Circuit : Rodney Byne, UK
Email :

Description
This is an experimental 60khz direct conversion receiver for reception of the UK MSF Time Signal from Anthorn, Cumbria. (The mast array can be seen on the right hand image.) For continued stability, the radio should be continuously supplied from a +13.8v bench power supply.

To achieve enough rf signal pickup, an outside long wire aerial is required, of at least 25 feet anchored to a pole. No aerial balun must be used.

Exploring the Anthorn MSF UK Time Signal.
This station which was relocated to Cumbria in 2007 from the once massive Rugby aerial site alongside the M1, is the atomic resonator wide-area standard time signal for synchronizing UK clocks and watches.

Here are some links to Anthorn information of background interest:

MSF link here: http://www.npl.co.uk/science-technology/time-frequency/time/products-and-services/msf-radio-time-signal
Wiki technical link here: http://en.wikipedia.org/wiki/Anthorn_radio_station
Anthorn area views here: http://www.visitcumbria.com/car/anthorn.htm


Notes
There are six NPN transistors all BC549C, but equivalents will be satisfactory.

Transistor Functions:

TR1 & TR2 form an asymetrical squarewave multivibrator local oscillator (LO), running at a frequency close to 60khz.

TR6 is the LO buffer stage, between the LO output and mixer/detector input.

TR3 operates as a LO-switched, mixer and detector.

TR4 is an af amplifier having a signal voltage gain of 10.

TR5 is an emitter follower, delivering a dc blocked, low impedance source to the pc mic input.

Prototype

RFC1 artificially lengthens the aerial wire and together with C2 acts as a low pass filter to broadcast stations. RFC2 inhibits detector rf leakage to the audio stages. RV1 reduces signal overload to the detector and helps to partially null the decoded MSF second harmonic.

The multivibrator output to the mixer is buffered, so varying lengths of aerial "long wire" or variations of signal strength, do not influence or load the oscillator. Its switching pulse output is designed asymmetrical, as formed by the circuit crossover components.

Ref the spectrum analyser sweep picture: As no signal information is needed at the lower and upper audio frequencies, then C4 limits the low audio frequency response and Tr4 & Tr5 together form a three-pole low pass filter, having a band stop at 10khz.

Operation notes
The DCF77 Component Module of the installed pc program SpectrumLab, will thus demonstrate to decode the MSF complex pulse sequence and indicate relative received signal strength and phase error. Also enabling the pc onboard clock to be re-synchronized to national standard, if the particular version of Windows operating system will allow.

While the whole circuit is centered on a 700hz output, the LO frequency unavoidably drifts with large changes in ambient room temperature. But afc action in SpectrumLab, tracks this drift without alterations in the program's performance. Manual variation of Ct1, allows compensation back to 700hz as required.

The prototype receiver proved particularly useful after building, as I needed an external reference source for adjustment within the ham radio slow scan picture program MMSSTV. This screenshot shows my laptop pc under test running the calibration aspect of that program and having precise reference MSF applied, to exactly set picture slant correction for true vertical straightness against the dotted green cursor line.

Sound Sample
When you press this button, you will hear a sixty second sample of what the MSF station actually sounds like as taken by the circuit above. The background noises heard are being picked up from around the outside aerial.