Trouble in measuring gate and drain voltage of transistor at 120 MHz
时间：04-06
整理：3721RD
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Hi,
I have to measure drain and gate voltages of power transistors at around 120 MHz. The transistors' casing are soldered on PCB and the PCB has other circuitry also which is causing great trouble in measuring the drain and gate voltages at the time of transistors' functioning. As expected the gate and drain voltages appear in rectangular pulses which have both dc bias and rf component. However, the oscilloscope shows the magnitude of drain voltage pulse varying if the wires (which are soldered on one end at drain and gate of transistor and at the other end are connected to oscilloscope) are moved or contacted or if you make a hole with your hand (see picture below) and let that wire pass through that hole.
Please share your ideas or suggestions or experiences in this regard. Also, please provide material in as much detail as possible.
I have to measure drain and gate voltages of power transistors at around 120 MHz. The transistors' casing are soldered on PCB and the PCB has other circuitry also which is causing great trouble in measuring the drain and gate voltages at the time of transistors' functioning. As expected the gate and drain voltages appear in rectangular pulses which have both dc bias and rf component. However, the oscilloscope shows the magnitude of drain voltage pulse varying if the wires (which are soldered on one end at drain and gate of transistor and at the other end are connected to oscilloscope) are moved or contacted or if you make a hole with your hand (see picture below) and let that wire pass through that hole.
Please share your ideas or suggestions or experiences in this regard. Also, please provide material in as much detail as possible.
Strange. Rather than your schematic, or a picture of the oscilloscope measurements, or a picture of your test setup, you show a picture of your hands. I hope someone finds that useful.
Keith
Hi,
Thanks for pointing out the mistake. The yellow pulse is of gate voltage and green one of drain voltage.
If 120MHz signals are passing through "wires" which are allowed to move significantly or pass through your hands, then you should expect inconsistent behavior. At such frequencies you either need to confine the signals to traces on a PCB, coaxial cables, or to very short, rigidly mounted wires.