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...
Posted 20th March 2013 at 04:18 AM byabraxalito Updated 20th March 2013 at 04:30 AM byabraxalito(Added FR)
Passive filters rock for SQ, no doubt about it but I'm still curious how good sounding an active DAC I/V post filter might be. So I've figured out an almost equivalent FR active version of my 7th order LC elliptic filter. This active elliptic has been designed using LTSpice's FilterCad program giving the pole/zero positions, then the Williams handbook of filter design helped me translate those numbers into a working circuit. Its using what Williams calls the VCVS 2nd order section based on a twin-T network to realize the zeroes.
My first attempt at an active elliptic filter was using gyrators but that proved very hard (practically impossible) to get stable with CFB opamps due to their HF gain peaking. VFB opamps I ruled out at the start for inadequate SQ - its not hard to make gyrators stable with them. Hence this approach which promises to work with CFBs though I'd guess I'll probably need to add series Rs between the stages in practice. Nothing built yet but thought I'd...
Posted 19th March 2013 at 07:02 AM byrjm Updated 20th March 2013 at 02:54 AM byrjm
Mid-range 1970's stereo receiver.
I was curious to find out a) what the phono circuit was, and b) how tight the RIAA response might have been.
The answer is "four transistors" and "pretty damn good", respectively.
We are impressed. These Japanese engineers knew a thing or two. I would like to see some of these old circuits resurrected as discrete phono stages with modern components to see just what they are capable of.
Results shown below, together with the LTSpice .asc file you can use to play around with this yourself.
First attached image shows FFT for the rectified DC (green), reference voltage (red) and X-reg output (blue) for the designed-for 10 mA output (top) and a more punishing 100 mA (bottom).
Second image shows an LTSpice screengrab for the LT1086 with bypassed adj pin under comparable loading. Input voltage in blue, output in green. This is a reasonable approximation of a "good" IC regulator.
Last image shows a plot of the exported LTSpice FFT data for the X-reg and the LT1086-12V (Cin 1000uF, Cout 100uF) both at nominal currents of 10 mA. The LT1086-12V is a reasonable substitute for a generic LM7812, i.e. a "bad" IC regulator.
A typical op amp will have sufficient PSRR to mop of the residual noise from the bypassed LT1086. The fixed LT1086-12V, on the...