yeah i now understand that it has to go to 0v at some point in order for the dmm to read it, but my question for your particular unit if the voltage does not go to 0 at any point what does the dmm show ? all zeroes or some seemingly random value ?
because mine without the 100nf cap it shows a seemingly random freq and a duty cycle of around 50-20% higher than the gm328 is generating
It actually shows 0, but as soon as I touch the test leads with my finger, it settles at 50Hz after a while. This is very sluggish, it stays at 50Hz for a while even after I let go of the test leads.
Do you have any interfering devices at your place?
The difference might be the blocking capacitor used (try other values for a different response).
Frequency on open circuit connection - any devices the leads are running close to (LED lights?) What about the signal generator, is it plugged into a noisy power supply? I would expect a high impedance input with loose leads connected to the jacks is working like an antenna for noise. Try using shorter leads and taking the setup somewhere away from other electronics.
A square wave is not the best type of wave for evaluation. Mathematically, it is the sum of many sinusoids at incremental harmonics. As the frequency goes higher, the device could potentially respond to harmonics more than the main frequency, because higher frequency = less sensitivity. The shape of the wave may be getting very distorted with your setup.
Is it possible for you to generate a sinusoidal wave, which is less likely to distort, ring, or contain harmonics?
What does the AC voltage measure at the different frequencies you are trying? Is the trend to get lower volts each time you double the frequency? Where exactly does everything fall apart, and how does that compare to the manual's response?
so i took special atention at Paul's advice about a possible noisy power supply, and turns out that was the issue, so for example i had the connector and cable just laying on the table next to the DMM as soon as i connected it to mains it would start reading a frequency, and as soon as in unplugged it from mains the dmm was back to zero
so i borrowed a 9v battery from somewhere and powered the gm328 from that and now it could actually read freq and duty cycle correctly on its own !
what i did noticed is that as the pwm frequency went up the duty cycle would read slightly higher, ie at 25khz 50% would be read as 50.7% but at say 10khz it would be read at 50.1%
for some reason at the pwm/square wave outputs on the pcb there were some blind spots in the freq generation for example 800khz wouldnt be read at all, but if i tried instead red probe on zif 3 and black on zif 1 it would read them correctly and even the pwm from k firmware was dead center
however at max on its own it could only read up to 2Mhz anything above it wouldnt, lowest i could get was a little over 1hz, i tried lower than that but it would show it on the dmm from like half a second then go back to nothing
adding the capacitor again it allowed it to read the full 4Mhz
but now another thing that has gotten me concerned is this:
so i had this 1st GM328 unit that came deffective and i have someone online that has been helping me diagnose it telling me what he sees on his and how and what to test
anyway one such test was to put red probe on zif socket 1 and black probe on ground
on old DMM which is a UNI-T UT33A (old non plus version that has hfe measurement) it reads 23.35Mohms
and this new DMM the ZT-225 using the exact same leads reads 5.42Mohms
but meassuring loose resistors both meassure about the same +- 1 or 2 ohms
so idk if i should be concerned or not
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