Signal bandwidth is the useful range of signal frequencies for a switch. NI switch modules are specified for frequencies down to DC so bandwidth is specified as the maximum recommended signal frequency. In many cases, this is the highest signal frequency that the switch can maintain <3 dB of insertion loss. Some RF modules specify bandwidths based on reflections instead of insertion loss. Refer to VSWR.
If a signal is purely sinusoidal, the bandwidth/insertion loss rating of a switch module can be directly applied. For signals that have multiple frequency components, like a square wave, the rating may be difficult to apply.
A square wave can be represented using a natural harmonic series, where only the odd harmonics are used. The frequency components that make up an ideal square wave are infinite, as shown below:
Where ω is frequency in radians per second.
A square wave may have a fundamental frequency below the rated bandwidth. However, harmonics above the rating may be attenuated by increasing insertion loss. For example, consider a 2 V peak-to-peak square wave. The square wave can be approximated by the summation of multiple harmonic sine waves. Three harmonics are shown in the following figure: the fundamental frequency (or first harmonic), the fifth, and the 13th harmonic.
Figure 6. Square Waves Represented by Sine Waves of Varying Frequencies
If the frequency content of a square wave is known, the required switch bandwidth can be determined based on the highest harmonic to be passed with minimal distortion. If the frequency content is unknown, the -3 dB point can be approximated using the rise time of the harmonic to be preserved. The rise time is related to the -3 dB point bandwidth by the approximation:
Where τ is the rise time from 10% to 90% of the signal amplitude. Using the fifth harmonic of the square wave from the previous example:
Figure 7. Rise Time of Fifth Harmonic
Where τ = 0.0515 s.
To preserve the rise time of the signal, a switch with an insertion loss of 3dB at 6.79 Hz minimum should be chosen. For a square wave, once the 5th or 7th harmonic is reached, the change in the rise time is minimal. In the previous example a switch with an insertion loss of 3 dB at 7 Hz will be sufficient to pass as a 1 Hz square wave. The rule of thumb is to choose a switch for square wave signals with a -3 dB point at a frequency 7 times the fundamental frequency of the square wave. If less attenuation is desired, the -3 dB point should be a higher frequency.