文档介绍：Philips Semiconductors munications Products Application note
A phase locked fiber optic system using FM modulation AN1434
Author: Les Hadley value of the capacitance across Pins 12 and 13. In this particular
example, the transmitter operates at with the VCO set to
.
SYSTEM OPERATION SUMMARY
The slope of the VCO transfer function is termed KO and is
The purpose of the fiber Iink is to transmit broadband video and measured in radians per second per volt or simply Herz per volt.
sound over moderate distances () with existing low cost Thus, to obtain the magnitude of the differential voltage for a given
components and plexity. frequency deviation the relationship below is used:
Figure 1 depicts plete system implementation. The
Df MHz
application makes use of a very wideband VCO to generate an FM V +
D(voltsDC) k MHzńV
modulated carrier at followed by a fast TTL LED driver to O
+
emit saturated 850nm light signals for entry in to the glass fiber. A IB2 Constant
PlN diode receiver is coupled to a 140MHz bandwidth
transimpedance preamplifier for increasing the detected signal KO is dependent upon the control bias generator current at Pin 2 as
amplitude and then fed to a phase-locked loop demodulator for is noted from the graph. Higher current into Pin 2 results in a higher
recovering the original modulation signals. conversion gain, KO. For a center frequency of 1MHz and an
800µA bias current into Pin 2, K is across Pins 4 and 5
The wideband FM sound subcarrier (150kHz deviation) is summed O
(V ).
with baseband video at and transmitted at a reduced level O
relative to the color reference signal. Cross modulation The value of KO also increases linearity with center frequency so
between sound and picture information is minimized in this way. FM that at 30MHz KO es 30X or 51MHz/V. Note that in this
demodulation of the sound subcarrier is plished after passing application the bias curr