A Selective and Robust UHF Front-End
January 27, 2015 4 Comments
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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Thank you for a very interesting QEX article.
There exists a certain interest for inexpensive VHF band-pass (saw) filters (kits and/or PCBs) tailored to reduce interference and receiver overload of RTL-SDR based AIS receivers (161.975 MHz – 162.025 MHz) from FM broadcasters (second harmonic) and nearby paging / trunk users. Many of these (AIS) receivers use RTL-SDR and Raspberry Pi units to create cheap network of internet-connected receivers around the world (e.g. http://www.marinetraffic.com/).
How hard would it be to adapt your circuit for 162 MHz center frequency?
73!
In principle, the design can be adapted, but this depends on the availability of a SAW filter that is suitable for the application. Searching digikey, I didn’t find ones that are narrow enough and with a passband that includes the AIS band. I did find two with a bandwidth of about 20MHz, but I don’t think that this will reject the paging/trunk users you mentioned.
Also, the SAW filter that I used had a 50 Ohm impedance, so it did not require any matching. At least one of the saw filters that I found on digikey for 162MHz did require matching. The matching network was specified in the data sheet, so no design is required, but it does mean a few extra components on the PCB.
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