it oscillates!

During this shutdown, I did a little pcb layout for my triggered common-collector Colpitts oscillator, and had some boards fabbed. A manufacturing person came in today and built a couple for me.

It's a triggered 125 MHz oscillator that will be used to time delays in a laser system. We want minimal time jitter so I tried to keep the Q up. The inductor is a Coilcraft Midi-Spring.

I used a BFT25 super-fast transistor, but it oscillates at more frequencies than I intended. Tons of jitter. The choice was to add a base resistor or go with another transistor. A BFS17 seems to work fine. That's a great little npn, fast but not too fast.

Here's the board

formatting link

and here's the roughly 250th rising edge after it's triggered to run

formatting link

at 100 ps/div. I haven't figured out how to get that scope to measure the RMS jitter on that edge; it's obviously smarter than I am, and lets me know. If I eyeball the p-p jitter and divide by 5 for RMS, I'm estimating 4 ps RMS jitter at 2 us out from start, which is a ratio of

500K:1. That's unheard of, so I may be doing something wrong.

We plan to phase-lock this to a good OCXO, but it will take a while to lock, so the better the open-loop Colpitts behavior, the less frantic we need to be about the DPLL. 2 microseconds is plenty of time to do the math.

It's out on the bench, so I'll put it in a metal chocloate box with some feed-thrus for better EMI shielding. Gotta empty the box first.

I need to temperature compensate it and play with the active guard idea.

Phil H helped me think about this. Thanks.

--
John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 
 Click to see the full signature
Reply to
jlarkin
Loading thread data ...

Too good a result feels good for a while always. Then the professional side starts wondering.

What does 'Trigger on positive edge with Holdoff by time' exactly do, is it arming on first edge, then triggering on the second edge after holdoff time ?

I think that would mean that you're triggering on the roughly 250th edge and thus seeing scope trigger jitter ?

How about triggering on the first edge and scrolling to trigger+2us ? I think the scope timebase should be quite good compared to oscillator.

--
mikko
Reply to
Mikko OH2HVJ

Going off on that point.... Many years ago I was working (peripherally) wit h a rather intimidating antenna designer. He had just built a radiating el ement using a 3dB 90 degree hybrid and was launching into the air from each output. He brought me over to the network analyzer and showed me his fant astic return loss. It turns out I had just been doing reading on hybrid co uplers and so I pointed out to him that any reflected power at the two laun ching ends would reflect back into the 4th port with a 50 ohm resistor. He paused for a moment and you could see his countenance drop like a rock. H e was nice to me for the rest of the time I worked there.

think

Reply to
blocher

Very interesting, show us the circuit!

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Very nice if true. (Nice scope, too.)

Glad to help. It's an interesting problem, for sure.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

What do you mean the first circuit oscillated at more frequencies than you intended? Isn't that circuit tuned?

Reply to
bloggs.fredbloggs.fred

LeCroy is always confusing. The oscillator makes a burst, a few us wide now, and my intent is that the scope triggers on the first rising edge and captures one burst. Seems to. If there's no holdoff, the display is a mess.

Holdoff usually means that once a trigger is accepted, triggers are inhibited for some time. If I set the holdoff greater than the burst width, I should trigger on the first edge of each burst.

I guess I should trigger the scope from the same pulse that gates the burst. I got the built board late in the day, and was pleased to see it oscillate. I need to spend a couple days carefully tweaking it.

LeCroy claims 1 ps RMS jitter. I checked it with our SRS clock generator as the input signal, and that has a lot less jitter than my oscillator, so the scope is good enough.

My old Tek 11802 uses an LC burst oscillator as its timebase, but the jitter is something like 1/20000 of the delay, which isn't good enough. The LeCroy just digitizes the input at a constant ADC rate, pokes it into RAM, and does all the rest in software, including finding the trigger. I think.

That's basically what's happening, probably. But I'll trigger on the oscillator gate, which is why I included the SMA loop-thru connectors on the gate input.

--
John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 
 Click to see the full signature
Reply to
jlarkin

I almost never trust measurements, especially good ones. Especially using an oscilloscope that nobody understands, even the LeCroy support people.

--
John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 
 Click to see the full signature
Reply to
jlarkin

Here's the non-secret oscillator part.

formatting link

One key thing to test and tweak is tempco. I cut an island out of the layer 2 ground plane as an active guard that can float, be grounded, or be driven from the Q1 emitter. That might mitigate some of the horrible FR4 capacitance. C3 can tweak tempco too. The real board will be 10 layers with just 8 mils from the top to the L2 ground plane, which means it will be different from this proto board. So I'm just learning principles now.

