Improving the Range of UK Channels 38-40 Wireless with the 788T

Note: While this article mentions Audio Limited products specifically, the facts and solutions apply generally to most professional wireless receivers.

Sound Devices has investigated reports from 788T users in the UK who have experienced reduced RF range when using a 788T together with Audio Limited wireless in a bag.

With the cooperation of Audio Limited, Sound Devices performed an RF analysis of this issue. The main cause of reduced range was found to be RF radiation coming from cables without their connector's shell grounded plugged into XLR inputs 1-4 of the 788T. Input channels 5-8 (TA3 inputs) and their cables were shown to have no effect.

Adding a short wire inside the XLR input connector connecting the shell to the shield-pin substantially reduces the RF emissions, significantly increasing wireless range. This connection ties the shell of the XLR connector to the cable's shield and ensures a thorough and effective circumferential shield all the way to the chassis of the 788T. Shortening the exposed wires also improves shielding. Figure 1 shows an XLR connector before the modification. Figure 2 shows it after, with an arrow pointing to the added wire.

Figure 1

Figure 2: after modification, wires shortened and short shell ground wire added (arrow)

Digital Equipment, RF Signals, and the 788T

All digital equipment emits RF signals with different frequencies and varying amplitudes. Whether these signals pose a problem or not depends on the surrounding equipment’s sensitivity to the frequency and amplitude of the unwanted signals. Receiver manufacturers strive to make their equipment extremely sensitive at the antenna. Therefore receivers can be affected by very low-level signals if they occur on a frequency that is susceptible to the receiver. There are two ways stray signals can be emitted from a piece of gear:  either radiating directly out of a unit's chassis through gaps or lack of shielding, or via cables. Any cable plugged into a unit has the possibility to act as a radiating antenna at various frequencies.

During the design of the 788T, Sound Devices took special care to minimize RF emissions. Some examples included RF gasket material used in chassis mating surfaces, since minuscule gaps in mating surfaces can act like antennas at specific frequencies, allowing internal signals to escape. Ferrite components are used on signal lines to reduce the signals emitting from the PCB traces. Additionally, the path of signal return current flow is controlled to reduce radiation from traces that formed loops. CE and FCC compliance testing at an accredited laboratory verified the 788T met both European and US standards for both conducted and radiated emissions.

Channels 38 to 40 RF Evaluation

Some users have reported problems of reduced range on Channels 38 to 40 at 606 to 630 MHz while using a 788T and Audio Limited wireless systems. Sound Devices performed an RF evaluation and measured low level signals being radiated from certain types of XLR input cables. TA3 input cables on channels 5-8 were also tested. No TA3 cables were found to emit 606 to 630 MHz signals.

While increasing the distance between the receiving antenna and the XLR can greatly reduce the effect, various other solutions were also evaluated. They included filtered XLR connectors, ferrite clamps, ferrite beads and shunt capacitors. However, the best solution proved to be adding a short wire inside the XLR connector between the XLR shield pin and the XLR shell connection. The RF spectrum analyzer screen capture in Figure 3 shows the 606 to 630 MHz range and the improvement of adding the extra ground connection. The yellow trace indicates the signal without the ground wire. The blue trace indicates the signal after adding the ground wire. The significant reduction of RF energy is almost to the noise floor of the spectrum analyzer. Channel 38 is approximately the three far left vertical divisions. Note the reduced resolution bandwidth of the analyzer that was required to lower the noise floor and display the very low level signals.

Figure 3: emissions without ground wire (yellow) and with ground wire (blue)

In addition to the lab measurements above, outdoor range testing was performed to compare performance with and without the ground wire. A significant improvement in range was observed when using the grounded XLR cable over the ungrounded cable. Sound Devices' test was a worst-case scenario with the XLR cable taped directly to the antenna of the receiver. With the cable fix, no difference in range was detected whether the 788T was turned on or off. Results may vary with other equipment and environmental variables.

Sound Devices recommends users experiencing reduced range in the Channel 38 to 40 at 606 to 630 MHz to verify whether a ground wire is connecting the shield pin to the shell of input XLR cables. If it is not present, wireless range could be negatively effected.