DCO/VCO Fine clock output?

Hello Alex,

Look at the National LM331.

Regards, Joerg

Use an op-amp. Feedback resistor R1 from output to Inv Input Shunt resistor R2 from Inv input to gnd. Positive feedback R3 from output to Non Inv Input. Capacitor from Non Inv Input to ground Inv input to ground. Let Vsat = op amp saturated output voltage. Let Vref = voltage at Inv input with op amp saturated. Then Vref = VsatR2/(R1+R2) Let Vs = Vsat + Vref. Let f = frequency of oscillation. f = 1/{2 Tau ln[(Vs-V)/Vs)]} Wwher ln(X) means "natural logarithm of X".

Ignore my last post. I accidentally clicked "Send" before I was through editing. ~ Use an op-amp. With +/- supplies: ~ Feedback resistor R1 from output to Inv Input. Shunt resistor R2 from Inv input to gnd. Positive feedback R3 from output to Non Inv Input. Capacitor from Non Inv Input to ground. Let Vsat = op amp saturated output voltage. Let Vref = voltage at Inv input with op amp saturated. Then Vref = VsatR2/(R1+R2) Let Vs = Vsat + Vref. Let Tau = R3C Let f = frequency of oscillation. f = -1/{2 Tau ln[(Vs-V)/Vs)]} Where ln(X) means "natural logarithm of X". If the +/- saturation voltages are equal, the ouput frequency will be independent of the supply voltage. ~ A pot can be substituded for any of the resistors. ~ Ignore my last post. I accidentally clicked "Send" before I was through editing. Regards, Jon

I'm guessing you want this for a digital audio clock. I would suggest a DDS chip such as AD9850 or one of the slower versions. They go down to 1Hz or less, and can be programmed with a PIC microcontroller such as a PIC16F84A, and you would also have to hook up a serial output ADC to digitise the pot position. I think that this solution would produce a less noisy (lower phase noise) clock than any analogue method that I can think of, and it would be very repeatable and manufacturable. If lower performance is OK, or lower power consumption and lower cost interests you then some kind of VCO like the one out of a 4046 IC might do what you want.

Chris

Hello Chris,

That one has quite poor linearity. The LM331 is much better. AD also offers a few V/F converter chips that are top notch but cost a bit more than the LM331.

It is also possible to linearize the 4046 via a feedback loop but then the cost approaches that of the LM331.

Regards, Joerg

Doesn't like being pushed beyond 10KHz...

Doesn't like being pushed beyond 10KHz...

hello Fred,

It can do up to 100kHz but you'd have to spring for an opamp if linearity errors really have to be under 1/10th of a percent.

Regards, Joerg

Hello Fred,

Does. Dunnit...

Regards, Joerg

Thanks guys for all the advice - I'm going to try a few of the methods suggested and see what works best.

I will update you with news of my progress! Best regards, Alex