A important learned lesson from the practical tests is that the THD+N is increasing very fast to an unacceptable value, as the disturbance signal power grows by e.g. 1 dB. An audio signal with a THDN = 5 % is useless in the same way as if the connection is completely disrupted (*CD), both signals are unacceptable.
The influence of the AM modulated disturbance signal on the pocket transmitters is depicted in Fig. 3 for the coarse frequency grid (400 MHz to 4 GHz in logarithmic stepping of 1 %) and for the fine frequency grid (530 MHz to 560 MHz) in Fig. 4. All three systems have different standard THD+N values, System A is around 0.23 %, System B is around 0.12 % and System C is around 0.14 %. All of them are well below the limit of 1 %, so they all fulfill the specification in the undisturbed as in the disturbed case.
System A does not show any influence, whether in fine nor in coarse frequency grid. System B show sensibilities in both grids, but still below the limit. System C exhibits rises of the THD+N, but not exceeding 0.2 %.
Figure 5 show the IMD related to the TX carrier power in [dBc] for 50 mW transmit power. System A has a quite stable IMD around −38 dBc, while system C has the highest IMD around −35 dBc. System B has the lowest IMD of all, below the TX frequency of 544.4 MHz it is around −40 dB and above the lower value of −50 dBc.
The IMDs of the pocket transmitters at the lower transmit power of 10 mW is depicted in Fig. 6. System A has the best performance with an IMD around −45 dBc, while system B is around −30 dBc below the TX frequency and around the lower value of −35 dBc above the TX frequency. System C offers two modes, Mode 1 is around −33 dBc and Mode 2 is around the lower value of −38 dBc. In both modes the IMD stays on the same levels on both sides of the TX frequency.
The disturbance test of the receiver units is interesting, as they have proven to be more sensible than the transmitters. The disturbance signal is AM modulated, the field strength is 3 V/m. Both are tested in fine and coarse frequency grid. Figure 7 shows the THD+N versus the coarse frequency grid. System A is exceeding the THD+N limit of 1 % in the TX frequency region of 544.4 MHz and has smaller peak around 1.7 GHz. System B has an unacceptable high THD+N peak of 1.7 % around 558 MHz. System C has a small peak of 0.6 % around 900 MHz.
Figures 8 and 9 show the THD+N versus the fine frequency grid for the receiver tests with the small step size. They contain the same data but use different scaling on the y-axis to show the behavior in lower THD+N regions better. These pictures reveal strong weaknesses for system B with a THD+N of 29 % close to the TX frequency and for system A with a THD+N of 70 % at approximately 559 MHz. System C has small THD+N peak at 0.6 % at a frequency of 537 MHz.
The immunity measurement results of the pocket transmitter when it is disturbed with a DVB-T signal, are depicted in Table 1.
System A is always below a THD+N of 0.93 %, while system B has the best performance, because it is always below 0.36 %. System C exhibits the weakest performance, because directly at the TX frequency, the connection is disrupted (CD).
The immunity measurement results of the pocket transmitter when it is disturbed inside the used transmit band (“Brute Force” test) with a disturbance signals using different modulations are depicted in Table 2. The disturber power is risen until a transmission system fails. They start to fail within small amounts of transmit power, like 0.5 dBm. Some systems immediately have disrupted connections, others have a very high THD+N. For practical usage, all these cases have the same effect, the audio signal is no longer usable and can be rated as fail.
As a simplification for the measurement, only the signal generator power levels are depicted in Table 2. Nevertheless, still a good comparability between the systems is given.
In the “brute force tests” shown in Table 2, the system A is weaker than all others with the FM disturber, as it stops the connection at lower power levels than the other systems. System C is weaker than the other systems when AM disturbers are employed.
All systems are quite robust against disturbances with DVB-T modulations, system B was the most robust as a THD+N value of 10 % could not even be reached with the available power amplifiers. In general, system B is the most robust of all systems in the “brute force” tests.