= ) 160~320mm supply or ground lead length including probe ground path length on cct board gnd location. This raises resonant frequency by inverse square root (LC), so to raise 25MHz to 200Mhz 1:8 requires reducing the ground lead length by a factor of 64 down to ~5~10mm from (est. Thus to expect an increase in resonant frequency requires better impedance matching of the probe 10RC+L : 1RC ratio by reducing the L value. The computed resonance is dependant on the length of the ground lead inductance. This is common with long ground lead inductance 0.5~1nH/mm and special coax used in 10:1 probe < 90pF/m. The underdamped square wave indicates peaks every 2/5 in 1cm at 50ns per cm=division approx. 3.Select Counter as ON or OFF in the right has given the correct solution.Īnalysis. 2.Push the Acquire button to show the right menu.1.Push Trigger Menu button, set the trigger type to Single, set the trigger mode to Edge, select the signal source.The counter is displayed at the bottom of the screen. Only if the measured channel is in Edge mode of Single trigger type, the counter can be enabled. The frequency range is from 2Hz to the full bandwidth. The counter can only measure the frequency of the triggering channel. If this internal counter uses the main timebase (having accuracy of 0.01%) then its least-significant digits are suspect. Your OWON 'scope has an internal counter claiming 6-digit resolution. If you need more accuracy, measure frequency with a frequency counter instead.a decent frequency counter should contain a more accurate time base. Your OWON oscilloscope has a time-base accuracy of \$\pm100\$ parts-per-million. If the MCU oscillator has an input pin and an output pin, probe the output pin. It will add some capacitance to the MCU oscillator, which will likely shift its frequency lower. Your oscilloscope photo indicates that you are able to measure the MCU clock at 16 MHz quite nicely. Set the oscilloscope to 50 ohm input impedance if using this approach. If the source cannot drive a coax cable directly a 470 or 910 ohm resistor at the probe point can make an ad-hoc 10:1 or 20:1 attenuator with very good frequency response (up to a few GHz). To get the best waveform I don't use a probe and instead use coax wired directly to the signal. Your oscilloscope probe accessories will probably contain some small springs that are used for this purpose. The probe should have an extremely short ground - a couple of inches is too long. The wave shape distortion you see is almost certainly because of your probing technique. If you want to measure the frequency accurately and your scope does not have a built-in frequency counter you will need a frequency counter/timer such as [Keysight 53230A 350 MHz Universal Frequency Counter 1 It may only have an accuracy of 1% or so.Īs points out in his answer some oscilloscopes do have counter/timers built into the instrument that can give high-accuracy (typically only for a single chanell) but the frequency displayed using the normal measurement facilities are usually only obtained after the signal is sampled and so will not be as accurate. However an oscilloscope is usually not the correct instrument to accurately measure frequency. The shape of the waveform won't affect the fundamental frequency of a signal.
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