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micropower hartley oscillator

S

Steve Taylor

Jan 1, 1970
0
We have an instrumentation application where we need to have a very low
power RF oscillator (30-50 MHz) sitting spinning on a small shaft. The
frequency of the oscillator is selected by a pair of capacitors which
are switched externally by a magnetically triggered reed switch. Our
exisitng brush-based system is a.) expensive b.) fiddly. c.)
unreliable. The existing system takes the RF back through the sliprings
to an amplifier for conditioning - we are only interested in the
frequency of the return, not its amplitude. The transmission distance is
about 12mm, 1/2".

We wondered whether we could use a solar cell to power the circuit, but
the typical output voltage is only around 0.7 volts, and there is not a
whole heck of a lot of space to get more in - the board diameter is no
more than 38mm (1.5 ")

Would LEDs of the same type as the emitters make efficient photo-cells ?

Ideally we would like to couple enough energy to modulate an LED
(different colour) at the RF frequency, to pick it up on a filtered
photodiode.

Any suggestions or comments would be gratefully received.

Thanks

Steve
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Steve Taylor
We have an instrumentation application where we need to have a very low
power RF oscillator (30-50 MHz) sitting spinning on a small shaft. The
frequency of the oscillator is selected by a pair of capacitors which
are switched externally by a magnetically triggered reed switch. Our
exisitng brush-based system is a.) expensive b.) fiddly. c.)
unreliable. The existing system takes the RF back through the sliprings
to an amplifier for conditioning - we are only interested in the
frequency of the return, not its amplitude. The transmission distance is
about 12mm, 1/2".

We wondered whether we could use a solar cell to power the circuit, but
the typical output voltage is only around 0.7 volts, and there is not a
whole heck of a lot of space to get more in - the board diameter is no
more than 38mm (1.5 ")

Would LEDs of the same type as the emitters make efficient photo-cells ?

I don't think so. But you can get proper photodiodes in very small
packages and put a few in series.
Ideally we would like to couple enough energy to modulate an LED
(different colour) at the RF frequency, to pick it up on a filtered
photodiode.

Why don't you just pick up the radiated RF directly? Even with a flea-
power oscillator, there is plenty to detect at a range of 12 mm. I don't
think you can modulate an LED at 30 MHz.
 
S

Steve Taylor

Jan 1, 1970
0
John said:
Why don't you just pick up the radiated RF directly? Even with a flea-
power oscillator, there is plenty to detect at a range of 12 mm. I don't
think you can modulate an LED at 30 MHz.

We tried coupling it off to air cored transformer, but we got damn all
signal, my RF engineering knowledge is almost non-existent, and we ended
up using MAR-6 or MAR-8 devices to get the signal up to the point we
could clip it for the next stage.

Its a marginal design at the moment, and certainly not one either of us
here likes !

Thanks for your comments

Steve
 
J

John Larkin

Jan 1, 1970
0
We have an instrumentation application where we need to have a very low
power RF oscillator (30-50 MHz) sitting spinning on a small shaft. The
frequency of the oscillator is selected by a pair of capacitors which
are switched externally by a magnetically triggered reed switch. Our
exisitng brush-based system is a.) expensive b.) fiddly. c.)
unreliable. The existing system takes the RF back through the sliprings
to an amplifier for conditioning - we are only interested in the
frequency of the return, not its amplitude. The transmission distance is
about 12mm, 1/2".

We wondered whether we could use a solar cell to power the circuit, but
the typical output voltage is only around 0.7 volts, and there is not a
whole heck of a lot of space to get more in - the board diameter is no
more than 38mm (1.5 ")

Would LEDs of the same type as the emitters make efficient photo-cells ?

Ideally we would like to couple enough energy to modulate an LED
(different colour) at the RF frequency, to pick it up on a filtered
photodiode.

Any suggestions or comments would be gratefully received.

Thanks

Steve

I've seen shaft horsepower meters (30" propeller shafts) that needed a
lot of power on the rotating side. They wound a bunch of turns around
the shaft as the secondary of a 60 Hz power transformer. The primary
was a line-powered coil on a C core that induced flux through the gap
and through the spinning secondary. Data was returned as capacitively
coupled RF. At lower power, the power transmission could easily be
capacitive, too.

But if all you're trying to do is sense a reed switch state, the
spinning stuff could be completely unpowered. Got any more details?
What does the switch sense? Does it have to work down to zero speed?

John
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Steve Taylor
We tried coupling it off to air cored transformer, but we got damn all
signal, my RF engineering knowledge is almost non-existent, and we ended
up using MAR-6 or MAR-8 devices to get the signal up to the point we
could clip it for the next stage.
Either recruit a good ham radio RF man as an adviser or get an ARRL
Handbook and look at the practical lore in it.

I notice that the subject line says 'Hartley oscillator'. The Hartley
circuit, with centre-tapped inductor, isn't usually used for 30 to 50
MHz. I'm not saying you can't make it work, but other configurations are
likely to be better.
 
