There are many, many ways to implement a frequency 'mixer', and the choice
of which one to select is based on even more considerations and tradeoffs,
so it's a little diccicult to recommend a topology without knowing exactly
what the requirements are.

However, from your short description, it sounds like you're trying to make a
metal detector that works on the principle of an oscillator whose frequency
changes when the search head comes into close proximity of a metal object.
The way these usually work is that the search head has a rather large
inductive coil that is part of an oscillator tuned circuit (usually at a few
hundred KHz). The search-headd oscillator frequency is 'mixed' with a fixed
oscillator a few KHz different in frequency, producing an audible beat note.
When the search head comes near a metal object, the change in the oscillator
frequency produces an audible change in the audio beat note.

For this type of application you can use a very simple digital 'mixer' since
there is no useful information in the amplitude of either mixer input - the
only thing that matters is the frequency of the beat note. Assuming that
your two oscillator signals can be formed into clean square-wave logic
levels, I would suggest an HCMOS exclusive-or gate for this application,
followed by a simple RC lowpass filter prior to the audio ampliifier.

A little trick you might try is to connect an unused exclusive-or gate as an
inverter by connecting one input to the supply. Then connect a large value
(100K is fine) from the remaining input to that gate to its output. This
will bias the gate into a linear mode and the output DC voltage 'idle' at
approx 1/2 Vcc). If you capacitively drive this biased gate with your
filtered mixer output, it will have quite a bit of audio gain. You can use
the remaining gates as amplifiers in a similar fashion.

I'm sure that this thread will produce many other very good suggestions, But
this one is simple and pretty idiot-proof.

Joe
W3JDR


"e2" wrote in message
oups.com...
please help me with my design on metal detector.i need to mix my two
sine inputs(few hundreds of KHz)and extract their difference.i opted
for the bjt.my questions a

1.biasing
-- using the buil-in transistor of lm389,how do i bias it such that
it operate in "mixing" mode?

2.diode-based
-- can u share any other simple circuit to implement a mixer in the
low frequency range?


thanks!!!!