Varicaps have bad tempcos too, so things will get tangled.

I'd like to make the board thicker and make the dielectrics thicker, but there are mechanical limits, including the Vbite connectors.

--
John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 
 Click to see the full signature
Reply to
jlarkin

I think the NPN was also oscillating at some microwave frequencies, all on its own. Fast emitter followers do that. It was probably still oscillating when the 120 MHz tank was quenched and the oscillator theoretically stopped. Touching various counter-intuitive nodes with a tiny screwdriver changed things a lot. I even stopped the extra oscillations by touching something that made no sense.

A series base resistor is the usual fix for an oscillating emitter follower, but that would need a hack and might reduce Q. Going to a slower transistor seems like the best fix. It's only 120 MHz.

--
John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 
 Click to see the full signature
Reply to
jlarkin

Modern DSOs also double as a portable gaming device; Doom runs even better on this Rigol than some of the Nintendo home video game systems it was ported to from the PC in the late 90s:

Reply to
bitrex

Am 16.04.20 um 18:48 schrieb bitrex:

I have booted my Infiniium 54846B scope with a Linux variant to make backups of its windows disk. It even ran open office, but sloooow. That looked weird.

Cheers, Gerhard

Reply to
Gerhard Hoffmann

...

Depends if that's the relevant parameter of the frequency jitter within the burst. Trigger on gate will also include whatever gating jitters there might be - which just may be the key performance criteria depending on application.

You might also be able just to use normal trigger delay ?

..

Wonders of modern scopes...

--
mikko
Reply to
Mikko OH2HVJ

The LeCroy is a Win7 machine.

You really need a mouse to use it.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

Well, you could test it using a couple of hundred feet of coax, a trombone line, and one of your fave PECL comparators or a Mini-Circuits mixer.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

The SRS clock generator has super low jitter, and using it as the source in a similar test, I see a couple ps RMS jitter. So it looks like the scope is good.

One way to check a scope's math is to drive it with an FM'd signal, where you can calculate the added jitter. Then measure it with the scope.

Around here, some people were measuring the standard deviation of cross times, on a Tek sampler. That's not jitter, because the cross times have already been computed from multiple samples. One of the LeCroy support people told us to do that too.

I have sucked raw Y-T sample points out of my Tek and done the math myself. We could do that with the LeCroy too. I'd have more confidence in that.

formatting link

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

The laser designator that I worked on just used the FPGA to do all the timing.. Laser diode firing and Q switch opening and range detection...

--
This email has been checked for viruses by Avast antivirus software. 
https://www.avast.com/antivirus
Reply to
TTman

Did the FPGA initiate each shot? What sort of timing resolution are you getting?

Our trigger is asynchronous to our main clock, and we have to time everything off that.

Some lasers just fire when they feel like, or are triggered by someone not-us.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

I've seen (LTSpice and bench) Colpitts oscillators that produce pulses of much-higher-frequency oscillation near the fundamental's zero crossing. So you might have 125MHz with synchronously amplitude-modulated 900Mhz overlaid.

Also beware of paralleling larger and smaller capacitors, as is often recommended for decoupling. The ESL of the larger capacitor can form a surprisingly high-Q resonant tank with the smaller capacitor, causing all kinds of interesting birdies.

Clifford Heath.

Reply to
Clifford Heath

Even worse, Spice transistor models usually exclude wire-bond inductance.

Turns out that I was still having erratic jitter with the slower transistor. Touching totally illogical nodes with a small screwdriver sometimes fixed it. As you note, it might have been burst oscillating on some parts of the main swing. Adding a 50 ohm resistor in series with the Colpitts base really seems to have fixed it. Exploring the solution space by soldering is a lot slower than Spicing. This lockdown does give me lots of time to fiddle.

I bolted it into a chocolate tin, which was a lot of work, but that didn't make any difference.

Here's the setup:

formatting link

The Tek scope is looking at the sine wave itself, and the LeCroy at the oscillator digital output. Looks like I have a bit too much differential-equation-initial-conditions current. The ideal waveform would be a perfect flat tone burst. More soldering.

I need a big black blanket to huddle under when I photograph that big scope.

--
John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 
 Click to see the full signature
Reply to
jlarkin

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.