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[Discuss-gnuradio] Understanding the interfence environment

From: Marcus Leech
Subject: [Discuss-gnuradio] Understanding the interfence environment
Date: Tue, 05 Sep 2006 11:13:21 -0400
User-agent: Thunderbird (X11/20060719)

I'm trying to understand the interference environment for my application (radio astronomy), using the DBS_RX daughtercard.

I've had an ongoing problem with a pulse source getting in via the feed spillover of my 12' dish. I initially thought this was due to the local 5.6Ghz doppler radar de-sensing my LNA. The radar is only 10km north of here, and has peak power of 250KW. Since my LNA is a GaAsFET device, it has gain up into the several Ghz. But I put a commercial 11-pole interdigital filter in front of the LNA, which covers from 995Mhz to 1600Mhz, with out-of-band rejection of at least 85dB, and likely much more. My pulsey little friend is still there. If it weren't for the fact that it's only weakly directional, and the pulse structure is rather "elastic", it'd be a good candidate as a pulsar--the main pulses show up every five seconds, with sub-pulses that have variable timing with respect to the main pulse. The pulses are 30dB out of the noise, which makes them pretty strong. Their duty cycle is quite small (another characteristic of a natural pulsar).

It doesn't appear to show up in my spectral display, at 4Mhz bandwidth, but the short duty cycle would probably
 preclude it, unless I turn the averaging way down, which I might try.

I've just modified my code to turn on the baseband filtering in the DBS_RX, but that seems to have made the total
 detected power *less* stable, rather than more stable.

My understanding of the way things are processed in the FPGA is that the baseband signal (possibly low-pass filtered by the DBS_RX) coming out of the DBS_RX is sampled at 64Mhz, then filtered and decimated down to my desired bandwidth (4Mhz in this case). How effective would the filtering in the FPGA be at keeping intermod products out of my
 passband, etc?

In the band that I'm observing, 1418-1422Mhz, there should be *no* deliberate transmission sources at all, but I'm starting to suspect that my pulsey little friend is in-band, rather than out-of-band. But the duty cycle makes it hard to observe
 on the spectral display.

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