Re: Streamer formation on the scope...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Paul,

At 12:32 PM 9/7/2002 +0100, you wrote:
>Terry wrote:
>
>> I feed a 100mAp-p square wave in and got this:
>> http://hot-streamer-dot-com/temp/02090601.gif
>> The data file is at:
>> http://hot-streamer-dot-com/temp/02090601.CSV
>
>> The input is in blue as picked up by a 60MHz 10:1 probe.  The top is
>> the output.  The output looks nice and stable but feeding the 50ohm
>> A33120a's output right into a 1 ohm resistor through about 3 feet of
>> coax causes a big spike... 
>
>Yup, you've quite a high ISWR there!  Perhaps a 50ohm termination
>would have been better :)
>
>Must be quite a bit of capacitance floating around somewhere to get
>a fundamental ringing with such a long period with only 3' coax.

I think that is more the signal generator's electronics burbing.  I think
they use high speed feedback loops in it for precise level control that may
be a little "hot".  I went to three inch wire leads and no difference.

Scope with 50ohm termination:
http://hot-streamer-dot-com/temp/02090711.gif

Scope without 50ohm termination:
http://hot-streamer-dot-com/temp/02090712.gif

Pulling signal right out of fiber probe port:
http://hot-streamer-dot-com/temp/02090713.gif

Here is the noise signal you asked about:
http://hot-streamer-dot-com/temp/02090714.gif
http://hot-streamer-dot-com/temp/02090714.CSV

And with no input:
http://hot-streamer-dot-com/temp/02090715.gif
http://hot-streamer-dot-com/temp/02090715.CSV

Not sure what that spike is??  Noise from something...

I think the bottom line here is that the present probes were only made for
2MHz.  The 40MHz "connector" does come in useful but it is too rough for
detailed work.  The transmitter should be ok (but I'll chech into it).  The
real problem is on the reciever side.  I need a nice clean signals right
out of the fiber modules that don't have a ton of other electroncs on them.
 Just a place to go directly to a scope probe...  There is just too much
junk in the way now that I don't think we will be able to work around.  So
give the above a look and tell me what you think.  I think I have all the
parts here for the HF fiber probe...  Starting on it now ;-))

Cheers,

	Terry

>
>The probe bandwidth looks to be a lot narrower than 40Mhz with a 
>pronounced roll-off from quite a low frequency.  Can you capture
>a white noise current at a timebase of 1uS/div?  Two traces would be
>nice, one with the noise source turned on, and another with the
>source off to get the background.  Use a 50ohm load to terminate
>the test current, and a 50ohm load at the scope end of the coax leading
>from the fibre optic receiver.  That'll take all the cable responses
>out of the picture and we'll be able to see the true amplitude response
>of the probe (although not its phase response!).
>
>So far it looks as if those ringing signals from the breakout current
>could be of rather higher amplitude than the low frequency calibration
>suggests.
>--
>Paul Nicholson
>--
>