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I have a microcontroller that outputs a 50% duty cycle PWM signal at 500kHz. That is then connected to the gate of a MOSFET. Now, I want to measure the PWM signal at the gate of the MOSFET. If I connect the probe, to measure it, I measure 15Hz and the signal is no longer a square, but a "sawtooth like". If I don't connect the probe, the MOSFET is switching at 500kHz.

I have a SIGLENT SDS1102DL oscilloscope, its channels are rated 1MOhm and 16pF. Is this the reason the frequency decreases?

How can I overcome this problem?

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  • \$\begingroup\$ Is there a x10 switch on your probe? If so, try using it. \$\endgroup\$
    – Majenko
    Commented Dec 21, 2014 at 14:18
  • \$\begingroup\$ @Majenko It gets better, but still.... I've noticed, that if I connect the probe to the microcontrollers PWM pin insted of MOSFETs gate, the frequency doesnt change. I guess there is a loss connection or something somewhere. Thanks anyway. \$\endgroup\$
    – Golaž
    Commented Dec 21, 2014 at 14:48
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    \$\begingroup\$ If you're seeing 15Hz (!), I would expect you're seeing aliasing. Try starting with a high sweep frequency and reducing it. It's unlikely the micro can drive the MOSFET gate directly at 500kHz, most MOSFETs have a LOT of input charge and your micro is capable of tens of mA or less- you probably need amperes of capability to drive the MOSFET smartly at 500kHz. That's a much worse problem than the breadboard. \$\endgroup\$ Commented Dec 21, 2014 at 14:59
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    \$\begingroup\$ @Majenko: No, that is very unlikely the problem. The issues with breadboards are overrated, especially at such a low frequency as 500 kHz. Maybe there is a extra 10-20 pF capacitance on that line. That shouldn't bother a micronctroller output producing a 500 kHz square wave. I think Spehro is on the right track. Even high capacitive loading shouldn't change the frequency since that net is not part of the oscillator itself. This is almost certainly a measuring artifact. \$\endgroup\$ Commented Dec 21, 2014 at 15:14
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    \$\begingroup\$ Combining Spehro Pefhany and Majenko anwsers worked. I soldered the circuit on a PCB and used a BJT push-pull MOSFET gate driver. Thanks! \$\endgroup\$
    – Golaž
    Commented Dec 21, 2014 at 16:35

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Combining Spehro Pefhany and Majenko solutions worked.

I soldered the circuit on a PCB to get rid of stray capacitances and used a BJT push-pull MOSFET gate driver, to supply more than a mere 20mA to quickly turn it on and off.

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