transmission line Colpitts oscillator

The signal doesn't go too negative for the comparator?

The FIN, like most LVDS input parts, is happy working around ground. Or Vcc. They are amazing.

If people called it a "1 ns rri comparator" they would charge 10x the price.

Have you ever built a TDR by any chance? I should imagine that would be right up your street.

Oops, switched the values for C1 and C2.

I did a board layout for what might be a 40 ps TDR, and had it built, but I haven't had time to play with it.

Old 11801 scopes and SD24 TDR/sampling heads are so cheap and good it's not worth making or selling your own yet.

In the USA maybe, not elsewhere. Perhaps if you can't sell your design, you could be persuaded to publish it?

Right, because the negative peak is the inverted reflection from the line.

Kevin Aylward is the man to answer your original question.

Clifford Heath

It'll affect the phase according to the dispersion of the delay line at the frequencies of interest. The 'square wave' input means you have not just 500 MHz, but 1.5GHz, 2.5, 3.5, etc. It also means that the delay line impedance is just as temperature-sensitive as D1 and Q1.

The phase noise might be dominated by a high harmonic in the delay line, is what I'm saying. If you could keep it in the sinewave oscillating region, it'd be better controlled (because it's unclear WHICH of the high harmonics is the important one).

The transmission line looks like an inductor below its 1/4 wave frequency, so resonates with the capacitors. Sorta like a quartz crystal in parallel mode looks like an inductor. The oscillation is mostly sinusoidal, squashed a bit on peaks by the clipping diode+resistor. I'm only interested in the zero crossings into the comparator, and near zero volts it looks like a good sine wave.

In real life, the txline losses go up fast with frequency, which further encourages a sine wave oscillation mode.

What's weird is that the clipping is bouncing back and forth inside the line, many times, but it stays near the peak and doesn't wander off into the rest of the sine wave.

With a different kind of gain element, this could be a square wave oscillator. Ctune would change the frequency by softening the edges.

It goes up both because of skin effect, AND because of the dielectric absorption (which is why CATV uses foam dielectric). There is extra time delay for the harmonic.

Unless the slew rate at zero crossing is peak voltage * f/(2*pi), the time of that zero crossing is being affected by the harmonics, for which Q is lowered.

Generally, the clipping won't make a difference to flatband noise. For the same supply current, AGC pretty much always results in worse noise. This is because its simply more bits and pieces generating noise.

The comparator will always be the dominant flat noise in a well designed circuit.

This paper gives a full analysis as to what clipping/limiting generates 1/f up converted phase noise.

A key idea is minimising non-linear time constants. Don't connect a varacter directly to the amp. Use a cap to block LF modulation. A pierce resistor can get you 10dB less 1/f close in noise because it damps out the non-linear output resistance of the amp. It reduces the HF gain though.

-- Kevin Aylward

Good stuff, thanks. I'll read the paper later.

I'm concerned with low frequency phase noise. The clock is used to time out delays in a laser system, and lf phase noise maps to more jitter for longer time delays. The comparator only affects jitter for short delays; longer delays are dominated by the oscillator itself... if I drive the comparator hard, I guess. The comparator can have its own 1/f noise, I guess. Especially an LVDS buffer which is hardly optimized for analog.

We should test the LVDS buffer separately. We were going to do that, for speed, but we should evaluate noise too. They are sure cheap.

SN65CML100 is a possibility too.

Where would that pierce resistor go?