Hi,
I need a measure a Vpp of a 30 us RF pulse of 100MHz, and have used edge trigger, with internal trigger at the pulse, at 10% at the beginning of the pulse on my scope. I just wonder is there a better method to trigger a RF pulse. I have a Tektronix THS 720 Scope. Also I am little confused about pulse trigger. Can someone explain it?

Thanks in advance

 

Have you got the manual to hand.

https://uk.tek.com/oscilloscope/ths720a-manual/ths710a-ths720a-ths730a-ths720p-user-manual

Pages 49/52 triggering on missing pulses or video signals...

 

Hi,
I need a measure a Vpp of a 30 us RF pulse of 100MHz, and have used edge trigger, with internal trigger at the pulse, at 10% at the beginning of the pulse on my scope. I just wonder is there a better method to trigger a RF pulse. I have a Tektronix THS 720 Scope.

Your scope isn't exactly well-suited for that task, the scope BW should ideally be at least 30% higher than the highest frequency content in the signal because of the scope's BW limiting filters.

Scopes with very low bandwidths like 100MHz typically have a gaussian response at their BW limit, i.e. frequency components approaching the limit will see increasing attenuation with increasing frequency. Therefore, scope BW is spec'd by the so-called '3dB point' which is the point where the signal becomes attenuated by 3dB (approx 0.707 times the actual voltage).

Now, The THS 720 has a spec'd BW of 100MHz, which means that a 100MHz signal will (in theory) appear with an attenuation of 3dB. That also means whatever you measurement results are, they'd be only 70.7% of the real value.

However, the real BW of scopes is usually a fair bit higher than what the specification says. Which means your THS 720 may well have its 3dB point at, say, 120MHz. Of course you don't know, which means your measurement uncertainty has now increased dramatically (the attenuation by the scope's BW filter can now be anywhere between 0dB and 3dB).

And that is assuming the 100MHz signal within that 30us pulse is actually a sine wave, because if not then you're looking into a 500MHz or better scope to get any meaningful results.

Besides, the THS 720 is a pretty old and slow handheld scope with a tiny sample memory (2.5k) and a terrible monochrome low-res display, which is fine for very simple tasks but performs poorly on anything more complex (thanks to the low sample memory you have to be careful as the sample rate drops pretty quickly at longer time bases, which affects its usable BW). It's pretty much the last scope I'd want to use for such a task.