Testing the TK Tar Baby

[picowatt]

TK,

Fresh minty breath and a NMP, wintergreen cool!

Any idea how much Ibias (current draw) changes with the zener removed? 

You might consider a trimpot for R3, element between batt+ and gnd and wiper to the gate.  That will let you set the gate voltage at will.  Also, I would again suggest a small source degeneration resistor from source to gnd just to improve Ibias stability as temp changes (2R-6R).

Caveat:  Without the zener, Ibias will change as Vbatt drops.  You could try increasing the zener voltage a bit, two 3.3V in series for example.  It may not be all that bad though to have Ibias decrease as Vbatt drops and the batt ESR increases.

Way to go TK and .99!

PW

[poynt99]

Yes,

The bias current increases to about 30mA (from about 20mA) without the zener installed, and as PW mentioned, it will most likely be more stable as the battery voltage changes with the zener in.

[TinselKoala]

What makes you think Vbatt is going to drop? It is still sitting there oscillating away and I see "no detectable change" in the battery voltage.

Why don't we have a tongue in cheek smiley?   -)



The thing is a nice little RF oscillator with enough radiated power to show up fine on the shortwave as a carrier, here on the bench. I'd still like to put some modulation on there for demonstration purposes. This would involve simply chopping the signal at an audio frequency; the osc duty cycle could still be 80 percent or more and the modulation would be hearable on the shortwave at around 3.6 MHz.

I do like the idea of having the Zener in there. But now... for some reason, now it _won't_ oscillate at all with the Zener in! I think maybe that earlier initial problem I had getting it started might have been a cold solder joint even, that actually got worse instead of better and by failing to keep connected it allowed the circuit to work! I've checked the Zener, replaced with another, still the same behavior. I think I will try a higher voltage and see what happens.

Hmm... a 5.1 Zener causes some interesting stuff.

[picowatt]

What makes you think Vbatt is going to drop? It is still sitting there oscillating away and I see "no detectable change" in the battery voltage.

Why don't we have a tongue in cheek smiley?   -)



The thing is a nice little RF oscillator with enough radiated power to show up fine on the shortwave as a carrier, here on the bench. I'd still like to put some modulation on there for demonstration purposes. This would involve simply chopping the signal at an audio frequency; the osc duty cycle could still be 80 percent or more and the modulation would be hearable on the shortwave at around 3.6 MHz.

I do like the idea of having the Zener in there. But now... for some reason, now it _won't_ oscillate at all with the Zener in! I think maybe that earlier initial problem I had getting it started might have been a cold solder joint even, that actually got worse instead of better and by failing to keep connected it allowed the circuit to work! I've checked the Zener, replaced with another, still the same behavior. I think I will try a higher voltage and see what happens.

Hmm... a 5.1 Zener causes some interesting stuff.

TK,

Most circuits operate on the smoke intrinsically contained within the components.  It may be that your new circuit operates on external volatiles contained in the mint container and they have all evaporated.  Possibly crumble a mint and sprinkle within.  Be careful not to mix flavors, results can be unpredictable.

You mentioned grounding/not grounding the can.  Are you still "ungrounded"?  What changes when you do gnd the can?

PW

[TinselKoala]

All these zeners test good, the magic smoke still inside, and in the mosfets too. I got 4 of the 3.3 volt Zeners, cut out the first one just on spec and replaced it with another new one. No change... it still just doesn't want to oscillate with the 3.3 Zener in there. I suppose it's possible that all 4 of these are bad... but they test the same on my diode checker. The circuit definitely oscillates with the 5.1 V Zener in there... quite a bit lower f0, big spikes, definite ringing but the waveform looks less like it will make the Negative mean power product. The only lowvoltage Zeners I have in stock are the 3.3 and the 5.1, but I found some different 3.3's in the box that I can try from another manufacturer.

The box... Right now I've soldered a 3" cliplead on the circuit's common ground terminal and can use that to clip to the box or not. I've also soldered a 0.01 uF cap to the box itself, so I can clip to nothing, to the cap, or to the side of the box directly. There seems to be an issue with the metal case of the battery. When I have the battery sitting in the box and I clip the circuit common to either the box or the little cap, the waveform gets really complicated and my scope can't trigger cleanly on it. But if I remove the battery from the box and just set it alongside, then clipping the circuit common to the box slightly smooths the ripples and I think gives the nicest looking and most stable waveforms. So there is some issue with coupling to the metal case of the battery. When I built the thing I put a sheet of thin plastic underneath the circuit board itself but didn't think that the battery area would need insulation... so it is too tight in there now to add any easily. But I'll tear it apart and get something lining the battery compartment to insulate the battery case, and maybe that will allow "grounded" operation with stability.