Receiver Design Details

 
 

From left to right, the main boxes are

  1. RF low-noise amplifier (LNA) and bandpass filters

  2. IQPro DDS VFO

  3. Audio signal processing


The little red box at the far right is the R2Pro downconverter. At the upper left is the power supply, which has a toroidal transformer with the usual rectifiers, filters, and three-terminal voltage regulators. In the center of the chassis is a small module containing output buffer amplifiers for the I and Q oscillator outputs that are routed out of the chassis to the transmitter.


The basis for the whole receiver is the R2Pro by Rick Campbell, KK7B. You can find information on it in his book, Experimental Methods in RF Design, and on the web. Kits have been available from Kanga, though he’s temporarily out of business. If you’re familiar with that design, my work will look very familiar.

What’s Inside

The front-end consists of a low-noise JFET amplifier with bandpass filters, exactly as designed for the R2Pro, which I upgraded to a multi-band front end. In consultation with Rick, I used five separate LNAs, followed by a buffer amplifier and attenuator. This extra stage provides a very stable 50-ohm source for the downconverter’s mixer. There is also a step attenuator right after the antenna for use when signal levels are extremely high.


I wrote two reports on this work that will tell you more than you want to know about front ends of this type. Part one covers the requirements and has lots of measurements on the basic JFET LNA. Part two covers the buffer amplifier and shows the overall performance as a system.

    R2Pro_LNA_Performance_1.pdf

    R2Pro_LNA_Performance_2.pdf


Since other folks are interested in multi-band applications of the R2, I teamed up with Craig Johnson, AA0ZZ, who went forth and laid out a beautiful circuit board. It’s a modular design that has space for two bands on a single board with an option for stacking several more for multi-band use. He also includes lowpass filters for a mating transmitter (up to about 100 W class), and decoding logic for automatic bandswitching when used in association with his IQPro DDS VFO (see below).


I built mine ugly style and stuffed it in the brass box shown below. Craig’s board set is much better but came along later. One day I’ll upgrade this. Electrically they are the same but mine doesn’t have automatic band switching. My S meter is mounted in the front panel; more on that later.

RF Front-End





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