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Analogue comparator spots three signal relations

29 Jul 2013  | Glen Chenier

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Several years ago we were developing a clamp-on photometer, a hand-held instrument that could be clamped onto a communications optical fibre to indicate the presence of light, its direction of travel, and the presence of combinations of low-frequency test tone modulation. Its purpose was to positively identify a particular optical fibre in a manhole or pole tent prior to cutting and splicing, since the accidental cutting of a misidentified live-traffic fibre must be avoided at all costs.

The fibre is clamped in a V-shaped block under light spring pressure at the bend. The bend allows a small amount of the internal signal to escape through the relatively translucent jacket material; a pair of photodiodes placed on either side of the bend detects the released light. The direction of propagation is determined by which of the two photodiodes receives the strongest signal. For the most sensitive detection of low-level signals, the amplifiers need to work in the microvolt region, necessitating chopper-stabilised op-amps for minimum input offset voltage.

At first it seemed a simple approach – apply the photodiode signals to a chopper stabilised analogue comparator to determine direction. However, this had a small problem. If there were bi-directional signals in a fibre such as used in coarse-wavelength multiplexing (1310nm and 1550nm propagating in opposite directions), we wanted both direction indicator LEDs to light up. This of course cannot be done with a one-of-two-states comparator.

The circuit of figure 1 gets around this problem by lighting only the appropriate direction indicator LED when there is a large difference between the two signal levels or when one signal is non-existent, but lights both LEDs when the photodiodes respond with signal levels closer together. The desired thresholds are dependent on the mechanical design of the optical clamp which is beyond the scope of this DI. If no signals are present in either direction, both LEDs remain off to indicate a dark fibre.

ICL7650S chopper stabilised op amps are used in this example as per the original hardware when this was in production (figure 1 omits the necessary external stabilising capacitors for clarity) – other types of op amps may be used. An approximate 1V bias is set up through series diodes D1 as the zero-signal reference. R15 maintains both comparators U3 and U4 low with about +2mV offset from this reference if there are no input signals present.

The circuit operation is straightforward. Each photodiode operating in photovoltaic mode drives the outputs of the corresponding transimpedance amplifiers U1 and U2 to a voltage depending on the intensity of the detected light. Each voltage is then applied to its corresponding comparator U3 and U4. The comparison voltage level is derived by combining the outputs of the transimpedance amplifiers through buffer U5 with slightly greater than unity gain. Therefore this level follows the maximum photodiode level divided by the minimum photodiode level through the voltage division of R5 and R12 to accommodate a wide variation in signal levels within the optical fibre. With the resistor values shown, both signals will indicate active as long as the lower photodiode current is not less than about 0.22x the larger photodiode current. Below this only the strongest signal will light its corresponding LED.

Since the optical levels need to be averaged to a DC value, the capacitors serve to remove data and test tone modulation from the detected optical signals (figure 1). C2, C7, and C10 ensure op amp stability. C4 and C8 are optional to slow flicker when the comparators U3 and U4 are close to their switching thresholds; alternatively, positive feedback resistors Rh can be used instead for a hard switchover with no LED flicker. If required to prevent op amp saturation for very strong optical signals, R2 and R11 can be reduced as necessary.

The simulation in figure 2 shows both photodiodes with a DC offset current of 0.5µA (2V on the left scale) and ±0.6µA variation to demonstrate the comparator responses with the resistor values shown in figure 1. Reducing the values of R8 and R9 will narrow the both-on range; reducing R6 and R17 will widen the both-on range.

About the author
Glen Chenier contributed this article.

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