Looking for suggestions on a CMOS RL Oscillator

[Steaphany]

I'm well aware of most non-frequency critical oscillators being based on the timing determined by a Resistance coupled with an Capacitance, but I'm looking for suggestions for a low power, wide operating voltage oscillator based on a Resistance-Inductance timing with a target frequency of 8 KHz.

One concept that I have been considering is the CD4047 CMOS Monostable/Astable Multivibrator which is designed to have the astable frequency set by a single resistor and capacitor. Reference design examples of CMOS RL osciillators that I've found indicate that the CD4047 oscillator circuitry should be compatible with the timing set by an inductor and resistor, but these reference designs were not specifically based on the CD4047.

The reason for exploring this direction is many capacitors are thermally sensitive and as the ambient temperature changes, an RC oscillator is prone to frequency drift, which would not be as pronounced in an RL oscillator.

For this application, I do not want to derive the 8 KHz from a higher frequency that has been divided down, the circuit must produce the 8 KHz directly.

Does anyone have any suggestions or experience in this direction ?

 

[crutschow]

I think an RL oscillator is not likely to be more stable than a RC oscillator built with a stable R and C. Inductors also can change inductance with temperature. If you want a more stable RC oscillator then use a low temperature coefficient resistor and an C0G/NP0 (that's 0 as in zero) type capacitor or other stable capacitor type.

 

[dr peppers]

You can get polystyrene caps designed to be used in the local osc section of old tranny radios, these tend to be really stable, maybe not so much at 8kc though.
A silver mica cap might be better as you can get them in higher values more suited to lower freq's like 8 kc, they are reasonably expensive.
I agree with cruts, even an air core inductor will vary with temp, a ferrite core even more so.
By far the most stable would be a xtal divided down, but you dont want that, if you really want an lc circuit you'd probably have to use a ferrite core coil to get high enough inductance for that freq, try and get an old timer tube radio local oscillator transformer can and rewind it for 8kc, you'll need a lot more turns to get that frequency its a 100 times lower, you'll also need a high value resistor to get the current slope slow enough, but that might be possible with a cmos chip.

 

[alec_t]

I agree with the others about the RL oscillator frequency stability. Added to that, you will need a relativey high inductance value for 8kHz if excessive current is to be avoided. CD4000 series circuits have only low current capability.

 

[colin55]

Use a PIC chip. It has an internal 8MHz oscillator that is very stable and costs only 50 cents.

 

[jpanhalt]

Or, if you want better stability, just add a crystal as shown on the last page here: https://www.electro-tech-online.com/custompdfs/2012/03/AN-118-3.pdf

Here are a couple I made awhile back:

View attachment 62735

Note: That is the CMOS 555. It also works well with a non-CMOS 555, if you add a pull-up resistor.


John

 

[ronsimpson]

Use a PIC chip. It has an internal 8MHz oscillator that is very stable and costs only 50 cents.

The internal oscillator in a PIC is a RC oscillator. How can it be much better than a RC oscillator?
Looking at a PIC12F508; 4 MHz Precision Internal Oscillator:- Factory calibrated to ±1% (+1 to -3% at -40C)
Typical 0.8% change in frequency over 2 to 5.5 volts supply.

 

[lebevti]

could it be you're looking for a Hartley oscillator?

 

[dr peppers]

Theres an lc osc on this page half way down:

https://www.electro-tech-online.com/custompdfs/2012/03/20051213190144250.pdf

Its designed to operate at rf freq's which you'd expect for this type of osc, you might be able to get it to work at 8kc by scaling the values, but you'd have to contruct an inductor with a moveable core to facilitate adjustment as you wouldnt be able to get a variable capacitor big enough for the job, hence my idea about a radio local osc can, I used one of these as a radio beacon I made for the cb band a while back.