We want to make < 1 Hz Frequency measurement with a 53220AN. We have a 33250A feeding this counter with a 100 mHz negative ramp, ampl: 4vp/p, offset by -2 VDC, Going into the counter with a 50 Ohm term. Counter setup as follows. Range 5 V, coupling: DC, 100KHz Filter: off. Trigger: -2.000V. My question is why does the counter need to have the 100KHz filter on for it to make the measurement? The counter will display in the 50V range, but not in the 5V range with the filter off. We have have 4 of these counters and all 4 seem to exhibit these same symptoms.