24 Nov 2012

Simple TRF based AM radio ICs

Many years ago when my sons were little I made them a couple of Medium Wave AM radios using just a ZN414 3-pin TRF receiver IC. It worked very well considering how simple the circuit was using just a small ferrite rod antenna and a crystal earpiece. This IC is no longer available but there are other similar parts available such as the MK484. Although I have not tried one of these as an IF stage I have no doubt they could be useful in simple rigs like the Tenbox currently under development. The advantages over a super-regen would be selectivity. These devices have input impedances of around 4Mohm so if a ceramic filter was to be used ahead of the IC a step-up transformer would be needed to minimise loss. As they only work to around 3-4MHz they would only be suitable as an IF in the Tenbox 10m AM transceiver.

A useful page I've just found on how to get the best from these simple ICs is http://theradioboard.com/best-of-the-best/mcgillis-mk484.htm.  The datasheet for the MK484 is available here.

A page showing the use of the MK484 as an IF stage in a simple 160m RX is  http://www.vk6fh.com/vk6fh/mk484radios.htm . See also http://www.youtube.com/watch?v=qdlXVKOITe4. I am told that a similar design appears in 'Radio Projects for the Amateur' Volume 4 by Drew Diamond VK3XU. 

I'm still likely to stick with a very simple super-regen RX in the Tenbox - they ARE the best in the intended application - but I may make it in a modular form so that different TX strips and RX strips can be used and compared.  I can see a whole series of simple AM designs in the pipeline, HI.


aa7ee said...

I have happy memories of the ZN414 IC Roger. I built a MW receiver in an England's Glory matchbox with one using a mica compression trimmer for the tuning, and a crystal earpiece. Sitting in the back of class, I used it to listen to the cricket scores, wrote them on slips of paper, and passed them around the classroom. Eventually, it was confiscated by the deputy head, and all that saved me from punishment was the fact that he was fascinated by what I had built. Good times!


David Cope said...

Glad you reminded me of my distant memories of TRF ICs. I love the sound of regens on AM and was only thinking the other day (inspired by your TenBox efforts) about a TRF IF application.

Look forward to your design.

David. G8JGO.