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Introduction

This page will describe a simple device you can build to test the susceptibility of audio equipment to radio frequency interference (RFI).

Anecdotal evidence suggests that audio equipment is most susceptible to RFI starting at approximately 500kHz through 30MHz to 50MHz. There seems to be little high powered transmission below 500kHz in the U.S. (that is where the AM radio band starts), and I speculate that above about 50MHz the cabling connecting audio equipment is likely a very poor antenna and so does not conduct much of the very high frequency RFI into a typical audio chassis. Note that I am only speculating on that last point, I have not conducted susceptibility testing of a typical audio system in a controlled RF test lab, which would be required to verify that speculation conclusively.

The goal of this tester is to provide an easy to use source of high level RF signal, modulated in such a way that when connected to an audio input, you can tell right away if that equipment could possibly have noise or distortion problems when operated in a high strength RF field (e.g. near a radio transmission tower). Even if your equipment is not being operated near a transmission tower, there is a possibility that RFI could cause a very low level of noise in your system as local RF transmissions are demodulated, and protecting against RFI may result in a cleaner sound. This tester will allow you to "torture test" the equipment. If a piece of audio equipment does not produce any ouput when this tester is connected to an input, it is likely that it will be noise free under almost any RFI condition experienced in typical use.

Theory and Design

The need to protect audio inputs from RFI stems from the fact that solid state amplification circuits have semiconductor junctions as an intrinsic part of the transistors which make up the circuits (either discrete or on the substrate of an integrated circuit). Transistors are non-linear devices which are typically used with feedback around the devices to linearize the transfer function. Feedback operates over a limited frequency range, and in typical audio equipment the feedback is not effective above anywhere from just over 20kHz to a couple of MHz. When presented with a signal far outside the operating range of the circuit, the junctions of the transistors making up the amplification circuit can act as rectifiers to form a simple AM demodulation circuit. The basis of the test is to present an amplitude modulated RF signal to the circuit under test and verify whether or not the signal is demodulated. In order to make the test easy to verify by ear, the modulating signal should be chosen to fall in an area of high sensitivity to the ear.

The tester functionally is a high frequency square wave oscillator, operating at approximately 7MHz, and modulated at 100% by another square wave oscillator running at 2.7kHz. It is built with a quad NAND gate with Schmitt trigger input. The first section is the 2.7kHz oscillator, which is used to gate the next section, the 7MHz oscillator. A third gate is used as a buffer (the last gate in the package is unused).

Construction

A prototype was constructed on perf board using a Philips 74HCT132N device as the gate package. I found the parts at a local Radio Shack, but parts should be available at most reasonably well stocked parts distributors. I like Digikey for mail order.

This is a link to the schematic of the modulated oscillator.
You will need Adobe Acrobat reader (or equivalent, such as Ghostview on Unix or Linux) to view the schematic in the link above.

Construction isn't very critical, although you should try to keep component leads short, and place the power supply bypass capacitor as close as practical to the gate. Not shown on the schematic is the power supply. You could use a 9V battery with a 5V regulator of some kind. I used a 5V nominal "wall wart" style supply, but found it actually put out a little over 8V when lightly loaded. A simple resistor/zener diode circuit takes care of clamping the voltage to 5V on my prototype.

perfboard construction prototype

Results

I used the tester to check my Tascam DA-30mkII DAT recorder. At unity gain, I could not hear any demodulated output from the DAT when in A-D-A monitoring mode. I could hear the 3kHz demodulated signal faintly when I switched to variable gain and cranked the gain to maximum.

My Soundcraft Spirit Folio Light did not handle the RFI quite as well as the DAT. At unity gain on either microphone or line input the demodulated signal was audible on the mix bus.

The R-C-R-C-R-C filter from the Phoenix crossover was built as identical sections (i.e. the filter in the Phoenix crossover design was built twice, identically for the hot and cold connections of a differential input). With the input XLR connector wired to the input of the RC filter with short wires (approx 1in./2cm), the demodulated output could be faintly detected when the input buffer was connected to amplifier and speaker.
Changing the connection from the XLR connector to the RC filter from a bare wire to a ferrite bead reduced the demodulated output to a level which could only be heard when my ear was within 6in/15cm of the tweeter.

Possible improvements

This tester was inspired by more formal tests which utilize an RF signal generator. Typically equipment is measured while varying the frequency and amplitude of the signal. The circuit of this tester uses a 100% modulation level because that is easy to achieve with inexpensive logic gates. A different approach might be to modulate the supply voltage of the output buffer gate. Variable frequency operation would be relatively easy to achieve with either a variable resistor or variable capacitor in the second (gated) oscillator. Operation of the circuit would have to be checked after it was built to ensure that the variable components chosen operated acceptably at the frequencies used.


Page last updated: 13 July 2002


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