What i have: an inductor of unknown value, but most likely in
the henry region, with resistance from 4K to 10K. It is a
grounded inductor, and has so many turns that it cannot stand
current thru the coil - as it would saturate. I would like to
make an oscillator that uses the inductor as one of the frequency
determining components,
[snip]
I wonder if it could be done with a variation of the
Baxandall sine wave oscillator. ie, Parallel-resonate
the coil and drive it with a switched constant-current
square-wave, with the polarity of the const-I being
swapped at each zero-crossing of the voltage across the
tank. Rough sketch below.
-+---+-----+- +Vs
| | R5
| R1 |
| | |/e
| +---|pnp C2
| | |\ +------||------+
| R2 | | |
0v--|\| | | C1 | RL L |
|S>---+ +--||--+--/\/\--////--+--0v
+-----+|/| | | |
| | R3 | | +------+
| | | |/ | | |
| | +---|npn | +-|\ |
| | | |\e | |O>--+
| | R4 | +----|/ |
| | | R6 |
| -+---+-----+- -Vs |
| |
+--------------------<--------------+
It's a pair of constant-current sources, alternately
switched by comparator/gate (S), which is driven by
the buffered sinewave across the resonant tank. C1
is a large dc-blocker and C2 is the resonating capacitor.
The Baxandall circuit requires a Q of roughly 5-10 for
best operation. So the design frequency will initially
be determined by Q = wL/R. With the rough numbers already
given this suggest an Fosc in the 6KHZ to 10KHZ region.
That then determines the value of C2, which will be around
500pF. C2 includes the Cstray of the inductor.
The impedance of the tank at Fres is Z = L/CR, so will
be up in the 300k region. With 15v supplies, and a few volts
across the tank, the required switched currents would then
be of the order of +/- 20-40 uA or so.
All numbers above winged on the fly, and to be confirmed.
An interesting possibility could be to use a single OTA as
the switching constant-current source. An OTA has a voltage
adjustable current output and this could be used to
stabilise the amplitude of the voltage across the tank.
C2
+------||------+
__ +Vs | |
0v--+---| \| C1 | RL L |
R1 |OTA>----||--+--/\/\--////--+--0v
+--+|__/| |
| | -Vs | +------+
+----|->--+ | | |
| +-------<---R2---|--+-|\ |
| | |O>--+
| C3 +----|/ |
+--R3--+---||--+ |
| | D |
| /|--+--+---R4---|<|--+
+--<O| |
\|--+ +---R5---|>|--+
| D |
0v-+- |
-+- -Vs
The same +1 buffer looks at the voltage on the tank
and now switches the polarity of the OTA's output
current via R2 and R1. R1 and R2 should probably be
sized for a 2V or so peak voltage across R1.
Actually I'm not quite certain of the last sentence.
An OTA has a linear (log) region of about 100mV or
so. So there may be a need to check that there is
enough loop gain for oscillation with R2:R1 ratios.
The additional opamp is an erroramp/integrator that
compares the half-wave rectified AC via R4 against
a reference current from R5, and sets the required OTA
output current via R3. R3 should be sized for about
100uA through it when the integrator is at pk +Vout,
(R3= 250k-ish with 15-0-15 supplies). R4 and R5 should
be sized so that the ratio of R5/R4 is about 4.7/1
(giving ACVpk= -Vs*2/3 approx).
Ummm... haven't a clue whether the above will work or
not. Suggest you SPICE it first.