Clip extracts signal from phone line

Using a capacitive-coupled clip, you can pick up the signal from a twisted-pair or -wire telephone line or from other unshielded analog lines without piercing the insulation. No line test can detect the clip's presence, and it leaves no evidence of having been attached. It needs no ground return. You can fasten the small, insulated pickup plates to the opposing jaws of an alligator clip for quick and easy attachment. Balanced lines from the plates connect to the inputs of a high-impedance differential amplifier (Figure 1). For this scheme to have satisfactory signal-to-noise and frequency-response parameters, the clip, connecting cable, and amplifier must m be attached parallel to the signal wire and must be as long as is conveniently possible—an inch or more—and preferably slightly curved to maximize the coupling capacitance. (For a twisted-conductor line, the plates should not be longer than the twist "wavelength" to avoid signal cancellation.) You should orient the clip for the cleanest signal output.

The clip, its connecting cable, and the amplifier must be shielded to minimize interference, typically comprising 60-Hz signals and their harmonics from powerline fields. The cable should have good electrical symmetry and low total capacitance between conductors and to the shield. Thus, the amplifier must be near the clip. The amplifier should have high input resistance, low current noise, and adequate common-mode rejection.

The clip's coupling capacitance and stray capacitance and the amplifier's input resistance determine the low-frequency cutoff of the detected signal. Stray capacitances in the clip and in its connecting cable to the shield are generally much larger than the coupling capacitance. Thus, voltage-divider action reduces the signal, but the stray component adds to the capacitance the amplifier's input sees and reduces the circuit's noise by the square root of the signal attenuation. The noise reduction accrues from reducing the needed input resistance. Therefore, you generally don't need the complication of an insulated "boot-strapped" shield. You can follow the amplifier with a nearby or remotely located postamplifier for more gain and bandpass filtering to optimize the signal-to-noise performance. A telephone signal has a bandwidth of approximately 300 Hz to 3 or 4 kHz. A sharp highpass cutoff at 300 Hz effectively rejects power-line noise pickup. A simple, two-pole, Sallen-Key Butterworth filter works well. You can trim it to provide some high-frequency peaking to obtain the most intelligible signal.

The amplifier of Figure 3 uses two quad J-FET or BiFET op amps. Thanks to stray capacitance on the input lines of the test model, a relatively low input resistance of 3.3 MΩ is sufficient. Input noise is mostly the Johnson noise of the 10-MΩ feedback resistors. Power-line noise pickup is usually the bigger problem. The output stages incorporate some highpass filtering to reject noise below 300 Hz. The output level depends on many factors but is approximately 50 mV. A postamplifier (not shown) can provide more equalization, filtering, and gain if necessary, as well as manual or automatic level control. Tests of models of both design approaches use readily available components and show perfectly clear telephone speech through a small speaker in the postamplifier box. The multiple-pad pickup system produces noticeably lower noise, and clip orientation is less critical.