A Clean Audio Installation Guide™

Application Note #1

6.0 RF INTERFACE

6.1 Open Shield Ends

6.2 Op-Amp Configurations

6.3 LC Filters

6.4 Common Mode Filters

6.5 Shielding Materials

6.6 Screen Rooms

6.0 RF INTERFACE
Often, RF related problems will remain after the above procedure has been followed. The presence of RF related problems may often be heard as an increase in high frequency noise, a "gurgling" sound, or as outright detection of the RF signal by a PN junction in an active device or by a poor solder joint. RF field intensities at a transmitter site may easily range from 1 to 100 V/Meter; as a result, the interconnect scheme described above may need modification to accommodate RF problems. With the advent of the wireless society we are experiencing, most cities now have significant RF field intensities - virtually everywhere.

6.1 Open Shield Ends
The open ended (one end) shield recommended above can cause the interconnect cable to act as an antenna. One possible cure for this is to tie the open end of the shield to its respective chassis ground through a 0.01 µF high quality ceramic capacitor (such as Erie Red Cap or other monolithic type). Note the C1/R3 network in Figure 4. This, in essence, provides a ground at RF frequencies while leaving the shield open at audio and power frequencies. However, all capacitors have their own self-resonant frequency, and you may need to parallel two or three capacitors of different values in order to have an overlapping of curves and full effectiveness at high frequencies. The 10k ohm parallel resistance assures a ground tie for the shield if this line should pass through a patch bay with a switched ground circuit. This is a place where it is possible for the line to have an ungrounded shield under patch conditions.

6.2 Op-Amp Configurations
The three basic op-amp gain configurations - non-inverting, inverting, and differential - each have different RF sensitivities. The non-inverting unity gain buffer amplifier is the most sensitive, and the well-balanced differential input stage is the least sensitive. Manufacturers sometimes use an instrumentation amplifier input stage which often consists of two unity gain non-inverting input buffer amplifiers followed by a well-trimmed differential input amplifier. This amplifier type will generally work well in all but the highest RF environments. In extremely high RF environments it may be necessary to replace the input buffers with jumpers and have a lower (10k ohm or thereabouts) input impedance for the sake of RF stability.

6.3 LC Filters
At both AM and FM frequencies, series resonant L-C networks can be placed from each line of a balanced pair to chassis ground to bypass incoming RF. These devices should be placed just inside the chassis and directly at connector terminals, observing good RF wiring practice. At FM frequencies self-resonant (parallel) chokes may be placed in series with the incoming lines, and then the lines should be shunted with capacitors (that yield a maximum of 2 ohm reactance to ground at the RF frequency of interest) after the choke. A typical self-resonant choke for 100 MHz would be a single layer solenoid approximately 1/8" in diameter and 1" long. It is tuned by varying the spacing between windings and thus changing the distributed capacitance. One can expect approximately a 45 dB reduction of RF energy from such a network.⁽⁹⁾

6.4 Common Mode Filters
A very helpful but not well known device in audio is the common mode filter. When inserted in a balanced line, this filter is effective in troublesome RF environments. A typical filter will consist of a common mode choke, which in turn consists of two highly symmetrical windings on a common toroid core, two 1000 pF capacitors, and two 10 K ohm termination resistors that tie to chassis ground. The filter shown below will have a differential bandwidth of greater than 200 kHz when driven from a low impedance source, but a common mode bandwidth of only 26 kHz. This is primarily accomplished by the common mode choke itself. When the choke sees a differential signal (equal amplitude and opposite polarity), the magnetic fields created in the core cancel and the inductor effectively disappears from the circuit. However, when the signal is of equal amplitude and of the same polarity, as is the case with the interference we wish to remove, both lines see the L-C low pass filter. As a two pole device with a cutoff rate of 12 dB per octave, the filter is down 60 dB at 1 MHz and thus prevents RF from reaching the active electronics. (See figure 5)

6.5 Shielding Materials
Additionally, further shielding may be necessary. Separate ferrous conduit for the audio cabling can be very effective with RF as well as with power line radiation. Tightening up the shielding of specific audio packages may be necessary. Numerous EMI type seals and gaskets may be obtained from either:

6.6 Screen Rooms
In a few extreme cases with which the author is familiar, entire studios had to be constructed inside RF tight "screen rooms". If you need to construct a screen room for your facility, use steel screen, if you can locate it, for all interfering frequencies including the FM band and below, since it is more lossy and thus is more effective than copper, not to mention the expense. Copper is necessary for the microwave frequencies, and there exists a gray area in between. You will need to solder the seams, and when your room is finished, be sure to use power line filters physically located at the point of power entry into the room to clean up the incoming mains.

Go to: Section 7.0