Simple Fox Hunt Attenuator


nanda kumar s:
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Dale Hunt:
Hi Nandu,

A couple of suggestions based on my experience using active attenuators.

First, there are two operating modes - one where the DC though the diode is used to form a
variable attenuator on the signal frequency, and a second where the oscillator is used to
offset the frequency.  Both modes are useful:  the first is better when you don't need much
attenuation, and the second when the signal is stronger.  In your design (and some of the
others I have seen) both modes operate simultaneously.  This can cause problems when the
desired signal is weak and there is a strong signal offset by a harmonic of your oscillator
For example, I was trying to hunt a signal on 146.55 MHz with a 4 MHz oscillator and the
signal was overpowered by the local weather broadcasts on 162.55 MHz.

The solution is to split the DC and RF operation of the mixer.  I use the same pot, but add
a switch to apply either 0.7V DC (from the battery and a series resistor) or the oscillator
signal to the top of the pot.  This way I do not hear the interfering signals on other
frequencies when I am using it in "straight-through" mode.  By the time I have to switch
to "offset" mode and retune the receiver, the desired signal is usually strong enough that
other signals are no longer a problem.  This simple change makes it much easier to use.
Ideally the DC mode would be set up to bias the diode into conduction (minimum attenuation)
then reduce the forward voltage to zero (maximum attenuation):  there may be some advantage
in arranging for reverse bias to increase the maximum attenuation, but that isn't as convenient.

You can put different coloured LEDs in series with the battery lead for the two options to
give an indication of which mode is active.

I like to use a double balanced mixer rather than the single diode, but I do not know
if the improvement is really that much.  Using a silicon switching diode (such as a 1N914 or
1N4148) rather than a germanium OA90 will give a better range of attenuation.

Happy hunting!

Dale Hunt:
When you do, you'll probably end up with a series resistor from each source (DC and RF) to the
top of the attenuation control.  Careful adjustment of the values of these resistors will
optimize the attenuation range.

First, in DC (or "through") mode, you have to know the forward voltage drop and maximum
current capability of your diode.  For a silicon switching diode this is about 0.6V, and the
forward current may be 25 to 75mA.  For some Schottky diodes it can be 0.3V and 10mA.
Adjust the resistor so that, with the control set for minimum attenuation, the voltage is
sufficient to turn on the diode and limit the current to a safe range - 5mA perhaps.  If
the current is too low the loss at minimum attenuation will be too high, while too high
of a current will destroy the diode, or at least run down the battery faster.

For RF, the series resistor limits the amount of RF applied to the control.  Having the
"through" range available means that the minimum attenuation required in the "offset"
mode is less.  Ideally you would have about 10dB of overlap between the two attenuation
ranges.  By limiting the top end of the range you gain more adjustment at the low end:
there is no sense having to turn the knob down half way before the circuit starts
attenuating.  Experiment with this resistor to find a good balance - I generally start
with a resistor that is larger than necessary and solder others in parallel with it to get
the right value.

If the hidden transmitter is too strong and you STILL can't reduce the signal far enough,
add a switch across the input to the attenuation control that shorts the RF to ground
(though the series resistor).  The switch leads usually have enough inductance that
there is still some small amount of RF applied, which allows the attenuation to be
increased even further:  with one of my attenuators at maximum attenuation I'm only
applying a few millivolts of RF to the mixer.  (Using a balanced mixer helps a lot at that
point, too, as it reduces the fundamental RF input so it is doesn't overload the receiver
as easily.)


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