Shown on the left (click it to enlarge) is the circuit diagram for the receiver part of the clock – click it to make it readable or download it. This schematic is essentially a sensitive 60kHz receiver and ASK demodulator. You could use it as a simple receiver for other frequencies by just taking the signal at point 4 and adjusting the Inductor (antenna) and shunt capacitor to match the frequency you wish to receive.
The circuit should be fairly self explanatory. Firstly the power for the receiver comes from the PSU/decoder, and can be around 13-20V. A 7809 9v voltage regulator brings this down to 9v for our needs, there’s no need for a heatsink on the regulator if it’s the 1A type. As far as receiving the signal is concerned, the ferrite antenna and two shunt (parallel) capacitors adjust the frequency that they resonate at, according to this equation:
We can’t really change L, it’s set by the ferrite rod antenna (code LB12N from Maplin) Once you have found a C value to set f as 60kHz (I found C to be around 840pF), they will resonate and you’ll be receiving any 60kHz signal in the air. This signal is obviously tiny though, so we amplify it. The first stage of amplification is a JFET (2N3819). This has an extremely high input impedance, which is exactly what we want. The JFET’s gain is set to 10 by the 1k and 100R resistors. After a DC blocking capacitor there are two identical stages of op amp amplification. The gain of each op amp is set to -214 (the – indicates that there will be a phase shift, we don’t care about phase so that’s ok). Each op amp also acts as a low pass filter, attenuating frequencies higher than 72kHz, which helps us a lot.
After this we’re at point 2 in the schematic and we have a big 60kHz signal. This signal is turned into a DC level by the use of a low voltage drop diode (a germanium type would be ideal, silicon types e.g. 1n4001 are not suitable) and a 10n capacitor. The resulting DC level is then fed into a comparator, a low pass filter to weed out any fast spikes, then another comparator. The levels of each comparator can be adjusted. Simply adjust each 10k potentiometer until you get a decent signal. The LED will flash at 1Hz when a good signal is received.
The best way of setting the circuit up is by the use of an oscilloscope. If you live near Anthorn then you can ditch one of the TL081 op amps. The shunt capacitor value next to the ferrite antenna will need fiddling with to get a decent signal, and you may find that the resistors around the comparators may also need adjusting, the 10k potentiometer may not have enough adjustment for your needs.
So there you have it – the schematic for a sensitive 60kHz receiver. Feed the output of this into part one of the schematic and you’ll have a perfectly working, completely accurate clock!
The circuit could be improved significantly by ditching the germanium rectifier diode in favour of a system that does not give us a voltage loss. There are plenty of example circuits around for op amp based rectification circuits, but I didn’t have the veroboard space to try some other rectification methods.
Hi Andy,
Thanks for sharing your work. I have a comment. In your schematic it appears as though test points 3 and 4 are pointing at the same node yet the pictures you drew of the output are different.
Thanks for the comment – good point!
If you were to remove the 10n capacitor then the trace would look like picture 3, however with the 10n capacitor there, you get the trace as shown in picture 4.
My mistake when drawing it from left to right!
I would like to build the circuits described, can you please email a schematic to me at Karl.kuker@thelincgroup.com or karl.kuker@gmail.com.
Thanks
I should probably make it clearer, but if you click the little picture of them on the post they should enlarge. Direct links are below:
http://www.burningimage.net/clock/wp-content/picclockparttwo.jpg
http://www.burningimage.net/clock/wp-content/picclockpartone.jpg
It pains me to look at the scruffyness of them now, but I’m sure you’ll be able to work it out!
Let me know if you have any problems, I’d be happy to help.
Andy:
Isn’t “MAPLIN” a UK-based parts supplier?
Would you have any Part Numbers for a ferrite core from either “Digi-Key” or “Mouser” here in the U.S.? (www.digikey.com and http://www.mouser.com).
Since publishing your original “60KHz Receiver”, have you made any improvements to your circuit? Or, do you know of any other 60KHz receiver schematics that would serve as a good “Receiver/Clock” circuit?
I would like to build a WWV receiver/clock that can demodulate the WWV code and then drive a series of 7-segment LED displays, showing the “Year, Month, Day, Day of Year, Hour, Minute, Second” and anything else that could possibly be displayed. Would you have any information on anything like that? If so, I would GREATLY appreciate in receiving whatever you may have.
Regards,
JBWilliams
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JBWilliams,
Maplin is indeed a UK parts supplier. I will have a look at the digikey and mouser websites and see if there are any suitable ferrite cores available – I’ll get back to you.
With regards the WWV code, it looks like it may be a little more complex to decode than the MSF (UK) code, because it is on a -100Hz subcarrier. The “Digital Time Code” of the wikipedia article may help you a bit: http://en.wikipedia.org/wiki/WWV_(radio_station). You will probably need some decent filters to isolate the subcarrier and ensure you aren’t swamped by the main carrier. There is a little information on this site: http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver36.html.
With regards driving the 7 segment displays, the best way to do that is with a purpose-made encoder device. I used a MAX7219. They communicate via the SPI protocol and are bound to be widely available.
I’ll get back to you with regards the ferrites.
Regards,
Andy
A better wiki for the US station – http://en.wikipedia.org/wiki/WWVB
The modulated subcarrier (IRIG) does not apply to the 60 KHz transmission
Hello Andy, what a super project..I am wondering though, can I DIRECTLY replace the reciever part here with a commercial “ready made” MSF reciever module?
Thanks
Jan
Yes, you can. You need to make sure you use the ‘inverted’ output of the receiver module. You want the signal to be high at the start of the second and low at the end. Every MSF module I’ve seen works in this way so you should be fine.
Hello, I was thinking of trying this circuit as a project in my 6th form, but I only have access to 2n7000 MOSFETs, and I was wondering if you knew if this would work for the JFET you used there? I imagine it would as you can set a gain on a MOSFET of 10. Otherwise, could I use a voltage follower MOSFETT then another op amp to set the gain to 10? Also, I was reading through your explanation, and I was wondering if the 100nf capacitors on the two op amps’ 47K resistors and the JFET in the receiver part of the circuit were just to smooth the supply, or what other purpose they serve? Thanks for the help.
The 2N7000 MOSFET isn’t really ideal I’m afraid as it’s an enhancement mode device with a threshold voltage of 2.1V. Given that we are feeding the input directly from an antenna, we are never going to see the input get up to 2.1V, so in all likelyhood the mosfet will never start to conduct.
The 2n3819 device in the circuit is an n-channel depletion mode JFET device. Ideally you would use a device similar to this I’m afraid!
To be honest, if you had to you could ditch the mosfet stage and drive directly into the 220r resistor at the input of the TL081. Ideally though you’d see if you could get your hands on an n-channel JFET! Is there a supplier you can use? Farnell/RS/etc. will stock lots of suitable devices.
I’m not sure which capacitors you mean. The 100nf capacitors that lead to pin 2 of the op-amps are DC blocks – they remove any DC offset in the signal at that point in order that the op amp amplifiers within its DC bounds. Simply speaking, amplifiers need to be fed with signals that centre around 0V, DC blocks ensure this.
The 47p capacitors on the 47k resistors for a low pass filter, filtering any signal over around 70kHz that we aren’t interested in.
Let me know how you get on.
Are you grounding the -Vcc of your op amps?
Yes, they are grounded. The op-amps are biased to around 4.5V however by the potential divider on the non-inverting input (pin 3).