Ring Oscillator with a set frequency at 10kHz (low)

[Ntonio]

Hello (this is not a homework)

I have a doubt about the ring oscillator. I have to create a R.O. with a frequency of 10kHz, now I know i have to link an odd number of inverters in a ring form, but using the formula of (f=1/2*n*Tp) it gives me a frequency in the Megas, I've red you can use a RC (f=1/1.2RC) but how to do so?! do I place it anywhere in between the first and the second inverters, and if i do that how many inverters do I use?!

If you could help me understand I would appreciate =)
thanks

 

[jsgruszynski]

Hello (this is not a homework)

I have a doubt about the ring oscillator. I have to create a R.O. with a frequency of 10kHz, now I know i have to link an odd number of inverters in a ring form, but using the formula of (f=1/2*n*Tp) it gives me a frequency in the Megas, I've red you can use a RC (f=1/1.2RC) but how to do so?! do I place it anywhere in between the first and the second inverters, and if i do that how many inverters do I use?!

If you could help me understand I would appreciate =)
thanks

A ring oscillator frequency is determined by inverter delay (T_p of the inverter) plus any parasitic caused delays. For typical ring oscillators on-chip, these parasitics will be small and ignorable. This is why the primary use of ROs is to characterize semiconductor manufacturing processes and do that by determining the prop delay of a prototypical logical gate.

If you build a ring oscillator from actual TTL/CMOS ICs, the parasitics can be significant. You may even need to explicitly add parasitics if you need to control the frequency. That's probably where the RC reference comes in. You need to modify the prop delay to get a low enough frequency.

The other issue is how you measure the frequency of the ring oscillator. Using a frequency counter or oscilloscope is generally fraught with accuracy issues if you are building an RO on-chip. For a TTL/CMOS RO, you can get by with them.

Both of these instruments are time-domain, which is a bad starting place for determining RO frequency. The frequency counter has to threshold properly to make a correct measurement which requires having a substantial signal level. The oscilloscope is generally low resolution in terms of vertical accuracy compared to other instruments; good for qualitative but not so much for quantitative accuracy.

For on-chip RO measurements, always use a spectrum analyzer as it has logarithmic vertical scale and can resolve multiple frequency components that can appear in a RO output and distort the time-domain signal. SAs are what most semiconductor firms use for RO testing on-chip.