![](http://4.bp.blogspot.com/_71-IkTH3uDw/TKhiade81uI/AAAAAAAADYk/GqFo56h94n0/s400/Microelectronics.jpg)
As regular readers will recall, the m0xpd beacon has been using the excellent SDR Kits USB synthesizer. Whilst being a perfect solution in a PC-based system, the USB Synth is not so handy in the context of microcontrollers (which don't have USB as a matter of course) or stand-alone beacons. An internal DDS would give me freedom from the PC on start-up (previously, I'd had to set up the synth via USB) and flexibility to make the beacon multi-band and frequency-agile.
I decided to use the same Silicon Labs Si570 device featured in the SDR Kits synth (not least because Jan, g0bbl, sells them). However, in making that decision, I was forced to accept a couple of challenges...
- the Si570 is 3.3V powered and so would need level conversion to interface to my 5V PIC environment
- the Si570 is a surface mount device - so I would have to enter the scary world of MICRO electronics - no mean feat for somebody who has worn spectacles since age five and has an intention tremor (at least I had an intention tremor - but that has been much improved since my physician prescribed Beta Blockers for migraine prophylaxis)!
![](http://1.bp.blogspot.com/_71-IkTH3uDw/TKhkjLnqCCI/AAAAAAAADYs/WOtsipIcWXM/s400/Level+converter.jpg)
I could only get hold of the MOSFETs in surface mount packages, so that really sealed the deal - I HAD to adopt surface mount technology! Anyway - here's the proof that the level converter circuit works as intended...
Although I'd never played with surface mount before (in an h/b context) my trusty Eagle PCB package could handle it effortlessly and a PCB design was soon produced...
![](http://1.bp.blogspot.com/_71-IkTH3uDw/TKhlSA4-oaI/AAAAAAAADY8/ybBVnjsF0Hw/s400/Si570+PCB.jpg)
(In truth this was the second design - I had not bothered to read the data sheet for the LP2950 LDO regulator I chose to generate the 3V3 line and so missed out the loading capacitor which stabilises it!)
Here's the finished board in the development context plugged into a breadboard...
![](http://1.bp.blogspot.com/_71-IkTH3uDw/TKhlzMlql2I/AAAAAAAADZE/yqRqu0xODs0/s400/DDS+on+Breadboard.jpg)
Once I'd confirmed operation of the hardware I could look at the software required to make the Si570 do its stuff...
The synth module was designed to interface to my controller board over the Si570's two-wire I2C interface. I was delighted to find that Andrew, g40ep had trodden this path before me and has generously posted a mine of useful information and software on the 'net. It is (as Sir Isaac understood) good to be able to hitch a ride on the shoulders of giants - thanks Andrew! Follow this link to see Andrew's Si570 resources.
I borrowed heavily from some of Andrew's I2C routines (simplified somewhat by my bi-directional level converter on the serial data line). I decided, however, to take a simple brute-force approach to setting the Si570's frequency to get things running quickly...
I had noticed that one of the PC utilities provided with the original SDR Kits USB synth produced register contents associated with any frequency setting...
![](http://4.bp.blogspot.com/_71-IkTH3uDw/TKhnxZVgLII/AAAAAAAADZM/kx98HI9tYz8/s400/Si570+Hack.jpg)
Accordingly, I simply programmed my new PIC 16F873 in the controller board (the original 16F872 has been replaced, as I wanted to duplicate the system on the external breadboard and the junk box only had one 16F872!)...
![](http://3.bp.blogspot.com/_71-IkTH3uDw/TKhqOMcAJFI/AAAAAAAADZU/Vzulb8WANGg/s400/Reprogramming.jpg)
to send the "brute force" register contents to the waiting Si570 and - Hey presto - the beacon is up and running with its own internal DDS...
![](http://2.bp.blogspot.com/_71-IkTH3uDw/TKhqmbz6e0I/AAAAAAAADZc/flUFToNj7lk/s400/DDS+in+situ.jpg)
Michael, dl5ocd, was first to spot the beacon in its new configuration.
Now I'm ready to extend the operating flexibility of the system - but first I have to arrange some switches to tell the beacon what I want it to do!
This has been great fun (so much so that I've ordered a complete set of 1206 resistors and a set of caps) and I've learned a lot along the way.
...-.- de m0xpd
does the first circuit diagram operate only from right to left?
ReplyDeletedave
g4fre
Dave
DeleteNo - it is bidirectional.
73 m0xpd
You say "I could only get hold of the MOSFETs in surface mount packages". I use 2N7000, from eBay (£2.00 + £2.50 p&p for 50x). They are 'traditional' through-hole devices. BTW, the design seems to originate from Phillips - see AN97055 - which specifies that device.
ReplyDeleteRegards
PeteS
Hi Pete. I meant I could only get the BSS 138 in SMT - I wanted to start with the specified device. Pleased to hear the 2N7000 works - I've got heaps of those! However, it has been nice to use the small parts in making "quad" and "octal" versions of the level converter :
Deletehttp://m0xpd.blogspot.co.uk/2013/05/octals-and-octuples.html