Showing posts with label mk484. Show all posts
Showing posts with label mk484. Show all posts

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.

21 Feb 2012

A 160m AM local natter rig

Further to my post a few days ago about possible bands to use for a very local "natter link" over a 3km path between G6ALB and here, my thoughts are turning to 160m, partly for nostalgic reasons. This is where I first started my ham radio operating with a small 400mW AM transmitter from Practical Wireless (Dec 67 I think).

Although 160m is VERY noisy now, especially at night, I figure that 100mW AM will be enough to kill the QRM/QRN over such a short distance. So, I am gathering ideas for something incredibly simple to build.

For the TX the most sensible rig would be a crystal oscillator (or ceramic oscillator) followed by a buffer and series modulated PA using the modulation circuit from the 2m AM Fredbox. On receive, either a regen receiver (operating just out of oscillation) or even an MK484 (like the ZN414) single chip receiver are worth a try.

This would be no DX rig, just a very basic transceiver to get me into the next village and not much further. I'll keep you posted. Incidentally I am unable to do much building right now as my house is in the middle of some redecoration. I can't find anything!