Like any other test instrument, an RF frequency counter has a number of specifications associated with it.
Knowing what the specifications mean and how they affect performance can enable the choice of the correct RF frequency counter to be made more easily and successfully.
Input coupling specification
The input coupling can be a key issue as it affects other specifications. It is often possible for the input to be coupled directly, or via a capacitor, i.e. DC or AC coupled. The selection being made via a switch.
AC coupling may be preferred when there could be a DC bias on the signal. DC coupling can give better performance so this is the state in which the frequency counter would normally be used.
Frequency range specification
The specification for the frequency range of the counter is also important. Some counters may only be required for use up to a 100MHz or so, whereas others may be needed to perform up to frequencies of 1GHz, 5GHz or more. Obviously more sophisticated circuitry is needed to handle the higher frequencies. Accordingly the frequency range of the test instrument is needed.
The frequency range specification for the frequency counter will be different dependent upon the coupling used. DC coupling typically gives a better response. The DC coupled option typically extents to 0Hz, whereas the AC coupled option will have a minimum frequency - often just a few Hz.
One key specification for RF frequency counters is their input impedance. For many RF applications matched impedances are needed and therefore it is necessary to understand what impedance the frequency counter will present, whereas for other applications a high impedance is more suitable.
- High input impedance: For many of the lower frequency instruments, up to a few hundred MHz or so, it is typical for the frequency counter to present a high impedance. This is specified in Ohms – typically a MΩ or so. A value of capacitance will also be quoted in the specification. This is the value of capacitance that exists across the input impedance. This is normally low, often around 25pF or so. These values often need to be known so that the amount of loading placed upon a circuit is known. Ideally the frequency counter should not load the circuit, but in reality any probe will have some effect. Knowing the amount of loading enables this to be accounted for in any measurement calculations
The high impedance inputs can often be used with a x1 oscilloscope type probe for easy measurement. For this style of testing, AC coupling may be appropriate if a DC bias may be present. For these inputs a BNC connector is normally used.
- Matched input impedance: For other applications a matched input may be required. This is normally the case for frequencies above 1 GHz or so. An input impedance of 50Ω is standard, and a BNC or an N-type connector is may be typically used.
For this style of input the signal is directly fed into the matched port and frequency is measured. Where only a small portion of the signal is needed, a power divider or coupler may be used. However beware of overloading the input as they are only able withstand small amounts of power.
May instruments will have both high impedance and matched impedance capabilities. The details of the impedances available will be given in the overall frequency counter specification.
Frequency counter sensitivity specification
Another important frequency counter specification is its sensitivity.
This is the minimum signal that the frequency counter is able to successfully count. A typical specification might be: Sinewave15mV RMS from 30Hz to 100MHz.
In this specification the type and size of the signal is specified. To obtain the peak to peak value of the signal the 15 mV is multiplied by 2.818 (42 mV peak to peak).
The frequency range is also specified as the sensitivity will vary with frequency. Outside the range specified it will fall away, although it may still be possible to use it outside these ranges, no values may be given.
Signal range or maximum signal specification
Although sensitivity is a key specification for frequency counters, the maximum signal is also important. This can be specified in a number of ways according to the type of input.
This is generally specified in one of a number of ways:
- DC voltage: For high input impedance inputs that are DC coupled, the voltage is generally specified as the voltages that must not be exceeded. For example one frequency counter specification gave a range of 0 to 5 V, i.e. the DC voltage must not be less than 0 V and it must not exceed 5 V.
- AC voltage: For high input impedance inputs that are AC coupled, the specification may be slightly different. Often the specification may be given in volts RMS (root mean square) or as a peak to peak voltage value. The peak value is 2.181 times the RMS value. One frequency counter specification read, 1 V RMS or 3 V peak to peak.
- Match 50Ω inputs: For matched inputs the specification may be given either in volts RMS or as a power level, often in dBm (deciBels referenced to 1 mW). One frequency counter specification read: +13dBm (1 V RMS) maximum.
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