Frequency shifter for 'close in' ARDF:
When a hunter gets close to the hidden 'fox', the transmitter's signal can enter the hunter's radio through
it's plastic case rather than through the DF aerial. As discussed in Beginners only, without a directional
aerial the signal will appear to come from all directions at once. If the 'fox' transmitter is fairly powerful,
then this could happen when the hunter is hundreds of yards from the 'fox', so it won't necessarily be obvious where he is hiding.
A neat solution to this is to have the radio operating on a completely different channel to that of the hidden transmitter.
A 'signal shifter' is used to convert the transmitter's signal to this new frequency. The radio can no longer receive the
signal directly as it is listening elsewhere. The only signal path is through the aerial and 'shifter' to the radio's aerial
socket restoring the directivity of the system.
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How's it work?
Your radio already uses a similar system inside it's works. Called a 'mixer' these internal circuits convert the high
frequency radio signals to a lower, more manageable Intermediate Frequency (IF). More manageable, because it is easier
to amplify and filter these lower frequencies, creating a more sensitive and selective radio set.
By feeding both the radio signal, and a separate radio frequency oscillator into our external 'mixer' circuit we will get
an output containing a 'mixture' of both signals.
In fact this mixture will contain four main components:-
a). The radio signal
b). the oscillator signal
c). the sum of the radio and oscillator frequencies and
d). the difference between the two frequencies.
For example if our signal was 145MHz and our oscillator 1MHz then our mixture would contain:-
a).145MHz
b).1MHz
c).146MHz and
d).144MHz.
So now with an external box containing a 'mixer', you can see that it would be possible to tune our radio to 144MHz
(or 146MHz) and hear the 145MHz radio signal. But now the radio itself would ignore the powerful 145MHz signal!
The circuit:
Despite the complicated 'goings on' in the electronics, the circuit is surprisingly simple, and can be assembled in a couple of evenings.
Running from a single 1.5 volt battery this circuit comprises an oscillator Q1 and an output buffer Q2.
L1 and C6 provide the tuning elements for the oscillator, C6 setting the operational frequency, (nominally 500 kHz).
The oscillator's signal is buffered by Q2 which has a potentiometer in it's emitter, this allows the operator to
vary the amount of oscillator signal fed to the mixing diode D1.
Signals from the antenna pass through C7 and are combined with the oscillator signal at D1. They pass through D1
and appear at connecting pin J4, this combined mixture of signals goes off to the radio aerial socket.
L2 provides a d.c. path for D1 to ground.
NEW *Updated to RoHS compatible part numbers*
| ID | Value | Farnell Order Code |
Q1 | 2N2222 | 980-1278 |
| Q2 | BC107 | 920-6752 |
| D1 | 1N4148 | 984-3680 |
| R1 | 100K | 934-1129 |
| R2 | 3K3 | 934-1749 |
| R3 | 1K0 | 934-1102 |
| R4 | 47K | 934-1960 |
| R5 | 1K0 | 934-1102 |
| P1 | 1K0 | Linear(e.g.879-1287) |
| L1 | 470uH | 118-6794 |
| L2 | 2.2uH | 118-6787 |
Battery Clips (Pair) 908-733
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| ID | Value | Farnell Order Code |
C1 | 10nF Ceramic | 121-6425 |
| C2 | 470pF Polystyrene | 952-0058 |
| C3 | 330pF Polystyrene | 952-0040 |
| C4 | 2.2nF Ceramic | 121-6428 |
| C5 | 0.1uF Ceramic | 121-6445 |
| C6 | 5-65pF Variable | 121-5712 |
| C7 | 120pF Polystyrene | 952-0171 |
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Assembly:
Once you have acquired all your components then assembly is just a simple soldering job. I have developed a double sided printed circuit board for this project which makes assembly that much easier of course. If there is sufficient interest then I may make boards available.
I have not soldered a potentiometer directly into my board but have put termination pins in the holes, as I wanted to mount my pot on the side of my box.
Once you have soldered all the components in place, check to see that there are no solder 'bridges' between tracks, or any components in the wrong place or the wrong way round. Once happy with your construction, you are ready to begin testing your 'shifter'.
Clip an AA size battery into it's holder and momentarily connect a milliammeter between the two 'switch' pins on the board. The circuit should take no more than 150 micro-amps. Remember to protect your meter, start this measurement with your meter on a high current range and increase the sensitivity once things seem fine!
If the current seems in order then the next step is to see if your circuit is generating signals. The easiest way to do this is to use a Medium Wave radio and listen for the 'shifter's oscillator. Tune your broadcast radio to around 300 metres, (which is twice 500kHz on the Medium Waveband), and switch on your circuit. Place your circuit next to the radio and tune the radio until you hear a whistle or quieting in the radio loudspeaker. Prove that this is your 'shifter by either switching it on and off or by adjusting the shifter's variable capacitor and listening for the signal change.
If you are fortunate to own an oscilloscope then connect it's probe at the junction of Q2 emitter and P1. There should be a fairly sinusiodal 500kHz waveform about 1.5Volts peak to peak. Now move the probe to the RF OUT pin and there should be a rather distorted waveform 150mV p-p.
Build your 'shifter' into an all metal enclosure with only holes drilled for the potentiometer, switch and connectors. It is very important to build your shifter in a metal box as screening is paramount. If you choose a plastic enclosure, the unit will reintroduce direct signal pickup, which is exactly what you were trying to avoid in your radio.
Operation:
The 'shifter' can operate in two modes.
1). Primarily designed to be operated with the radio tuned to the shift frequency, the shifter can provide quite a degree of variable attenuation via the control potentiometer. By reducing the amount of oscillator signal, the shifted signal can be made weaker and weaker until it cannot be heard. Operating the unit in this way gives a double benefit, no direct signal and variable attenuation of the shifted signal.
2). Switch the shifter off and retune the radio to the signal frequency. The potentiometer still acts as an attenuator when you are further away from the hidden transmitter.
In practice:
As long as you have followed the instructions the device should function reliably. I have built several of these units without any problems.
The 'shifter works equally well on 433MHz as on 145MHz and is particularly useful for operation in on-foot foxhunts.
copyright © silent design associates. 2001. all rights reserved.
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