A crystal calibrator injects a harmonic-rich signal into the analyser signal path, and is very useful for calibrating the frequency (horizontal) axis of the display. It also provides an excellent test signal. The 10MHz crystal oscillator used here generates strong odd harmonics at 30, 50, 70, 90 etc. MHz. The even harmonics at 20, 40, 60 etc. MHz are also visible up to the 100MHz harmonic.
The output of the crystal calibrator output is injected directly into the signal path at the output of the input attenuator. The calibrator is controlled by switching the +12V power supply to the module from the front panel. When switched off the circuit does not appear to adversely affect operation of the analyser.
The calibrator is built into a small box fixed to the back of the input module. One reason for doing this rather than building a separate module was that I was starting to run out of SMA connectors. I wanted to keep a few spare for possible future enhancements.