MSF detector - turning the radio signal into data


Posted on Feb 5, 2014

The MSF transmitter sends time data bit-by-bit over the 60 seconds of each minute by dropping the 60 kHz carrier. It`s a basic CW signal [1]. Two bits are transmitted every second by modifying the length and number of carrier drop(s) per second (see the second article in this series). As is often the case in timing circuits a negative logic is use


MSF detector - turning the radio signal into data
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d. When the carrier is ON the data being sent is a `zero` and when the carrier off it is a `one`. It is the drop in career that is accurately timed i. e. the ON to OFF state rather than the OFF to ON state. I suppose dropping the carrier (ON to OFF) is a clearer transition than the OFF to ON transition, as fading might confuse the exact ON transition but the carrier going fully OFF is quite distinct. Reading the data sheet from the NPL it appears that they do more than key the carrier ON and OFF to get the precise signalling. This may be because the high Q antenna / output circuit will `ring` for too long after the transmitter keys off. They imply that the ON / OFF data is a combination of data pulse and actual switching of power to the antenna system [1]. In principle we can take the changing voltage from the receivers automatic gain control (AGC) i. e. the S-meter circuit as the basis of the data detection signal. The S-meter voltage can be feed into a comparator / trigger to convert the change in signal strength into logic level (0 and 5V) data pulses. If the MSF signal is very steady and strong (i. e. if you are relatively near to the transmitter) then this will work well. This is shown at the top of the first picture. A little bit of hysteresis built into the comparator circuit will help to stop `chattering` at the transition edges at switch ON and at switch OFF. The main problem is when the signal strength is weak or...




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