EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Dekatron on April 23, 2016, 09:12:30 am
-
Hi All,
I am quite new to using the 33120a that I bought recently. I have checked this forum and other sources but can't find an answer to my question.
I have hooked it up to two different scopes, one at a time, one being an old HP 1740A (still in very good order) and also to my Lecroy Waveace 2004. Both scopes show a quite nice (some ringing) signal on the sync-output up to 15 MHz, but the main output shows more of a sine wave signal on both scopes the higher the frequency gets.
I have used an RG-58 cable with a T-connector at the scope end with a 50 Ohm terminator (also tested the 50 Ohm input on the 1740A but got same results, without the terminator then) when making these measurements.
Am I making something wrong or what could be the cause for the square wave output looking like a sine wave when it gets above 5-7 MHz (approximately)?
Best regards,
Martin
-
Am I making something wrong or what could be the cause for the square wave output looking like a sine wave when it gets above 5-7 MHz (approximately)?
First understand how a square wave is composed from sine waves - the Fourier series.
7MHz fundamental => 21MHz 3rd harmonic (no second harmonic, of course).
33120 square rise/falltime <20ns. 20ns => bandwidth of 18MHz (rule of thumb BW=0.35/tr).
21MHz > 18MHz, so the 20MHz will be attenuated.
-
The sync output is only concerned with timing rather than wave shape so probably has a faster rise time but more ringing and no control over amplitude and duty cycle.
From the data sheet the rise time on the 33120a is < 20nsecs so up to 20 nsecs. If we assume the fall time is the same and we have a 50% duty cycle then at 10MHz with a period of 100 nsecs, a half wave must happen in 50 nsecs and 20 nsecs rise plus 20 nsecs fall will take up 80% of this so it will be looking very rounded at 10MHz.
At 15MHz the half wave length (or half period) will be 33 nsecs so there won't be time for a complete rise and fall so don't be surprised it looks very sine like.
-
Thank you tggzzz and jpb, that explains it!
I didn't think about the rise time.
I also found the schematic and saw that the Sync output is a simple logic output but the main output has a proper driving stage.
-
I didn't think about the rise time.
With any data sheet or legal contract, always look at all the Ts&Cs or parameters, understand why they are there, in what circumstances they are relevant, and what they aren't saying.