# Digital Oscilloscope to view high frequency signal

1. ## Digital Oscilloscope to view high frequency signal

Hi,

I read your reply in one of the posts and thought you might be able to help me. I am using Keysight 2.5GHz oscilloscope to view a 121.5MHz signal across a 15pF capacitor. The data sheet says it should be a square wave but I am getting a sine wave of 121.5MHz. The probe bandwidth is 500Mhz with 10MG and 9.5pF capacitance.

Could you advise what am I doing wrong?

Kind regards

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2. ## Re: Digital Oscilloscope to view high frequency signal

Maybe the problem is the bandwidth. When you have a square wave you have energy in all odd harmonics of its fundamental frequency. So your bandwidth is not large enough and only pass the firsts two harmonics.

3. ## Re: Digital Oscilloscope to view high frequency signal

It is not your bad test method. I assume you have a std 10:1 probe with long ground wire and probe clip. the probe input is say 3~10pF //10M but the cable capacitance at roughly 60pF /m for ultra low C coax instead of typ 50 ohm coax at 100pF/m is a problem with probe ground inductance.

Next your std 10cm ground clip at 1mm is 100nH (102) and 150nH if 0.5mm thick or 70nH if 5cm long. Now calculate the resonant frequency of LCR and you get a sine wave.

So your probe must be SMA from a pad from x Ohm source to 50ohm 1:1 coax direct to scope or try spring wire 5mm long wrapped around probe barrel and remove clip to expose <5mm tip to get maybe 300 MHz bandwidth . coax quality and proper probe use is critical. If source is 50 Ohm CMoS and you make a 10:1 50 ohm pad, the scope must also be terminated to 50 ohms for textbook waveforms with compensation cap added as required.

I don't know your probe but it needs to be 50ohm terminated to prevent LC resonance , then scale accordingly.

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4. ## Re: Digital Oscilloscope to view high frequency signal

Indeed, your probe's impedance and reactance is so similar to the impedance of the signal load you are measuring on (15pF cap) that it is significantly affecting the waveform.
With these sorts of measurements, you may have to resort to an active, FET input probe for a more accurate image. The lead length and grounding tips mentioned earlier are good to follow and may help you to get closer to not affecting your measurement with your measuring device, but these sorts of sensitive measurements are often done with the active FET probe to minimize measurement error

5. ## Re: Digital Oscilloscope to view high frequency signal

its not that simple with a long ground wire and it's actually more complex than this and much higher impedance than the signal imedance, unless he has 10cm bread board wires
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6. ## Re: Digital Oscilloscope to view high frequency signal

The real circuit is even more complex because the coaxial cable is intentionally lossy, using a resistance wire as conductor.

The 100 nH ground clip + around 10 pF probe input capacitance circuit is however sufficient to explain why harmonics of the 120 MHz signal are attenuated. The probe manual fine print would probably tell that 500 MHz bandwidth can be only achieved with a very short (e.g. coaxial or "bayonet" tip) ground connection.

500 MHz or GHz bandwidth with "flying wire" connection is only possible with an active probe.

7. ## Re: Digital Oscilloscope to view high frequency signal

They do have a passive 500Mhz probe option but they don't tell you how to use and the equiv. circuit has compensation more complex than I have shown.

Obvious to me is to use tip and barrel in a test socket or twin wire tips <<1cm

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