Oscillator circuits
Oscillator circuits
(OP)
Hello. Can anyone help me I have a bit of a proble. My electronics skills are quite limited but I have a grasp of the basics.
I want to create a high frequency oscillator circuit in the range of around 10MHz. But I also need to be able to affect the frequency of the oscillator using an external varaible capacitor. Can I just buy any normal crystal osc oand if so how would i go about making my addition.
Any manafacturers part numbers and details of how this can be achieved would be very useful.
Many thanx.
I want to create a high frequency oscillator circuit in the range of around 10MHz. But I also need to be able to affect the frequency of the oscillator using an external varaible capacitor. Can I just buy any normal crystal osc oand if so how would i go about making my addition.
Any manafacturers part numbers and details of how this can be achieved would be very useful.
Many thanx.





RE: Oscillator circuits
RE: Oscillator circuits
A Hartley or Colpitts circuit is probably the best for a variable frequency oscillator but keeping the frequency accurate and controlling 'drift' is quite a challenge. I've never seen a Wein bridge used at 10MHz though.
RE: Oscillator circuits
"Normal" packaged oscillators of either sine or square output generally have a buffer stage to prevent - or at least reduce - the "pulling" effect of changes in load and power supply, so you won't get much scope for adjustment there.
If you build your own oscillator with a separate crystal it is possible to adjust its freqency by a few percent with a variable capacitor. Since the crystal is tuned by the manufacturer to oscillate with a specific load capacitance, small variations can usually be made without stopping oscillation. The variable capacitor can be an external trimmer capacitor, or electrically adjusted with a control voltage applied to a varactor diode.
RE: Oscillator circuits
Ok I need the frequency to be altered as much as possible really with only very small adjustments in capacitance.
My aim is to produce a capacitance based micro-sensor that can monitor the movements of a human finger. As the finger moves toward or away from a fixed plate capacitor it will affect the frequency of an oscillator. The frequency of the oscillator is then feed into a phase lock loop chip to get a voltage out that I can then calibrate into movements.
The main problem is that I need really small movements to be monitored. In the range of 0.1mm.
Am I best of with a LC or RC oscillator and how can I test mathmatically which will produce the best frequency change over small capacitance changes. Also do I need to add the variable capacitor in series or in parallel with the other components?
Thanks again for your help.
RE: Oscillator circuits
RE: Oscillator circuits
One down side is that there's generally more phase noise with an RC oscillator.
Parasitics must also be considered; with the finger removed, the capacitance will not go to zero.
Can you build a test jig so you can measure the range of capacitance you'll be dealing with?
RE: Oscillator circuits
0.1mm resolution is not a problem if the total range is say 1mm. If you are trying to get to 5mm or 20mm then the scheme will probably become unworkable quite rapidly.
RE: Oscillator circuits
Correct spellings of the oscillators you mention are:
Wien Bridge Oscillator
Colpitts Oscillator
Might help someone find 'em on Google or wherever!
RE: Oscillator circuits
To me the primary factor is how much percent change in
capacitance happens for a movement of 0.1mm. If there is
say 10% capacitance change it is dooable.
By the way motorola just came out with a new chip for
doing e-field capacitive sensor type stuff.
Search online for E field , motorola
You must shield the sensor carefully to eliminate capacitance changes from outside the sensor interfere.
good luck
Rodar
RE: Oscillator circuits
RE: Oscillator circuits
We used a 74HC14 connected as a schmitt trigger, driving the counter input of a Philips flash 8051 controller and monitored the frequency shift. The oscillator ran between 100 kHz and 250 kHz. The frequency depneded heavily on circuit layour and the Schmitt trigger's hysteresis.
The smallest detectable change was a few tens of femto-farads - we could sense a hand with an approaching finger 10 to 20 cm away.
The circuit and some of the operation is explained U.S. patent 6,583,676 which can be searched for a www.uspto.gov
There are lots of fine points, most of them aimed at keeping the oscillator purely dependent on the capacitance, so the pickup plate has to be shielded from noise sources (like the rest of the digital circuitry, and in one case, a flourescent lamp backlight), and the power supply has to be kept very clean.
RE: Oscillator circuits
First the finger is quite likely to induce a lot of 60Hz (50 Hz ?) mains hum into your circuit which might cause trouble. Also high voltage electrostatic discharges might possibly damage or destroy components.
Its the old zap from the doorknob after walking on the synthetic carpet syndrome. While many simple circuits can be made to work, keeping them working, or mass producing them might not be so easy.
Touch circuits are quite commonly used in lifts, and other vandal proof applications. What is it you are trying to do exactly ?
RE: Oscillator circuits
I recall reading something describing the pad used in laptops to mimic the mouse action.
Suggest to try that in Gooogle or similar.
My experience with one laptop in Equatorial Guinea: high ambient humidity created the hell of havoc to the pad. Just in case, keep it mind. Also the opposite: too dry enviroment (static?) had similar effect but less pronounced.
Buena suerte. Feliz Año Nuevo
Agustín Tomás
RE: Oscillator circuits