In some cases, both frequency and amplitude ranges have to be specified Signal generators and frequency synthesisers are such types, and one has to specify the output frequency range and maximum power output levels. Power meter is another such instrument where one has to specify the range of frequencies for which power has to be measured and the range of power level to be measured. The measurement range needed may be from a few microwatts to a few watts. The ranges may be covered with different power sensors.

Accuracy. Typically, the term ‘accuracy’ in measurements refers to the closeness of a measurement result to the true value. The cost of the instrument increases with increase in its accuracy level.

The requirement of accuracy level depends upon the nature of measurement. For example, laboratories engaged in calibration and issuing certifiates need highly accurate instruments. A company that makes consumer articles need not have very accurate instruments.

Let’s consider another company that supplies crystal oscillators having stability of the order of 1E-7/day. Naturally, the counter used by the company should be at least ten times more stable than this. The measurement accuracy of the frequency counter depends upon the stability of the crystal oscillator used as reference for time-base generation in the counter.

Fig. 1: Power vs frequency display with different RBW settings

It may use an oven-controlled crystal oscillator or temperature-compensated crystal oscillator. Counters having stability of 2E-8/day to 5E-10/day are available and you can select a particular one depending upon the purpose of measurement. Basic counters have a low order of stability but offer options for higher-order stability. One has to mention the option in case a higher stability is required. For example, if the crystal stability is 1E-7/day, with option 01 it may be 1E-9/ day. So while ordering a higher stability, specify the option number.

Sensitivity. This gives the minimum required signal for an instrument to function. It is definedfor instruments that accept signal for making measurements. Frequency counters, power meters, voltmeters and spectrum analysers are such types of instruments.

In the case of a spectrum analyser, sensitivity is a measure of the ability to detect and display a small signal, and it is equal to the noise generated by the instrument itself. A signal level equal to the noise level will give a 3dB display. Signal levels less than the sen-sitivity cannot be detected.

Sensitivity of spectrum analysers depends on the resolution bandwidth. Sensitivity at 10kHz resolution band-width will be 10 dB better than sensitivity at 100kHz resolution bandwidth. So while definingsensitivity of spectrum analysers, resolution bandwidth is also to be specified.

Sensitivity of spectrum analysers is also called ‘displayed average noise level.’ It can be increased with the use of an additional preamplifiersupplied by the manufacturer. The preamplifie will have a very low noise figurecompared to the spectrum analyser, thus increasing sensitivity.

In the case of frequency counters, sensitivity is the minimum signal level at which the counter can read the frequency. For signals below this level, the counter will not respond.

For oscilloscopes, sensitivity is define in terms of defection factor. Vertical volts/division gives sensitivity.

In the case of power meters, sensitivity is the minimum power level that can be measured. A power meter consists of a power sensor, which is connected to the power meter through a cable. The frequency range of measurement and sensitivity depend on the power sensor used. A single sensor may not cover the entire range of frequencies as well as power levels. For higher power-level measurements and very low power-level measurements, separate sensors may be required to cover the power-level range. The cost of the power sensor may be high. So select a power sensor depending upon your application requirements. Future requirements can be met by procuring an extra power sensor whenever the requirement arises.

Maximum input. This specificationis needed for instruments that accept signals for making measurements. For power meters, the maximum input is the signal power level that can be fed to the power sensor without damaging the sensor. If signal level beyond maximum input is fed, the sensor may get permanently damaged. In spectrum analysers, the maximum input signal is the level that will not damage the firstmixer of the spectrum analyser. Signal level beyond it will damage the firstmixer permanently and sensitivity of the spectrum analyser will be poor.

Resolution. This is the ability to resolve the two values being measured as two different and distinct values. For example, in the case of spectrum analysers, two signals separated by a given frequency should appear as two different signals. This will happen only if frequency separation is more than resolution bandwidth (RBW) of the spectrum analyser. If frequency separation is less than resolution bandwidth, the two signals will merge and appear as a single signal.


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