ETTC'2003 - SEE

phase noise floor with the synchronized oscillator
approach remains constant and lower than -165 dBc/Hz.
Phase Phase Noise Noise (dBc/ (dBc/ Hz)
-100
-110
-120
-130
-140
-150
-160
-170
OCXO
Photo-oscillator
-180
1 10 102 103 -180
1 10 102 103 Filtered
photodiode
Frequency Offset (Hz)
Amplified
photodiode
104 105 104 105 106 106 Figure 2
Phase noise at the output of the different 10 MHz optical links, realized
with the Alcatel laser module and with 4 dB optical losses in the link ;
comparison with the reference OCXO phase noise.
In the 800 MHz application, the phase noise
requirements are not as stringent as for the last
application, and a conventional optical link may be used if
the optical losses are not too high. However, the
synchronized oscillator is still interesting in order to
maintain a constant output power and to filter the spurious
signals far from the carrier. Such a circuit is under test
today and the results will be presented at the conference.
V. OPTICAL LINK : THE MICROWAVE RECEIVERS
For many applications at microwave frequencies, a
classical optical link [9] will met the phase noise
requirements, with the exception of the transmission of
very high spectral purity signals, such as signals dedicated
to frequency metrology. The optical link will degrade the
phase noise of a synthesizer only if the distribution factor
(the losses) is very high. Therefore, such as for the 800
MHz application, the other advantages of the photooscillator
(constant output power and spurious filtering)
should be pointed out.
Also, at these frequencies, the circuit compacity is of
importance and it is interesting to investigate oscillators
involving InP phototransistors. These transistors are
certainly noisier than silicon or silicon-germanium
transistors for oscillator design, but they may be used at
very high frequencies (millimeter waves) and they are
able to detect directly the modulated optical signal.
Two types of photo-transistors have been used in a
preliminary 3.5 GHz experiment. The first one is an InP
based photo-HBT [3], the second one a InP based HEMT
[4]. Contrarily to the HBT device, for which an optical
window has been designed, the HEMT device is not
specially designed for optical applications. But it is
sensitive to a 1.5 µm radiation through direct illumination
of the gate region. Both devices have been measured on a
probe station, using optical and microwave probes.
The HEMT optical responsivity has been found to be
weaker and, above all, much slower than the one of the
photo-HBT. The design of a photo-oscillator with this
device is possible in the low microwave range (a few
gigahertz), but not in the mm-wave region, contrarily to
the results of some other researchers [10]. Above 10 GHz,
a solution could be in the sub-harmonic synchronization
of the oscillator, but the efficiency of such a process has
still to be evaluated.
Finally, a third experiment using indirect injection
locking has been carried out. To this purpose, a
commercially available microwave photodiode (Discovery
DSC30S) is associated to a 3.5 GHz oscillator which uses
a silicon-germanium HBT device (Infineon BFP620).
The three synchronized oscillators are realized using
discrete elements and the same resonator, a low Q factor
resonator (loaded Q of 150). The photo-transistors are
maintained on the probe station, and are illuminated as
already explained. The oscillators are modeled using an
analytical approach of the receiver involving, as an input
data, the measured residual phase noise and the electrooptic
responsivity of the transistor (or the photo-diode),
and the theory described in section II. This approach as
proven to be very efficient and most of the characteristics
of the optical link can be evaluated with it. The Table 2
gives the synchronization bandwidth and phase noise data
measured on these oscillators.
The 9 dBm optical signal at the emitter (the Mitsubishi
laser) is amplitude modulated by the microwave signal,
with a modulation index of about 0.25. In case of the
HEMT oscillator, this optical power is used directly to
illuminate the transistor. In the two other cases, a 10 dB
optical attenuator is added in order to get an observable
locking bandwidth. Moreover, if the optical power is too
strong, it can modify largely the oscillation, and
sometimes cancel it. With respect to the optical sensitivity
and the synchronization bandwidth, the Photo-HBT and
photodiode solutions lead to similar results.