OK - granted, my `just for play' 30 turns of 26ga. around a tube with a sliding 4mm dia. ferrite rod-150pF tank ( osc. freq.~ 1MHz) has a large inductance range.
As I slide the rod in/out I observe that CFB breaks into `really bad' harmonic-like ampitude spikes of mostly 2Vp-p - dropping down to normal 1Vp-p every dozen cycles. (CFB=22pF)
There's also a `sweet-spot' where it's mostly normal 1Vp-p and stable - but still popping up a 2Vp-p cycle every several hundred cycles.
Inbetween those extremes are rod-positions where CFB cycles ramp up to 2Vpp (over 3-4 cycles) and then drop - sawtooth like. There are also positions where the jump to 2Vpp is sudden ( not ramping up ) and `clean' - with the cycle period going from 19uS (53kHz) to 25uS (40kHz). Sometimes there are 2 consecutive 2Vpp cycles.
I observed that my total `all in' to `all out' sensor freq. oscillator range goes from 1 to 2.5MHz. The half-wave tank cycles rise to about 2-3V. I noticed that amplitude is modulated up with a logrithmic-ramp shape at the same period as the CVB signal. The short drop from peak to valley seems to be a straight line.
The working range of sensor freq. seems to be from 1.05 to 1.15 MHz (100kHz). By `working' I mean that I see CFB cycles and conversions are being done.
I have 100uF tant., 10uF ceramic and .1uF ceramic for bypass caps - decoupled from a 5V USB supply (which is also filtered with 10uH) with 20 ohms -> Vcc=4.9V. Power is clean. Crystal osc=8MHz.
10 min. later... I just put 5 ohm in series with my coil - just to see what that would do. It oscillated to 1.5V half-wave peaks (Amp = 0x10, RPMIN=0x3F) but that seems to be less than necessary to generate a CFB signal. I'm a little mystified as to how others speak of having Rs=10 ohms- and it works- while my 5 ohm test does not.
Is this jitter a problem? Do you have any ideas about wherein lies its cause?
