A zero intermediate frequency radio frequency digital mixer includes a low noise amplifier, a first RF mixer stage for mixing an RF signal from the at least one LNA with a first local oscillator signal operating a first predetermined frequency. The flexible spring member can be at least partially formed from elastomeric material. The pH measurement system can improve the response rate by reducing the time constant depending on the dynamic characteristics of the glass pH sensor, and it is possible to diagnose whether the pH measurement system is defective or not by calculating the equivalent internal resistance and the supply voltage of the glass pH sensor. When the final diameter of the laminate tube is reached, the cladding has been laminated to the exterior of the substrate tube by a bond generated from the rolling andor cold drawing operations.