Let's Design and Build a (simple) Analog Theremin!

Posted: 1/5/2021 8:22:51 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

"Strange different resistors in current mirror. A kind of magic?"  - Buggins

The resistor difference multiplies up the reference so it doesn't need to be so high (thus lowering total current for the oscillator) - kind of a Widlar in reverse.  I believe emitter resistors lower the dependence on beta, and also tend to increase the impedance of the collector.

"What is drive swing? Is 1.88V on scope for 2V or 3.3V?"

That's 1.88V drive with 3.3V VCC.

Posted: 1/6/2021 3:44:18 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

The latest 8 transistor oscillator is pretty nice, though yesterday my bench was inundated with some sort of noise that made it rather unstable out at 16.666ms delayed sweep, like 100ns or so of phase noise.  I swapped in the 6 transistor version and it was acting the same, so it doesn't seem to be the new buffer at fault, and it calmed down later last night.  This morning there is ~20ns of phase noise @16.666ms with 60Hz bursts superimposed on the drive wave.  If I kill the power the bursts are still there, so the coil is ringing to external mains interference.  But there seems to be a second interference component at work, and I've seen this sort of thing periodically through the years (crazy days and calm nights).

LC oscillators generally return one end of the coil to a low impedance, which might make one think would preclude this sort of behavior, but perhaps they are all susceptible to being pinged like this (i.e. an inadvertent ring down Q measurement).  I suppose sticking an antenna on a high Q coil and expecting calm seas is just sort of asking for it in the first place.

[EDIT] Here's a scope capture of the pinging / ringing:

This is just one example, it can and does happen at random locations anywhere on the drive waveform.  Zooming up, I see 4 cycles in 150ns, which is about 27MHz.  There are fairly long leads on my coils, perhaps this stray inductance is doing the ringing with the capacitance of the coil (above SRF L => C)?.

I need to get this off the breadboard and onto proper vectorboard.

Posted: 1/16/2021 7:29:38 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

8 Transistor ECL-LC Oscillator : The Vectoring

OK, finally got the oscillator on proper vectorboard:

And here it is connected to a 6mH coil:

With 3.3V regulator it's drawing 6.75mA, 220Vpp at antenna, 592kHz.  When I power it down the frequency just barely budges as it flatlines, really amazing.  Difficult to stall w/ insulated rod antenna, no starting issues.  Still seeing the pinging / ringing described above.  My 4 xistor buffer is boffo!  (It's a subsection of an audio amplifier I designed and built for an EE lab assignment in college.)

Posted: 1/28/2021 11:25:17 AM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

Ringing Source Found

It's my fluorescent desk lamp.  Turn it off and the oscillator waveforms are smooth as glass.  The bursts are at ~23us intervals, which is ~43kHz.  So it's not some random internal instability.  This is a great little oscillator, I turn it on most days just to watch it run for a couple of hours.

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