Hi.
I'm back in the UK for a few weeks and thought a fun project would be to build an MSF Time receiver. This is a signal sent out by the National Physical Laboratory (not far from where I live!) which encodes the date and time on a 60kHz frequency. Details of the spec can be found here
http://www.npl.co.uk/upload/pdf/MSF_Time_Date_Code.pdfNow, I'm interested in finding the best way to make the antenna. I've built an antenna and tuner for an AM radio before by putting turns on a ferrite core etc, but never for anything this low frequency, and I know much less about RF than I would like.
Currently I don't even have an LCR meter but will probably buy one soon so suggestions would be welcome ($150ish budget).
Is it as simple as putting 20-30 (more? less?) turns on a ferrite rod, measuring the inductance and calculating the correct capacitor for a 60kHz resonant frequency, and then hand tuning it a bit?
Best,
Matt
Ah, it depends where you actually live, the signal originates from the NPL but it's broadcast by the Anthorn Radio Station in Cumbria (Edit: Used to be Rugby, you could see it from the M1).
The antennas used by MSF controlled clocks these days tend to be short ferrite rod antennas with a the sort of number of turns you would find on a LW coil but with a relatively large value film capacitor (FKP) strapped to it. I suspect that the final tuning is done by adjusting the coil position on the rod.
I'm also working with DCF77 which is only 31Km away from me but changing to 60 KHz should not be a problem:
1) Get the biggest ferrite rod you can find.
2) Apart from 1cm at either end for mounting fill it with a single layer of fine gauge wire then measure the inductance. Because of the ferrite you will find that this is surprisingly high.
3) Work out the required capacitor for resonance where C = 25330/(F
2 x L) where C in uF, L in uH, F in KHz.
4) Use the next lowest standard value and keep in adding capacitors until you get resonance at 60 KHz
Edit, some shortcuts here:
http://www.6pie.com/electronic-theory/resonant-frequency.php
I don't want to buy a module because the whole point of the project is to develop it myself and learn in the process!
I'd like to start off by saying I'm a bit of a noob in these areas but I really want to learn.
@German_EE
So I managed to dig an old ferrite rod out of a radio, about 5cm long. I put about 50 turns on it and am getting ~95uH out of it. Therefore, I am putting it in parallel with 74nF. Actually the closest i could get was 75.2nF. I have a couple of questions.
1. Is this rod likely to be too small to get a reasonable signal? I'll probably go out tomorrow and buy a 12cm, 300uH ferrite rod aerial from maplin.
2. What is the best kind of amplifier to use to amplify the signal? My guess would be that most op-amps would struggle around 60kHz, so it would be normal to use some kind of transistor based op-amp.
3. I guess generally what would be the next steps to detecting this signal?
Sorry for the basic questions.
Don't worry about the questions, it's the best way to learn stuff:
1. Is this rod (5cm) likely to be too small to get a reasonable signal? I'll probably go out tomorrow and buy a 12cm, 300uH ferrite rod aerial from maplin.
Given your distance from the transmitter a 5cm ferrite rod is VERY small. I use a 30cm ferrite rod and I am only 31Km from the transmitter.
2. What is the best kind of amplifier to use to amplify the signal? My guess would be that most op-amps would struggle around 60kHz, so it would be normal to use some kind of transistor based op-amp.
A lot of modern op-amps such as the 5532 have a gain bandwidth figure of about 10 MHz so 60 KHz is not a problem. For my DCF receiver I use the following circuit
http://www.techlib.com/electronics/antennas.html but with lots of modifications. There is no Hi/Low switch and no varactor tuning, I just use the ferrite rod plus a suitable fixed tuning capacitor. Also the 1k resistor across pins 2 and 7 is replaced with a 77.5 KHz crystal to make a VERY narrow bandpass. Finally I use both output pins of the chip and run a balanced cable to the main electronics box.
3. I guess generally what would be the next steps to detecting this signal?
The amplifier above will give you a reasonable level of signal down the cable to overcome losses. The next step is decoding MSF and there you're on your own because I don't know how to do this step.
There are loop stick calculators (ferrite stick) out there somewhere .
The basic formula is the larger the better but for memory of those 12cm ferrite rod a first main loop /coil/antenna is around 200+ turns for VLF and down where you need it it'll be more (300+?) , then one generally uses a pickup coil of around 5 to 8 turns spaced around 5 to 10mm away (adjustable) on one end adjacent to the main coil , all those sizes can be experimented with .
Normally using a FET is the go for a simple circuit as it will not load anything down as the gate on the FET is high impedance which is attached to the pickup coil on one end ie:one end the earth and other to the gate and can either be via a capacitor or direct .
I'd suggest looking about for the easy stuff first via the net and go from there to what you want to do or is after , cheers ..
Currently I don't even have an LCR meter but will probably buy one soon so suggestions would be welcome ($150ish budget).
If you want a cheap do it your self LC meter I have this following one and it works really well and for me is quite accurate. I mainly wind up toroids for my radio projects (BITX20 etc) and it has been really useful. All the coils I wind (using an online calculator to work out the number of turns) always measure really close to what they should be - mostly within 1 uH. This is perfectly accurate for me and gets me very close to the required inductance (the circuits always need a tweak to get them spot on but this gets me off to a good starting point).
http://www.banggood.com/DIY-M8-LC-Digital-Inductance-Capacitance-Meter-Kit-p-976437.html?p=2D100314686672015046You do have to put it together yourself but it does not take long as all the values are marked on the board itself.
Just thought I would mention this as an option if you are looking for something pretty accurate but not expensive.
Ok so today I got a 12cm ferrite rod (
http://www.maplin.co.uk/p/ferrite-rod-aerial-lb12n). I've switched to a circuit using the TL592B, schematic attached.
I've ordered an LCR meter, I ended up going with an LCR45 and that should arrive in a few days. Until then I'm not confident in the value of the inductor, so I'm not really confident that I have managed to tune it well.
I thought I had tuned it ok, but I'm still not managing to detect anything useful when using the circuit above. It turns out I'm about 600km away from the transmitter which I guess is pretty far. I tried connecting 5 turns of about 15cm diameter in parallel with the input and that definitely boosted the signal, but still nothing noticeable in the 60kHz range.
I guess the next thing I'll do is wait for the LCR meter to come and verify that my tuning is about right.
Thanks for all the help so far.
Not sure about that link between pin 2 and pin 7. The Wenzel design has a 1k ohm resistor and I'm using a 77.5 KHz crystal.
The datasheet for the TL592B says "The differential gain is typically 400 when the gain adjust pins are connected together, or amplification may be adjusted for near 0 to 400 by the use of a single external resistor connected between the gain adjustment pins A and B."
So by joining them together I am achieving a gain of ~400. The amplified signal is not reaching the 1V peak so is not clipping. A 60kHz crystal will work too, but I don't have one to hand at the moment.
[edit] I suppose a crystal works because at resonance it is effectively a short (ie maximum amplification) and away from resonance it is a higher impedance (ie a lower amplification)? [/edit]