Nearly all of the present-day communication radio receivers are structured as superheterodyne types.
Figure 6 shows a simle blockdiagram of a single-front-end frequency conversion. This basic design has been popular since the 1930s.

To eliminate the disadvantage with the low selectivity in conventional eddy current equipments, a similar sophisticated Frequency Conversion principles and method was developed by Törnbloms Kvalitetskontroll.

Frequency Conversion is a very important base function in DELTATEST.

Using figure 6 a simplified explanation can bi given. When the incoming signal (RF) from the sensor reaches the mixer it is heterodyned with the oscillator (OSC) frequency to establish an IF (intermediate frequency).
The IF can be the sum or differens to the two frequencies (RF and OSC). The IF filters sets the overall selectivity for the signal handling line in the DELTATEST measuring unit. Therefor the Frequency Conversion improves selectivity performance approx 20 - 30 times, depend on the function of the frequency ratio RF / IF.

The "receiver" / ampifier required for satisfactory reception of eddy current sensor signals is generally similar to that used on the radio HF-bands, except that more care is required with the overall noise performance.
It is very important to provide sufficient selectivity in the RF amplifier and mixer stages to ensure freedom from image interference.
The ability of a eddy current front-end receiver/amplifier to reject unwanted signals is a function of both the selectivity and the linearty of the early RF-amplifier stages.
The linearity of an amplifier or mixer is always best at low signal levels, so to design a amplifier with the highest cross- and intermodulation performance, it is important to keep the RF-gain to a minimum. This conflicts with the normal requirements of high front-end gain to achieve the best sensitivity.
The answer to the problem is to look at gain distribution in the signal handling line, and optimize it for both high sensitivity and good signal handling.