A Selective and Robust UHF Front-End

schamatics

Quite a few modern wideband receivers do not have a selective front-end that can receive weak signals while rejecting strong out-of-band signals. An article I published in the Jan/Feb issue of QEX explains the issues involved in the design of external front ends for such receivers and describes a concrete front-end unit and its performance. The unit we describe was designed for a Universal Software Radio Peripheral (USRP) N200 radio with a WBX RF daughter card and for 431 to 435 MHz signals, but the design can be easily adapted to other bands. It is also suitable for many other radios, including low-cost USB dongles based on the RTL2832U (so-called rtl-sdr dongles).

Fairly unique features of this design are the use of a low-cost but highly selective SAW filter and the use of a limiter to protect both the receiver and the SAW filter. I also used a fairly expensive helical filter, but I now think that replacing it with a simpler and lower-cost filter would not hurt performance much.

The full article is available on my university web site, with permission of the publisher (ARRL). If I get interesting feedback on the article (I am sure the design can be significantly improved), I will post it here.

Earlier posts in this blog described the mast-mounted LNA that I normally use with this front end (and a small improvement to the LNA), how I prototyped the front-end, on designing and manufacturing PCBs for it, and on reflow soldering in an unmodified toaster over, which is how I build the front-end units. As you can see, it’s been a long learning process for me.

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