Weak Signals with XB-200?

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txjacob
Posts: 12
Joined: Fri Apr 14, 2017 10:12 am

Weak Signals with XB-200?

Post by txjacob » Mon Jul 31, 2017 8:38 pm

I just got a XB-200 and I started playing with it in GQRX, however signals seen to be really weak. I set the filtering to custom, and have it hooked to my J-Pole, and Broadcast FM signals are pretty weak indoors even with the gains turned up all the way. I tried the NOAA frequencies and they're alright, but I would have thought I wouldn't have to turn the amplifiers up all the way. I threw a 30 dB LNA on the antenna and it seemed to help a lot. Does the bladerf just have some weak LNAs, or are my coax runs too long?

bglod
Posts: 56
Joined: Thu Jun 18, 2015 6:10 pm

Re: Weak Signals with XB-200?

Post by bglod » Tue Aug 01, 2017 12:42 pm

There are 3 amplification stages built into the LMS6002d: the LNA, VGA1, and VGA2 for a total of 61 dB of RX gain. There are no external gain stages on the bladeRF or the XB-200. There will be some loss on the boards, but it shouldn't be anything too crazy.

How long are your cable runs? If you use the bypass custom filter path without a filter (just a straight SMA cable to jumper J8 to J9) on the XB-200, do you see any improvement if you connect a basic "rubber ducky" or telescopic antenna directly to the RXANT (J12) on the XB-200 while the bladeRF RX port is connected to RXIF (J3) with a short (1-2 ft.) SMA cable?

txjacob
Posts: 12
Joined: Fri Apr 14, 2017 10:12 am

Re: Weak Signals with XB-200?

Post by txjacob » Tue Aug 01, 2017 12:51 pm

I will have to try, but last night the cable I was using was probably around 6 feet long. Also had some adapters on it for doing connector conversions.

bglod
Posts: 56
Joined: Thu Jun 18, 2015 6:10 pm

Re: Weak Signals with XB-200?

Post by bglod » Tue Aug 01, 2017 1:05 pm

A 6' cable should be fine assuming it's an okay cable. Those adapters can introduce losses, especially if any of them are splitters.

Did you have something else connected to this antenna that could receive signals better than the bladeRF? I'm just wondering why you think they are "really weak". (I'm not dismissing your conclusion, just wondering how you arrived at it -- there could indeed be something wrong with one of your boards).

If you have access to a signal generator and signal analyzer, you could try injecting a known signal into the XB-200, like 100 MHz at -30 dBm, and probe throughout the RX path to determine if there are any components with excessive losses.

txjacob
Posts: 12
Joined: Fri Apr 14, 2017 10:12 am

Re: Weak Signals with XB-200?

Post by txjacob » Tue Aug 01, 2017 1:15 pm

Well in my experience broadcast fm stations totally saturate my RTL-SDR stick. On the blade I could only get a couple. Then I hooked it up to a 20m antenna I had laying on the ground and I couldn't get anything on the 20m band, or on the am broadcast band. My HF radio can pick the stuff up just fine.

I guess I am just underwhelmed by what I can pick up. Unfortunately I don't have any equipment on hand. Maybe I could find someone in our local amateur radio club that does.

bglod
Posts: 56
Joined: Thu Jun 18, 2015 6:10 pm

Re: Weak Signals with XB-200?

Post by bglod » Tue Aug 01, 2017 1:19 pm

Bummer about the test equipment, but totally understandable. Not everyone has that kind of stuff laying around.

I was about to test this, but my Gqrx install is totally broken (thanks glibc update!) and I can't seem to get it to build at the moment. I'll try to get this sorted on my end and see what kind of results I get compared to an RTL-SDR.

txjacob
Posts: 12
Joined: Fri Apr 14, 2017 10:12 am

Re: Weak Signals with XB-200?

Post by txjacob » Tue Aug 01, 2017 5:57 pm

So I tried the antenna thing you suggested, and the results were... surprisingly similar. These were all done with the amplifiers on the bladeRF maxed out.

This is with the rubber duck antenna by my computer (-33 dB):
Image

And this is with the j-pole on the other side of the room (-33 dB):
Image

And for fun, rubber duck on the other side of the room with a 30 dB LNA (-14 dB):
Image

bglod
Posts: 56
Joined: Thu Jun 18, 2015 6:10 pm

Re: Weak Signals with XB-200?

Post by bglod » Fri Aug 11, 2017 2:07 pm

My FM stations are substantially stronger and clearer with my XB200 versus my RTL-SDR. The signal at 162.4 MHz for me looks very similar to yours on the XB-200...about 20 dB above the noise pedestal. The same frequency is about 30 dB above the noise on my RTL-SDR. So yeah, that could be a limitation with the XB-200. :(

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