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Re: [Discuss-gnuradio] Practicality of DC-light RF front-end?

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] Practicality of DC-light RF front-end?
Date: Fri, 17 Dec 2004 12:24:46 -0800
User-agent: Mozilla Thunderbird 1.0RC1 (Windows/20041201)

Marcus D. Leech wrote:


It seems, though, that doing a filter bank for an entire 3000Ghz swath
  would be expensive, if done with high-quality in mind.  In the diagram,
  I show 4 filter control lines, giving 16 different filters--which gives
an average bandwidth per filter of 187.5Mhz.

Well, the filters could be tailored to the monitoring bands of interest, such as VLF, AM BCB, HF, low-band VHF, broadcast-FM, commercial air band, VHF-HI, mil-air, etc., on up through similar sub-bands in the UHF and SHF spectrum. These ranges tend to have similar loaded-Q requirements, so they aren't equal subdivisions of the spectrum, but rather (roughly) proportional to the band center frequency. For example, the filter for VHF-HI would be 136-174 MHz, or about 25%, and UHF-LO is is 400-500 MHz, which is about the same. A modular approach would let someone plug-in a pre-selector board w/pre-amp dedicated to some subset of bands, or some uber-expensive one that covers all the bands of interest from DC-3GHz.

The diagram obviously is missing a LOT of detail, but it gives some of my
  thoughts.  Kind of a combined SSRP and RF front end.  I think it's entirely
  practical to build something like this onto a single 4 layer board without
too much trouble.

I do prefer a modular approach, as mentioned before. In your diagram, having a separate "filter-bank + MMIC AMP" module would be of value in that you could use it with a wide variety of receiver architectures. (I'd love to have this just to put in front of my conventional scanner!)

Secondly, having an analog RF-to analog IF module using the MAX2116 would allow someone to choose a different filter-bank, perhaps a narrow one for a dedicated spectrum segment vs. a selectable one as shown. A third natural module boundary is the analog-IF to USB-IO. Finally, there would be a controller module that knows how to twiddle all the control lines for the prior three.

If you wanted to get fancy, these modules could have a defined "backplane" pin arrangement for power and control signals, then you could build them as daughter boards to the controller module.

(Oops, now we've ended up with the USRP, at least for the DAC, controller, and USB modules :-)

So I'd focus on the filter bank + down converter and not reinvent the excellent work achieved so far with USRP.

(That is of course, you buy into the modular approach. A monolithic single board design as you describe in the block diagram would be very useful, and much cheaper, too.)

Anyway, the "DC-to-Daylight Down converter" concept is what interests me; the analog IF-to-USB-to-PC path has now been well-trodden.

-Johnathan, AE6HO

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