least 20% range increases, I'm over 175 miles in 3/4 sectors.  I think I'll try that w6pql filter you use, it looks nice but I'm not sure I can top my new setup??  maybe I can, you've been right on the money about the filters and noise floor. 

Cheers..

btw, what do you think about SAW filters??  I've had good luck with them filtering out GPS signals for my Radio Control hobbies.

=================

I am glad that we sort out the problem.
See how easy was to do that 

I agree, more important is the good filter than the LNA in case where the cheap DVB-T dongles should be used.

Why is good to use the LNA with the filter and DVB-T dongles?
Well, if you go through the DVB-T tuners datasheet you can notice the quite high noise figure number (from 4-6dB).
This is quite high number.
Adding the losses in the filter before the tuner, this number is even bigger, depending on the losses in the filter.

If you add the LNA before the receiver and the filter, just after the antenna your overall noise figure of the system will be much lower.
You can download free ACAD Agilent RF software to simulate and calculate the gain and the NF of the system.

Adding the LNA will result lower NF and the signals that are buried in the noise will be received with the lower noise figure, means more packets and more plains. This weak signals will not be masked with the higher noise figure (without the LNA) and can be decoded.

So the main purpose of the LNA in the DVB-T setup is not to amplify the signal but to improve the overall nose figure of the system.
Anyway the gain and the sensitivity of the DVB-T dongle is quite good.

JUST USE THE FILTER AND YOU WILL BE FINE 

Adam

Just to add, I have the latest lab measurement of the LNA that you are using, reporting NF 0.7dB and gain 17.1dB @ 1296Mhz, so you should have even better figures on 1090MHz

[9a4qv]

I am not using the W6PQL filter, I just mention that it is easy to build and good performance for the time/money invested.

I have a professional filter ( 7 pole I think) silver polished...

SAW filter may help a lot, it is true that IL is quite high, but who care, there is the LNA covering the losses in the filter at the end.

SAW is the simple plug and pray option,

Adam

[thekubiaks]

I am not using the W6PQL filter, I just mention that it is easy to build and good performance for the time/money invested.

I have a professional filter ( 7 pole I think) silver polished...

SAW filter may help a lot, it is true that IL is quite high, but who care, there is the LNA covering the losses in the filter at the end.

SAW is the simple plug and pray option,

Adam

Adam,
     Can you tell me about your 7 pole filter please?  I am interested in building and testing projects like that.  By the way, I am peaking at a framerate of about 18,000 frames/min.  I'm not sure if that is traffic related or E4000 related.  I did try turning the AGC off and manually adjusting gain.   Running with Tuner AGC provided highest framerate.  Thanks.

[9a4qv]

If you have a filter inserted then yes, the AGC turned on will give you the best results.
Meantime I check my filter, it is 8 pole filter.
The filter is professional one that I took from the cellular base station running on the 960 Mhz. I do have also the original LNA that goes together with the filter but I am not using that one.
So after my attempt to tune the filter on 1296Mhz I decide to re-tune it on 1090 which gave much better results then on 1296 which is too high for such a filter.

I do not have the design for this filter but this software can help you to construct a really sufficient filter for you.

Just a spectrum analyzer may not be enough to tune the filter properly. Time domain will do the job properly, some network analyzer will be nice.. if you have such a possibility.
Two or three pole filter can be tuned used the method tune to maximum, but more than that it is very difficult and require a lot of practice and lot of time.

check this out:
http://www.vk3um.com/interdigital%20filter.html

Adam

[Saudistu]

Don't want to go too far off topic, but would the LNA4ALL be any good as a pre-amp for the SBS-3 ?

[9a4qv]

Strait from the heart, I do not know.
I never check the architecture of the SBS-3 or any other receiver for the ADB-S.

If you have some schematic or block diagram, then i can tell you yes or no. Without it, it is not easy to predict what will be the result.

The LNA4ALL have no problems working with the any receiver, it can be used also like transmitting amplifier producing up to 150mW over all range.

The problem may appear in the receiver itself, too many gain can saturate the receiver. If poor filter or no filter at all, then another problems...

You do not have to spend the big bucks to find out that, use the cheap terrestrial TV antenna amplifier to see the difference. If SBS-3 can handle that, then it will handle most probably LNA4ALL.

Adam

[thekubiaks]

If you have a filter inserted then yes, the AGC turned on will give you the best results.
Meantime I check my filter, it is 8 pole filter.
The filter is professional one that I took from the cellular base station running on the 960 Mhz. I do have also the original LNA that goes together with the filter but I am not using that one.
So after my attempt to tune the filter on 1296Mhz I decide to re-tune it on 1090 which gave much better results then on 1296 which is too high for such a filter.

I do not have the design for this filter but this software can help you to construct a really sufficient filter for you.

Just a spectrum analyzer may not be enough to tune the filter properly. Time domain will do the job properly, some network analyzer will be nice.. if you have such a possibility.
Two or three pole filter can be tuned used the method tune to maximum, but more than that it is very difficult and require a lot of practice and lot of time.

check this out:
http://www.vk3um.com/interdigital%20filter.html

Adam

Adam,
     I like that filter, I think I will build it, I have the test equipment to tune it.  By the way, my framerate for the E4000/LNA4ALL/filter combo exceeded 21,000 so I still haven't found the upper framerate limit. 

Regards

[9a4qv]

Hello,
at the end, I don know what filter you have presently 
there is another trick you can try, but this  is just if you want to have reception from the sector.
Use some highly directional antenna. I did some test with the 20dB dish antenna with a very interesting results.

Attached my setup with the GP antenna, Filter and LNA4ALL

Adam

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[Oldfella]

Would be very interest in trying this program. One question I need to know is can it be used to feed BaseStation.

Thanks in advance.

[Anmer]

Would be very interest in trying this program.

What Program?