EEVblog #306 - Jim Williams Pulse Generator

[BravoV]

Allright, here's all the cal certificates (shees,h i'm missing out some quality time floating in the pool...)

Finally, thank you ! 

Don't worry, that pool ain't going anywhere. 

@BravoV
  Will you be nitpicking about the Standard deviation in the values of Risetimes of Free's workmanship

Nahh.. I'm content, FreeElectron did the cherry picking for me, the one I got has the fastest Tr , close to that Agilent DSA-X 93204A limit, a braggin sub < 100ps. 

[c4757p]

Got mine today!  Thank you very much. But you forgot my 220R resistors! I want a refund! 

I won't insult it with a scope shot right now - I'm still waiting on a proper SMA-BNC adapter. It's clearly way too fast for the sad excuse for a cable I bodged it to the scope with for a quick "does it work" test. (It does.)

[free_electron]

Got mine today!  Thank you very much. But you forgot my 220R resistors! I want a refund! 

I won't insult it with a scope shot right now - I'm still waiting on a proper SMA-BNC adapter. It's clearly way too fast for the sad excuse for a cable I bodged it to the scope with for a quick "does it work" test. (It does.)

really ? darn !. ok i'll refund you 0.1 dollarcent. but you need to pay the paypal charge

there should be a strip with two resistors in it. those are the 220 ohms.

[c4757p]

but you need to pay the paypal charge

Damn!

there should be a strip with two resistors in it. those are the 220 ohms.

Nope. I've got a hundred or so of the things, though, so not a problem 

Very nice job with it, by the way. PCB looks quite professional, and I like the case too. Can't really comment on the actual functionality - not going to get quicker than a handful of nanoseconds down a clip-on China Brand coax with a five-inch ground lead... I could see the pulse dancing around as I twiddled the lead. I'd fart around with a proper probe on the back side of the SMA jack, but I've got a direct SMA-BNC (M-M, so no cable required at all) in my last order to Mouser.

[BravoV]

Teasing c4757p    C'mon, you should be properly equipped with these adapters.

[c4757p]

I think this is the first SMA connector that's ever been in my house! Damn mail takes too long, too...

[plesa]

Thanks FreeElectron!
Today I assembled the generator, everything works like a charm.
I connected it to my DSOX3054 and measured rise time about 940ps.
I also connected the N2792A 200 MHz 10:1 Differential Probe and it has rise time 2,94ns, which is about 136 MHz. Which dissapointed me slightly, but still according to spec.

[fmaimon]

Just came back home from a 30 day vacation and the package was here waiting for me. As soon as I clear the jet lag I'll assemble it. Thank you.

[c4757p]

Beauty! 800ps on a TDS380. 

[EV]

Looks fine! The overshoot is missing from my TDS3032 curve maybe because I must use attenuator to see the whole curve.

[c4757p]

The overshoot is probably due to the less than perfect termination. I didn't have a feedthrough handy so I used a T adapter with cap-style terminator. (Unterminated, it was almost completely washed out by reflections.)

[EV]

The overshoot is probably due to the less than perfect termination. I didn't have a feedthrough handy so I used a T adapter with cap-style terminator. (Unterminated, it was almost completely washed out by reflections.)

Does't your scope have 50 ohm input impedance? I did not find it from TDS380 manual. Rise time was 875 ps in the specs for TDS380.

[c4757p]

No, it doesn't have selectable impedance, it's always 1M. My 2445A does have 50 ohm inputs, but I didn't bother taking a picture of it because it has much lower bandwidth and I hate farting around with a camera in front of it 

Rise time was 875 ps in the specs for TDS380.

So it appears they're using the standard Gaussian 0.35/rt=bw formula - 0.35/875ps = 400MHz. Then I get 438MHz by the same formula. Nice 

[w2aew]

Here's a picture of #6, taken through a 20dB attenuator, into a 33GHz real-time scope:

[jpb]

My kit has just arrived - thank you Vince!

Now all I need to do is find some time over the weekend to make it up.

Edit: I can't make up my kit until I acquire some tweezers! (This is my first foray into SMD soldering.) I've just ordered some from Farnel but they won't arrive until next week - somewhat frustrating.

