Spectrum measurements in the E band – harmonic mixers are essential
Spectrum analyzers are the most suitable instruments for these sophisticated measurements. However, commercially available spectrum analyzers have a continuous frequency range of up to 67 GHz only. To carry out spectrum measurements in the E band, they must be used together with external harmonic mixers. The mixers multiply the spectrum analyzer’s local oscillator output signal and use a suitable harmonic to downconvert the millimeter-wave signal to be measured to the analyzer’s intermediate frequency. However, the large number of harmonics created in the mixer and the input signal’s harmonics produce a multitude of signals in the spectrum. The image frequency is not suppressed because there is no preselection.
This will not create any problems as long as only CW signals are present at the mixer’s input. With this type of signal, the spectrum analyzer can tell the difference between real signals and unwanted mixing products and their image-frequency signals that are present at the mixer output. To enable this distinction, the analyzer conducts a reference measurement prior to the actual measurement. During the reference measurement, the local oscillator frequency is increased to a value that is twice the intermediate frequency. Only signals that are detected in the reference measurement and in the actual measurement are real signals and are displayed in the spectrum.
If modulated signals are present at the mixer input, the task is more complicated. The real signal and the signal received on the analyzer’s image frequency may overlap each other, especially in the case of very wideband signals, so that it is no longer possible to tell them apart.