# Benchtop Frequency Counter - what does the AC specification mean?

Newb here. A friend and I are working on a small hobby project, trying to learn and mess around with some old electronics stuff he has. We have a need to accurately (+-0.1Hz) measure frequency of an AC square wave from the 1Hz to 5.5kHz range, 2-60v peak to peak.

When looking at something like the Victor Precision Frequency Counter 0.01Hz to 2.4GHz Digital RF Meter it lists the following specs:

• Freq range: DC couple 0.01-100Hz ; AC couple 100Hz-50MHz
• Sensitivity: "DC" 0.01-1Hz = 500m Vp-p, 1-100Hz = 80m Vrms, "AC" 100Hz - 50mHz = 80m Vrms
• Input impedance: 1 M? Channel B (50MHz - 2.4GHz)
• Freq: 50MHz - 2.4GHz
• Sensitivity: 50MHz-1.2GHz = 80m Vrms, 1.2-2.4GHz > 80m Vrms
• Coupling: AC only

Being a newb, I really don't know how to make sense of that.

My question is: Given my previously stated requirement:

need to accurately measure frequency of an AC square wave from the 1Hz to 5.5kHz range, 2-60v peak to peak.

What specs should I be concerned with?

• Freq range: DC couple 0.01-100Hz; Can't use DC coupling. AC couple 100Hz-50MHz. AC coupling OK.
• Sensitivity: "DC" 0.01-1Hz = 500m Vp-p, 1-100Hz = 80m Vrms, "AC" 100Hz - 50mHz = 80m Vrms. Your signal exceeds minimum sensitivity so the counter will detect it be we need to make sure you 60 V signal won't damage it.
• Input impedance: 1 M? [Looks as though whoever typed it couldn't figure out how to insert an Ω.] 1 MΩ is typical for an oscilloscope and should be suitable for most measurements.

Channel B (50MHz - 2.4GHz). Channel B doesn't appear suitable. Freq: 50MHz - 2.4GHz Sensitivity: 50MHz-1.2GHz = 80m Vrms, 1.2-2.4GHz > 80m Vrms Coupling: AC only.

Need to accurately measure frequency of an AC square wave from the 1 Hz to 5.5 kHz range, 2 - 60 V peak to peak.

• Use channel A.
• There is no maximum voltage listed for channel A so it would be wise to use a 10:1 probe or resistive divider. This would take the voltage down to 6 V p-p which should be fine.
• Do you think "80m Vrms" is a sloppy typo and should be "80 mV"? If so my signal would be far outside that range. Am I wrong? When you say "This would take the voltage down to 6 V p-p which should be fine." - what spec are you referring to that indicates 6vp-p is safe? Commented Oct 9, 2016 at 8:27
• 80 mV is the sensitivity of the internal trigger. Any lower and it may not be able to detect the transition from high to low and back and will read zero or be dodgy at best. There aren't any specs given for maximum but 5 to 10 V would be a normal requirement. Commented Oct 9, 2016 at 8:30

If you want to measure 5.5 kHz with a precision better than 0.1 Hz, the error of the time base of the frequency counter should be better than 18 ppm (part per million). If there is an oven controlled xtal oscillator, you should look for the warm up time necessary for precise results.

You would need to switch from DC coupling to AC coupling when your sampled frequency exceeds 100Hz. There may be some range overlap but this seems not to be the case from the specifications.

The AC range on channel A is your best bet but may give poor results with weak signals at low frequencies.

You may need to attenuate signal to 1 to 10 V as required by max input of counter. An opposed pair of shunt Zenner diodes and series resistor combination may be sufficient

If you have a scope probe with a 10:1 divider for BNC connector, you may connect directly to CH A

• use AC input mainly until low frequency cutoff (<100Hz)
• then switch to DC trigger
• adjust trigger to either side till it stops and set to mid point.