lambda diode

hello

can anybody expalin me this circuit. it is called OLIVE-MARTINI BUG TRANSMITTER. because it can be constructed in a small space. but, i don't understand the operation of tunnel diode in his circuit. also please, tell me whether the circuit is correct.

Assuming that the 125 mV bias voltage is somewhere in the middle of tunnel diode negative differential resistance range (it actually is, as far as I know), the oscillator should work. Furthermore, the characteristic of the used electret microphone must provide some gain at the rather low operating voltage. Cause the buffer is a depletion FET usually, this can be expected for typical devices, I think.

I saw some reports at the internet from DIY experts, that where unable to operate the circuit. But they said the same about a standard transistor oscillator, so I think, it isn't a matter of the circuit.

If you can find the diode you may try it. The big disadvantage is the microphone strongly controls the bias. While in highschool I made a Hartley transmitter with microphone connected DC connected in the transistor base and it was very unstable after a powerfull sound, i fixed it by coupling the mic signal in AC with a cap.
I think it's better to do bias with a potentiometer in the transistor base, bias separately the microphone through a resistor, and couple the modulating signal with a capacitor, not to disturb the bias setting needed for oscillation.

Another negative resistance oscillator is the so called lambda diode, formed from a p jfet and n jfet. This may be simpler to construct and may give higher power.


http://web.telia.com/~u43200663/blocks/lambda_diode.htm
http://users.tpg.com.au/users/ldbutl...esDipMeter.htm
http://en.wikipedia.org/wiki/Lambda_diode
http://www.zen22142.zen.co.uk/Theory...nce/negres.htm

The lambda diode circuits are interesting, however none of the shown would be able to operate from an 1.3 V battery. Clearly, Ugs,th (depletion FET cut-off voltage) is the determining parameter. Even with a low threshold values, you won't ever reach the characteristic voltages of a germanium tunnel diode. A single JFET in contrast can have a considerable gain also at low supply voltages.

Your right regarding the disadvantages of biasing the oscillator through the electret microphone, I think. Minimum part count is apparently motivating the said circuit, but at the expense of a rather unstable bias point.

As an additonal remark. Operating a tunnel diode transmitter from 125 mV DC with a 1.3 V battery means discarding 90% of available efficiency irretrievably. Actually no good idea. A better performance can be expected from any single transistor oscillator.

Tunnel diodes have interesting properties in special applications, e.g. for very fast pulse generators.

To minimize size you may try a strange thing - make a microphone that direcly modulates the LC tank.
You can get a flexible membrane (egg - from small earpieces, remove the winding) use a thiner wire for L and glue a few of the turns on the membrane, and the rest on a fixed plastic surface. Also a capacitive microphone with conductive membrane may be tried.
Frequency stability will be very bad, though.

have you ever made a micro transmitter. here is an interesting link on the famous surveillance expert HALL LIPSET.

http://www.spybusters.com/History_1965_Hal_Lipset.html

here' a paragraph from the above site,on OLIVE-MARTINI BUG.

"When we came up with the "bug in the martini olive" idea, it didn?t seem all that unusual. We used a large bulletin board with all sorts of gadgets stapled or drawn or pasted onto it., and the martini glass was simply another example of how ingenious these devices could be. The glass held a facsimile of an olive, which could hold a tiny transmitter, the pimento inside the olive, in which we could embed the microphone, and a toothpick, which could house a copper wire as an antenna. No gin was used - that could cause a short."

tunnel diode S59.50! Guess I won't build one of these