Re: [cosmacelf] Ceramic Resonator for 18-2 oscillator

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• ... The value of R2 includes both the external resistor and the internal output resistance of the inverter. The capacitance also includes the stray capacitance
Message 1 of 4 , Aug 5, 2013
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bill rowe wrote:
> thanks lee. So I would need like 1M and 220 ohms? that would be an RC
> of 130 ns which seems odd.

The value of R2 includes both the external resistor and the internal
output resistance of the inverter. The capacitance also includes the
stray capacitance to ground, besides that inside the resonator itself.
So it is rather hard to predict.

I was using a 4093 as the oscillator inverter. With a selection of
different brands of 4093's, I got

R2=0 2.0 to 3.4 MHz at 5v
1.4 to 2.3 MHz at 4v
0.5 to 1.2 MHz at 3v

Notice that 1.8 MHz (my resonator's frequency) is *inside* these limits.
If the oscillator just happened to be close to 1.8 MHz, it "locks in" on
this frequency, and wants to stay there. For example, if I powered it up
at 5v, it might start oscillating at 2.5 MHz. As I lowered the supply
voltage, the frequency drops until it gets to 2 MHz, when it suddenly
"jumps" to 1.8 MHz. Once at this frequency, it stays there even if I
raise the supply voltage back to 5v.

Next, I studied the range of values for R2 that would make it start at
1.8 MHz and stay there.

5v supply R2 = 500 to 2.2k
4v supply R2 = 300 to 1.5k
3.6v supply R2 = 0 to 1k

The actual value for R2 isn't quite that wide; it varies perhaps 2:1 for
each chip. For example at 5v, one chip might need R2 = 500-1k, another
needs 1k-2k, etc.

The internal inverter inside the 1802 probably has a higher output
resistance than the 4093, because it is optimized to be used as an
oscillator. So, you'll have to experiment. But I'm quite sure you can
find a value for R2 that will work with all 1802's at 5v.
--
Failure is only the opportunity to begin again more intelligently.
-- Henry Ford
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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