Friday, 23 December 2016

Forty-9er Shield

I've been tinkering with a quick lash-up of Wayne Burdick, n6kr's famous 'Forty-9er' receiver, implemented on an Arduino shield and tuned by one of my DDS systems.

Regular readers will remember how I tried running my Kanga / m0xpd Sudden-inspired receiver shield under the control of the new DDS on the Internet of Things board and found the latter to cause a lot of noise problems. I stripped out unnecessary active stages in the Rx shield to try to manage some of the receiver's susceptibility to the hostile EM environment generated by the ESP8266 and realised that I was left with something which resembled not only a Sudden but also any other SA602 / LM386 receiver - including the Forty-9er.

Interestingly, there's a guy in Asia who produces a CW transceiver with an ESP8266, available as a kit or as an assembled unit through our favourite auction site. This transceiver is rock-bound, and is offered with an app for an Android phone, which allows automated sending of CW. There's some implication that it was supposed to allow automated copying too but, from what I gather, that side of things isn't working too well. The transceiver is based on - you guessed it - the Forty-9er. (He also makes units based on Rock-mites etc.)

I figured that the combination of my beacon work (which does all the transmit side stuff) and my WiFi controlled VFO already adds up to way more than the simple CW transceiver mentioned above, which was what motivated me to investigate the IoT DDS / Receiver combo. But - as readers recall - I found it impractically noisy.

Well - if the commercial unit is offered and (one idealistically supposes) works with the Forty-9er, perhaps I should try the Forty-9er circuit too...

I found that I could take an empty prototype Sudden Rx shield PCB (with tracking for the DIL format 602, rather than the production version SMD) and bodge it for the Forty-9er circuit. The Sudden and the Forty-9er receivers are so similar that the PCBs can be re-worked to serve both roles with almost no effort.

Here's my new kludged Forty-9er receiver, atop the Sudden Tx prototype shield (which is only serving as a power supply, so most of the pins aren't connected) - the new ESP8266 / AD9834 board lurks behind the stack:

Here are details of what I've implemented on the shield:

Those familiar with the original Forty-9er will notice I've been forced (by the PCB) to deviate from the original recipe in a few ways:
  1. My input filtering network is implemented off-board
  2. I'm taking the output from the 612 OUT_B (pin 5), rather than OUT_A; simply an inversion
  3.  I'm applying the input to the 386 inverting input (pin 2) rather than the non-inverting input; another inversion
  4. I've added the 10uF capacitor between pins 1 and 8 of the 386; experiment showed I needed more gain to drive the speaker 
 All of these measures are not significant departures from the letter or spirit of the Forty-9er design (excepting 1 - which does have an impact which cannot be avoided in terms of layout).

The limited experiments to date show that the Forty-9er is better than the original Kanga / m0xpd HF shield (with its extra active stages) and possibly marginally better than the 'bare' Sudden. However, there is still a clearly audible clicking, which isn't yet - by any means - perfect. But, at least we're now at the point where I have a usable receiver.

I've not yet got a big enough inductor on the input to the 386 (the 'junk' box didn't stretch that far), so there is some performance gain to be expected in the original circuit when I get the correct magnitude inductance. Also, there is more to be done in experimentation with shielding / positioning, etc..

I've also searched the 'net for any mention of the clicking in discussion of the performance of the receivers - but to no avail. These devices have a very narrow (crystal) filter on their input and may derive some benefit from that strategy. If any reader has any direct experience of these units - or can point me to any evidence - please get in touch.

Now I'm off to continue with the tinkering, to listen to more clicking and - in between it all - to get lost in last minute preparations for Christmas,

...-.- de m0xpd

No comments:

Post a Comment