[musafir]

Hello
Are any more kits available ?

[sdscotto]

Here's a picture of #6, taken through a 20dB attenuator, into a 33GHz real-time scope:

Show off!  ;-)

[SeanB]

Here's a picture of #6, taken through a 20dB attenuator, into a 33GHz real-time scope:

Show off!  ;-)

He does work at Tektronix......................

I somehow doubt they buy Agilent scopes for internal use, though they do use Fluke as meter suppliers.

[jpb]

I've built up my kit - and it is not working


Having got through two batteries I've connected it to my lab power supply. It draws about 10mA. The voltage at the end of the diode chain is a healthy 106V.

The voltage at the transistor across C2 is 35V. The Voltage across the 50 ohms is 0.33mV which corresponds to 6.6 microAmps. The voltage drop across the 10M R1A is about 71V
so the current through this is approx 7.1 microAmps.

This means, I guess, that Q1 is leaking too much current to reach avalanche at 40V?

The voltage across R5 is 252mV where as it perhaps should be 106V*20k/(10M+20k) =209mV. In the circuit diagram R5 is only 10k whereas on the board layout
and as supplied in the kit it is 20k?

Any suggestions as to what Voltages and currents I should be seeing would be most welcome. Is 7 microAmps very leaky? (I've no idea.)

Should I be connecting a 10M R1? (No R1 came in the kit, I thought R1A replaced it?)

If I added a 10M as R1 then the voltage drop would be halved and the Voltage across Q1 would rise to about 53V potentially (though presumably it would
do what it is supposed to do and breakdown at 40V).

It's nearly midnight so I'm off to bed but any help would be much appreciated.


EDIT (following morning) :

I've added a 10M as R1 and it is now working! At least it is avalanching at a rate of just under 40kHz or every 25 microsecs. Q1 looks like around 5M when it is not avalanching, so the circuit is
two 10M in parallel then a 2pF in parallel with 5M so it is not quite as simple as just RC for the time constant - the effective R is 12.5M - I suppose the last bit of charging is stretched out. I've just looked at it with a probe but will properly terminate it and get some plots when I get a bit of time later on today.

The current draw is still about 10mA so most of this is probably the current of the led (around 8mA or so) - so if you want to save battery life disconnect the led.

[jpb]

Here are some initial screen shots.

The first is for a 50ohm pass through to 1M/12pF input of the scope.

The second is for a direct connection to the 50ohm input of the scope.

The nominal rise time should be 1nsec (it is a 350MHz scope).

[EV]

At least it is avalanching at a rate of just under 40kHz or every 25 microsecs.

Pictures look good. Avalancing rate is about 24.6 kHz with my pulse generator when the battery is installed.

[jpb]

Here is a comparison with the calibration curve measured by Vince on the Agilent super scope.

What I might do as an experiment is take FFTs of both and see if I can get some sort of approximation to the WaveJet's frequency response
(a little ambitious I know, but fun to try!)

[jpb]

At least it is avalanching at a rate of just under 40kHz or every 25 microsecs.

Pictures look good. Avalancing rate is about 24.6 kHz with my pulse generator when the battery is installed.

By adding both R1 and R1a I've reduced the effective resistance from 10M to around 5M so the time constant should be approximately halved so this (very roughly) fits
with your figure (my slightly leaky transistor in parallel with C2 looks like 5M itself so this increases the charging time so it becomes less than the 50kHz that
a simple halving of the resistance would suggest).

[EV]

Here is a comparison with the calibration curve measured by Vince on the Agilent super scope.

The shape of the curve is nearly like with my Tek scope. The overshoot which is in Vince's picture is missing.

[jpb]

Here is a comparison with the calibration curve measured by Vince on the Agilent super scope.

The shape of the curve is nearly like with my Tek scope. The overshoot which is in Vince's picture is missing.

Looking at the overshoot, and estimating the frequency of a sin peak that might fit into it, it looks like a peak width
of around 0.2 nsecs corresponding to a period of 0.4 nsecs or 2.5GHz so it is not surprising that a 350MHz scope
misses it. I guess it must be at least 17dB down probably rather more (17dB was a back of the envelope estimate
based on a single pole response).