ES10/ES26
Page 13/104
Document status: released
Operating Manual
rev. 01, 03.11.08
To suppress the Stimulated Brillouin Scattering (SBS) the wavelength spectrum of the optical
signal is broadened. Two technologies are used:
•
Broadening the optical spectrum by modulating the laserdiode operating current
•
Broadening the optical spectrum by driving an optical phase modulator
These SBS circuits are mandatory to avoid stimulated Brillouin scattering in optical fibers and
allow operation with optical amplifiers feeding at least +13 dBm of optical power into standard
single mode fibers. For the XL-version of the transmitter all microwave signals can be adjusted in
amplitude via the push-buttons on the front panel to optimize the SBS and SPM (self phase
modulation) performance.
The coupling of light from the laserdiode into the modulator is performed by using polarization
maintaining optical fiber. The optical modulator provides two optical outputs. The signal of one of
these outputs is tapped to an InGaAs photodiode. The electrical signal of this photodiode is
evaluated for two reasons to supervise a proper working of the cw laserdiode. In case of optical
output power drop of 2 dB of nominal power an B-grade alarm (=warning alarm) is generated, in
case of optical output power 0 dBm an A-grade alarm (=urgent alarm) is generated.
When switching on the ES10/ES26, the transmitter starts up adjusting the bias point of the
electrooptical modulator by output power regulation, it’s called “Output power based”. After warm
up sequence, regulation is performed by checking the CSO beats and minimizing them, which is
called “CSO based”. Both states are shown in the parameter menu. When internal working
temperature is reached, the bias point of the electrooptical modulator is evaluated by one of two
methods:
1. Channel controlled:
A detector circuit measures CSO and CTB distortions to optimize the bias point of the
electrooptical modulator. For a proper operation of the detection circuit at least two TV
carriers with a frequency spacing of 24 MHz have to be present. Using this standard
software setting of the detection scheme all known European and Chinese frequency
plans are supported: CENELEC frequency plan, all regular 8 MHz spacing frequency
plans as well as the German 7/8 MHz frequency plan. Additionally it is possible to change
the standard software setting to work with regular 6 MHz frequency plans (NTSC) or pure
7 MHz frequency plans via push-buttons on the front panel or via the NMS interface.
2. Pilot controlled: (*)
An internal pilot tone is used to check for distortion products out-of-band. Service signal
frequency range 47 to 862 MHz is not targeted by pilot tone control.
The ES26 transmitter provides the same functions as ES10 transmitter and moreover a SAT-IF
signal input in order to additionally transmit SAT-IF frequency band. Its block diagram is depicted
in Fig. 3.2.2. RF signal input for SAT-IF signals is separately controlled by an amplifier also
providing AGC option. Again, AGC stabilizes the SAT-IF output signal of the preamplifier to
maintain stable RMS- (root-mean-square) optical modulation index (OMI) of the optical modulator
regarding SAT-IF signal part. Input level variations are compensated as long as the AGC circuit is
working in its nominal gain range.
It is recommended to switch on SAT-IF AGC, but it can be turned off for a constant gain
operation in order to allow for special user requirements.
*) Note:
This function is protected by US-Patent 5,400,417 from 21.03.1995. In case of usage the
permission of patentee is required.