Talking Signs Specifications

Technical Data

Talking Signs

Love Electronics Inc.
March, 1991

System Overview:

880 to 950 nM Lightwave carrier, amplitude modulated with 25Khz (and/or auxiliary frequencies to 300 Khz) FM subcarrier.

Carrier:

880 to 950 nanometers pulsed or sinusoidally driven, single or multiple IRLED source sinusoidal drive reduces intermodulation products in a multiple subcarrier installation at the expense of transmitter efficiency reducing duty cycle to 10% while maintaining constant average current in pulsed mode produces a 30% to 60% increase in effective radiated power for maximum range, single subcarrier applications.

Subcarriers:

Fc = 25 Khz (or auxiliary frequencies as described above)

Fm = 300-3300 Hz (telephony voice bandwidth)

Fdev = 2.5 Khz

Index = 0.76

B. W. = 6.6 Khz

Receiving System:

TRF or superheterodyne receiver architecture.

Low noise PIN photodiode with Wratten 87C near IR filter.

Modified transresistance preamp designed for high dynamic range and full-sun background illumination.

TRF design uses conventional limiter and Travis discriminator with noise squelch.

Audio output level 85 dB max at 1 ft., 10% distortion ("A" weighting).

Maximum receiver sensitivity 0.3 nW optical power in.

Worst case full in-line sun induced shot noise reduction of receiver range 5 to 1 (eg, 200 ft. to 40 ft.).

System range for 50 degree beam, 16mW/steradian source and 12 dB input s/n ratio: 10 meters (no receiver optics).

Comments:

The conventional concept of output signal to noise ratio enhancement for large index (>5) FM links is not as significant for telecommunications (typical index 1.6) as for music quality, high dynamic range broadcasting. In reality, most frequent operation of the LEI Talking Signs is near the onset of the s/n enhancement and not deeply into hard limiting. Capture of the receiver by the stronger of two signals in the receiver field of view requires a received power ratio on the order of 20dB for negligible interference, adjacent transmitter frequency tolerance of 50 Hz to 100 Hz improves the intelligibility of interfering signals.