1audio

A few quick notes on using a tuner for digital audio clock jitter analysis (before I forget the details). Someone here (I have forgotten who) suggested an FM tuner for monitoring jitter. After a little research and experimentation I went pretty deep into figuring this out.

First the internal clocks on ADC's and DAC's have strong harmonics into the FM band. Second, FM tuners are very sensitive to modulation to carriers. modulation and jitter are closely related. The other advantage of this is that the jitter/phase noise is multiplied by the ratio of the actual carrier and the harmonic you are looking at. E.g. a 22.5792 MHz clock becomes 90.3168 MHz with the noise amplified by a factor of 4.

This is very simplified but covers the essentials-You need a really low noise FM tuner. Sound good is not an issue. The Yamaha TX-930 and TX-950 are possibly the lowest noise tuners ever made. They can be had for around $100 on eBay. What I did was to take the output directly...

While doing some research on making an ultra low distortion analyzer I stopped to look at the Fluke 510A that has been siting on my shelf for years. Lots more on the 510a here: http://assets.fluke.com/manuals/510A_AD_imeng0000.pdf

In short a low distortion (.005% or better) 10V +/- .002% reference oscillator. There are very good numbers and by 1970 standards exceptional.

I was looking at it because I was researching AGC circuit for oscillators. This is an interesting implementation with its standards lab precision accuracy and stability. However its not simple and the critical parts are expensive (an ultra stable voltage reference, ultra stable resistors etc.)

In any case I was wondering if the distortion can be reduced still further. At the time it was built .002% was about as good as possible in practice and better would be hard to verify.

In short, after a number of experiments I got the distortion down to .0003% THD (30 KHz...