T

Tim Wescott

Jan 1, 1970
0
What about using coils on the PCB and stationary magnets (you'd want to use
little rare-earths, probably) to power your circuit? This would take power
off your shaft which may be entirely unacceptable, and it would subject
whatever else is on your shaft to a varying magnetic field which might also
be unacceptable, but if you could live with it you'd have more power for
your circuit than you could use.

Why not couple it capacitively? You should be able to make a pair of
slip-ring-like air-dielectric capacitors that might work better. You could
either make these drum-style, or you could etch them into your PC board if
you could afford the space.

How are you arranging your coils, and are you inadvertantly shorting them
out? You should probably have a coil that's coaxial with the shaft, on a
segment of shaft that's not conductive, with no unnecessary conductors
within a coil diameter of either the inner or the outer coil. I assume that
your carrier needs to range from 30 - 50MHz. If the signal that you're
trying to send doesn't occupy this full bandwidth then you could boost your
signal by loosely resonating the coil to 40MHz, but this won't substitute
for making the driver and receiver impedance low enough to keep the system
transmission loss low.
 
T

Tim Wescott

Jan 1, 1970
0
Use a Colpitts with a parallel cap, or a series-tuned Colpitts. The first
one will lose you some tuning range, and the second some linearity, so they
may not be acceptable, but they'd do it.
 
S

Steve

Jan 1, 1970
0
John said:
But if all you're trying to do is sense a reed switch state, the
spinning stuff could be completely unpowered. Got any more details?
What does the switch sense? Does it have to work down to zero speed?

The switch is controlled by an external solenoid (reducing the power we
need to feed to the rotating bit) and flips one pair of plates into the
oscillator circuit, or another. We measure the ratio of the two
frequencies as our output.

Steve
 
S

Steve

Jan 1, 1970
0
John said:
I notice that the subject line says 'Hartley oscillator'. The Hartley
circuit, with centre-tapped inductor, isn't usually used for 30 to 50
MHz. I'm not saying you can't make it work, but other configurations are
likely to be better.

John,
We need to adjust the frequency by means of the external capacitor,
which has a grounded plate. Does that leave me with many other choices ?
I can't tap the oscillator off a single capacitor can I ?
Thanks

Steve
 
J

John Woodgate

Jan 1, 1970
0
(in said:
John,
We need to adjust the frequency by means of the external capacitor,
which has a grounded plate. Does that leave me with many other choices ?
I can't tap the oscillator off a single capacitor can I ?

You are getting a lot of advice from others. I don't want to confuse
you, but some version of the Colpitt's circuit is more usual for 30 to
50 MHz.

However, the classical Hartley circuit *doesn't have one side of the
tuning capacitor grounded. I suggest you put the schematic of the
oscillator part of your rotating unit on alt.binaries.schematics.electro
nics (a.b.s.e) and tell us here that you've done so.
 
S

Steve Taylor

Jan 1, 1970
0
John Woodgate wrote:

However, the classical Hartley circuit *doesn't have one side of the
tuning capacitor grounded. I suggest you put the schematic of the
oscillator part of your rotating unit on alt.binaries.schematics.electro
nics (a.b.s.e) and tell us here that you've done so.

John,
I have placed a copy of the schematic there as "rotating oscillator"
Thanks for your comments
Steve
 
S

Steve

Jan 1, 1970
0
Jamie said:
Hmmm.
you may need to do some machine work but you can use
inductivce coupling where is you excite the stational
coil with some fix hz and thus use the secondary side ( the board
that has the OSC) as the power source rectified.

Tim has suggested that too. We may well try it.
have you ever thought of using an Eddy Current surface effect?
the OSC is on the fixed arpiture and uses surface change to alter the
Frequency..

No, we haven't thought of it. I've never heard of the effect before !

Steve
 
N

N. Thornton

Jan 1, 1970
0
John Woodgate said:
I read in sci.electronics.design that Steve Taylor


If you put a lil pancake coil on the top of the osc, and a stationary
one above that... Now of youre lucky you can transmit your 30-50MHz
one way and while transmitting a lower f the other way for power
transfer.

Regards, NT
 
J

Jamie

Jan 1, 1970
0
Hmmm.
you may need to do some machine work but you can use
inductivce coupling where is you excite the stational
coil with some fix hz and thus use the secondary side ( the board
that has the OSC) as the power source rectified.
we do this also for a piece of equiment but we are only using much
lower frequency.
have you ever thought of using an Eddy Current surface effect?
the OSC is on the fixed arpiture and uses surface change to alter the
Frequency..
 
B

bg

Jan 1, 1970
0
I'm not sure what you want to accomplish (a frequency shift?), but for
reliability , you can use a rotating transformer like that in a VHS VCR to
extract your signal. The tank circuit could be mounted on the shaft with
everything else stationary. Use hall effect switches to switch in capacitors
or varactor diodes for the frequency shift.
bg
 
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