transmission line Colpitts oscillator

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I've been thinking about this

https://www.dropbox.com/s/yuu737wb3cqizph/TX_Line_Osc_3.JPG?dl=0

as a roughly 500 MHz clock oscillator for some digital stuff. My FPGA
guys claim they can accept that as an LVDS clock input. The comparator
would be LVDS in and out, FAN1101 maybe, if it is really fast enough.

So I have a question for some more RF-ey guys than me. Q1 furnishes
negative resistance so oscillation builds up, pretty much a sine wave.
At some point R1 and D1 load the negative swing and add real loss, so
it stabilizes at ballpark 1 volt p-p centered on ground, just enough
net negative resistance to match the losses of the transmission line.

If the gain of Q1 is high (low R2) and R1 is small, it clips pretty
hard and the sine is flattened. It's flattened identically on both the
positive and negative peaks, which confused us for a few seconds.

So, given all that, how does hard clipping affect the phase noise, as
opposed to softer limiting or, ultimately, some super linear AGC loop?
I want a lot of swing into the comparator, 1 volt p-p maybe before its
ESD diodes conduct, and I want the thing to oscillate reliably, which
suggest sorta hard clipping, with some visible flattening of the sine
wave.

No matter how much gain we have in Q1, and how hard we clip, the
txline still sees exactly as much negative resistance as it takes to
equal its equivalent resistive losses. So as regards phase noise, does
it matter how hard we clip?

I did google this some but didn't find a good answer.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: transmission line Colpitts oscillator
John Larkin wrote...
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 The signal doesn't go too negative for the comparator?


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    - Win

Re: transmission line Colpitts oscillator
On 14 Apr 2019 12:52:26 -0700, Winfield Hill

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

https://www.dropbox.com/s/0wccw9kffqxf5r2/FIN1101_Vcm.JPG?dl=0

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


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: transmission line Colpitts oscillator
On Sun, 14 Apr 2019 13:39:10 -0700, John Larkin wrote:

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



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Re: transmission line Colpitts oscillator
On Sun, 14 Apr 2019 21:12:41 -0000 (UTC), Cursitor Doom

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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.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: transmission line Colpitts oscillator
On 15/4/19 7:22 am, John Larkin wrote:
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In the USA maybe, not elsewhere. Perhaps if you can't sell your design,  
you could be persuaded to publish it?


Re: transmission line Colpitts oscillator
On Sun, 14 Apr 2019 12:27:36 -0700, John Larkin

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Oops, switched the values for C1 and C2.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: transmission line Colpitts oscillator
On 15/4/19 5:27 am, John Larkin wrote:
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Right, because the negative peak is the inverted reflection from the line.

Kevin Aylward is the man to answer your original question.

Clifford Heath

Re: transmission line Colpitts oscillator
On Sunday, April 14, 2019 at 12:27:45 PM UTC-7, John Larkin wrote:
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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).

Re: transmission line Colpitts oscillator
wrote:

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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.






--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: transmission line Colpitts oscillator
On Sunday, April 14, 2019 at 3:50:12 PM UTC-7, John Larkin wrote:
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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.

Re: transmission line Colpitts oscillator
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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.

http://www.kevinaylward.co.uk/ee/phasenoise/PhaseNoiseOscillators.xht

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
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html


Re: transmission line Colpitts oscillator
On Tue, 16 Apr 2019 20:10:18 +0100, "Kevin Aylward"

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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?



--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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