JunkBoxMsf is an MSF Radio Receiver / Clock built from parts I've had lying around in my Junk Box for several years and put together in an extremely short amount of time. It is a grossly simple TRF receiver based around Microchip's dsPIC30F2010 DSC MCU - 4K of program words, 512 bytes of RAM, 30MIPS and a 10-bit ADC.

The project was put together for the practical assessment of my Intermediate Radio Licence. I spent six weekends designing and building the hardware and writing the majority of the firmware for this. Ordinarily I specify and order parts as I need them, as well as get PCBs fabricated by a professional board house, but the turnaround was too tight for this project. Thus, all parts are what I had lying around in my Junk Box and not ones I'd have chosen if I'd done this 'properly'. The dsPIC30F2010, for example, is a motor controller from a previous project involving stepper motors, and the PCB is mainly through-hole and was made at home. The whole project cost £0.00, with the membrane switch donated by Dave (thanks !)

Since submitting the project for the exam I have spent a further two weeks adding a menu system, the clock synchronisation logic and generally testing, tweaking, polishing and turning the device from a demonstration receiver into a proper clock. The initial short turnaround has meant that both the hardware and firmware have been hacked together, thus no TDD :(

This is mainly a digital project - the analogue portions are nothing more than op-amps and a power supply. I even omitted filtering to save time, although there is a header on the PCB for adding a daughterboard for this purpose. The simplicity due to the lack of filtering comes at the expense of decreased SNR and increased aliasing, and I rely on the selectivity of the ferrite rod, choice of sampling frequency and the fact that the MSF signal in this area is relatively clean. All the real magic is DSP.

The 60kHz MSF signal is decoded via an undersampling (80kHz) Goertzel Detector algorithm, which also synchronises the on-board 32.768kHz crystal with the MSF minute marker to achieve a time (typically) accurate to within several tens of milliseconds of UK time. Periodic resynchronisation is done every few minutes or few hours (configurable via the menu system) to correct for the tolerance and potential drift of the local crystal (30ppm) and any intervening leap seconds that may have occurred.

The current time and date are displayed on the LCD, along with a drift indicator and any synchronisation information such as ADC channel and whether the last attempt was successful or not. There is a single push-button for menu navigation and turning on the LCD backlight.

The whole thing runs from a (nominal) 12Vdc wall wart and draws about 90mA when receiving, 15mA in normal operation, and 11.5mA if the LCD is also turned off. This equates to about 140-180mW at 12V, plus wall wart losses - there was no provision (or thought) about turning off the analogue receiver to reduce consumption further. The parts in my Junk Box are typically old DIPs and not for low power operation. The backlight on my LCD draws about 120mA.