Tuesday, February 5, 2013

Metal detector part 3, Darlington emitter follower

One thing I suddenly realized when trying to figure out what frequencies we have in the oscillator nodes, the probes will affect the circuit. The Tektronix probe has 10M\(\Omega\) and 10pF. Our nodes have about 2 pF and between 20k and 1.5\(\Omega\), so they will change the effective values. What we need is the Darlington emitter follower, which will introduce about the same impedance and capacitance as the probe, but with the difference that the follower will always be a part of the circuit.
The circuit now looks like this:

 The output from Q4's emitter is seen here:

The two outputs show the mixed down frequencies when the local oscillator is tuned to its two extremes.
The lowest frequency is about 13kHz, which is probably a bit high. But let's wait until we can listen to the signal until we fine tune it some more.
The next step will be to define a push/pull amplifier for the loudspeaker. By the way, a loudspeaker might draw a bit to much power, so I'll see if I can use a summer from a PC motherboard instead. I'll need to measure its impedance first, to make the calculations for the amplifier.