Frequency accuracy varies when I change Volts/Div on Hantek scope

[Alanalan]

I am an intermediate beginner in electronics and brand new to digital scopes. I have the Hantek DSO5102P

I noticed that when I hook the scope to the signal generator and pump in 10V at 1.7 KHz (sine wave) that the scope's onboard frequency counter loses accuracy when I lower the Volts/Div from 5v/div past 500mv/div . That is to say, that the last division the Hantek's onboard scope is accurate is at 500mv. When I go below that to 200mV, 100mV, 50mV, 20mV, and 10mV the frequency counter becomes way off.

However, when I view the Hantek's measured frequency it is spot on.

What gives?

Addendum: I lowered the Voltage out from the function generator to 1V and raised the frequency to 1MHz. Now the onboard frequency counter works all the way down to 10mV/Div with absolutely no problem.

What I don't understand is that the V/Div is only a "monitoring" function, yet when I change it it seems to affect the onboard frequency counter which, to my mind, should not be affected at all. Anyone know what's happening here ? I noticed the same effect on my analog scope as well.

Alan

[radiolistener]

Frequency is measured with hardware trigger, and trigger needs some min signal span, if you reduce vertical gain the trigger may not work when signal span goes below some limit. Usually it's about one grid cell on the display, but it may depends on oscilloscope type.

When you increase vertical gain too much, it may lead to a false triggering due to noise. Also DC offset may prevent trigger to work, so you're needs to check if trigger level is at the center of signal.

[Alanalan]

Thank you!
That makes sense.