Meeting Review, January 20, 2004
Brown described several
simple principles that can be used to prevent radio frequency entry into
microphones and microphone preamplifiers.��
He showed the results of research that he conducted over the last few
years to identify causes of the problem and the effectiveness of the
solutions.� Mr. Brown studied the
problem first in controlled laboratory conditions, and then verified his
observations in field trials in the Broadcast radio signals
and cellular telephones are the most common sources of interference, but
fixed and hand-held two-way radio transmitters are also potential
sources.� In tests conducted in the
downtown Key elements of Brown's
research are the extension of the work of Neil Muncy,
published in the June 1995 JAES, that identified two
mechanisms for the coupling of audio frequency interference into audio
systems. They were 1) the mis-termination of
shields within equipment (the "pin 1 problem"), and 2) non-uniform
magnetic coupling of shield current in balanced audio cables to the two
signal conductors, a mechanism he called shield-current-induced noise
(SCIN).� Brown's research,
published during 2003 in a series of four AES papers, shows that these
mechanisms are also principal causes of RF interference to audio systems to
at least 1 GHz. Brown explained that RF
interference enters equipment due to five principal design defects, ranked in
importance from most to least. 1) the pin 1 problem; 2) Insufficient
differential-mode filtering of input and output wiring; 3) Insufficient
common-mode filtering of input and output wiring; 4) Insufficient shielding
of the equipment itself, typically an unshielded enclosure; 5) Insufficient
filtering of power and control wiring.�
Brown's work has so far focused on the first two of these mechanisms. Muncy's
"pin 1 problem" is simply another name for common impedance
coupling.� Two steps are needed to
avoid it.� First, within equipment,
each cable shield (pin 1 of an XLR connector) must have a very short, low
impedance path to the shielding enclosure of the audio equipment.� While not a component of the pin 1 problem,
a connection to the outside of the enclosure is best, both because skin
effect keeps RF on outer surface of the enclosure, and also because the
connection, however short, cannot act as an antenna inside the enclosure. A short connection alone
is not a solution - if the shield is also connected to circuit common, the
voltage drop produced by RF shield currents will be added to the signal,
detected (rectified) by signal circuitry, and heard as interference. Thus the
second component of the solution is that circuit common must be connected to
the shielding enclosure, not to the shield.�
Brown showed photographs of the connector construction of many popular
and expensive microphones where this principle was either neglected or poorly
implemented.� The construction of the
audio cable can have a large impact on the RF signal that the equipment
sees.� Brown often �accidentally�
referred to audio cables as antennas, reminding attendees of their dual
nature.� Using an RF generator to
impose an RF current through the shield, Brown measured
shield-current-induced noise (SCIN) in more than 20 cables up to 4 MHz, and
some to 300 MHz. His data showed that RF interference coupled by
shield-current-induced noise (SCIN) in foil/drain shielded cables was
typically 20-30 dB greater than with braid-shielded cables below about 4 MHz,
but that foil shields are superior above about 30 MHz.� His research suggests that a foil/braid
shield comparable to that commonly used for VHF/UHF antenna distribution can
provide the best performance at all frequencies. Brown also showed
examples of new XLR-style cable connectors that are being developed by two
companies to control RF interference.�
The designs are based on work by Brown and members of the AES
Standards Committee.� The cable-mounted
connectors have a concentric capacitive termination of the shield to the
connector shell, a dc connection of the shield to pin 1, and a ferrite bead
around the pin 1 connection.� Connecting the cable
shield directly to both pin 1 and the shell would help equipment with a pin 1
problem, but would add noise if the connector were used with a grounded
wiring panel in a building.� The
concentric capacitive connector avoids this dilemma.� Brown showed this connector greatly reduces
interference in all of the products tested, provided good contact is made to
the shell.� The results were the same
both in the lab and in the field.� Another prototype
connector is intended for use with in microphones as the output
connector.� It connects pin 1 to the
microphone case via a very short strap and places ferrite around all three
pins. No test data was available for the latter connector. Brown played a recording
made on a DAT recorder that had very poor VHF immunity, made in a downtown The second essential
step in controlling RF interference is to block the radio frequency signals
carried on the signal pair from entering the equipment. Audio equipment is
often designed with excessive bandwidth, allowing AM broadcast signals
coupled onto the signal pair (by SCIN) to be amplified and detected. Brown
urges limiting bandwidths to the minimum required for good phase response,
and certainly no more than 100 kHz.�
Filtering with simple bypass capacitors is sufficient for interference
below 10 MHz, but additional small capacitors and series chokes are needed to
extend immunity to VHF and UHF.� Brown
noted that carefully selected ferrite beads could provide the inductance. Brown showed the results
of RF susceptibility measurements of over 50 microphones, microphone
preamplifiers, and mixers.� By using
appropriately designed RF networks, he was able to selectively inject current
into the shield path or into the audio path.�
His measurements confirmed that microphones and input equipment built
with poor grounding techniques were very effective as radio receivers.� The worst microphones had 80 dB more audio
interference than the best.� The price
and prestige of the microphone often had no relation to its RF immunity.� Several microphones that had poor immunity
were modified by Brown to create excellent immunity by applying the
principles described above. Finally, Mr. Brown
showed how hand-held VHF and UHF transmitters and cell phones can be used for
a quick check of susceptibility to interference.� He noted that the legal use of such
transmitters is limited, and must conform to FCC rules. The localized nature
of the transmitted radio field can help the operator locate the point where
RF enters into an audio system.� Brown
explained that audio cable is very lossy at VHF and
UHF, so the locally generated RF signal is greatly attenuated after traveling
a wavelength or two through audio cables. Many of those present at
this meeting work in the broadcast industry or for audio equipment
manufacturers.� They found Mr. Brown�s
informative talk particularly relevant to their work. |