Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-06-22
2002-05-21
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S226000
Reexamination Certificate
active
06392219
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to reference receivers and more specifically to discrete filter-less optical reference receiver and output amplifier having a system frequency response curve predominantly established by a sampling circuit.
A reference receiver is a measurement instrument that is bounded in the frequency domain by defined tolerance limits. Ideally, the frequency response curve should be Gaussian but such a response can be difficult to achieve, especially at high data rates, such as 10 Gb/s and beyond. In the telecommunications industry, the typical reference receiver has a has a system frequency response curve that matches a Bessel-Thompson filter response. The predominant frequency response of previous reference receivers has traditionally been established by discrete reference receiver filters at the input of the receiver having frequency responses that match a 4
th
or 5
th
order Bessel-Thompson frequency response.
The telecommunications industry is continually establishing standards for the ever increasing data rates used in the industry. Most telecommunications and data communications industry standards define acceptable scalar frequency response characteristics that test equipment must have when performing eye-pattern conformance testing of fiber based optical signals. The industry standards define the scalar frequency response as having a Bessel-Thompson shape (near Gaussian) with the −3 dB electrical power response rolloff point at ¾ of the bit rate. For example, a reference receiver for a 2.488 Gbps SONET signal would have a nominal −3 dB point at 1.87 GHz.
FIG. 1
shows a graphical representation of the Bessel-Thompson shape scalar frequency response for a generic reference receiver. The frequency response has upper and lower tolerance limits that the reference receiver must meet for telecommunications and data systems.
As new standards are being proposed, the telecommunications standards committees approach test equipment manufacturers to solicit proposals for establishing the upper and lower tolerance limits for the proposed standards. Tolerance limits proposed by test equipment manufactures are based on the reference receiver design and the components used in the design. The data profile in
FIG. 1
shows aberrations in the data that may be caused by a number of factors. A significant cause of data aberrations is commonly the discrete reference receiver filter.
Referring to
FIG. 2
, there is shown a typical reference receiver
10
designed for measuring optical signals by the telecommunications industry. The reference receiver has an optical-to-electrical (O/E) converter
12
that receives the optical signal under test. Typically the O/E converter is a photodiode
14
that converts the optical signal to a corresponding electrical signal. A transmission line couples the electrical signal to a first switch
16
that is connected via a second switch
18
to a series of discrete reference receiver filters
20
,
22
. Transmission lines are also used to connect the switches and filters together. The O/E converter
12
includes a termination resistor or resistors
24
that reverse terminates the converter
12
in the characteristic impedance of the transmission line. The reference receiver filters
20
,
22
are selectively switched into the reference receiver depending on the data rate of the optical input signal. For example, the discrete reference receiver filters may have a Bessel-Thompson shape scaler frequency response for 622 Mbps and 9.953 Gbps data rates. The filtered electrical signal output of the respective discrete reference receiver filters
20
,
22
are selectively coupled via a third switch
26
to a fourth switch
28
that also receives the unfiltered electrical signal from the O/E converter
12
via a separate transmission line. The fourth switch
28
selectively couples the filtered electrical signal and the unfiltered electrical signal to a sampling circuit
30
. The sampling circuit
30
includes a termination resistor
32
that terminates the transmission line in its characteristic impedance. Generally, the transmission lines have a 50 ohm characteristic impedance requiring 50 ohm termination resistors. The sampling circuit
30
includes two series connected diodes
34
,
36
having a common node
38
receiving the filtered and unfiltered electrical signals. Opposing negative and positive strobe pulses are applied to the opposing sides
40
,
42
of the diodes for gating the diodes
34
,
36
into conduction. Positive and negative bias voltages are applied to the respective diodes via resistors
44
,
46
to bias the diodes
34
,
36
for selected turn on and turn off times.
A number of drawbacks are associated with current reference receiver designs. One major drawback is the generation of reflections from the discrete electrical filters requiring reverse termination of the optical-to-electrical converter. The reverse termination resistor absorbs a substantial portion of these reflected signals, for example 95% of the reflected signal, and provides a good temporal response in the presence of discrete filters. If the discrete filter produces a 10% reflection, then there is a ½% reflection that is coupled into the sampling circuit. Such reflections cause aberrations in the sampled signal as represented by the aberrations in the data in FIG.
1
. These types of aberrations are taken into account by test equipment manufactures in making recommendations for the tolerance limits for reference receiver standards. Another drawback to current reference receiver designs is that the reverse termination resistor generally tends to flatten the frequency response of the system and produce a much steeper roll-off than a non-reverse terminated system. The result is further deviation from the ideal Gaussian response and renders the system with more aberrations when using it for non-filter applications when all-out bandwidth is desired. A discrete electrical filter in the high frequency path also increases the possibility of group delay distortion caused by the filter itself.
FIG. 3
is a schematic representation of a prior art output amplifier circuit
50
associated with a prior art reference receiver
10
. Operational amplifiers
52
,
54
have their respective non-inverting input terminals connected to bias voltages +V
bb
and −V
bb
. The inverting input terminals of the operational amplifiers
52
,
54
are set to the respective bias voltage levels of the non-inverting input terminals by feedback through resistors
56
,
58
. The bias voltages on the inverting input terminals are coupled to the sampling circuit
30
as the biasing voltages for the sampling diodes
34
and
36
. An offset voltage may be applied to the sampling diodes
34
and
36
that shifts the bias voltages levels in common mode. The feedback resistors
56
,
58
in the operational amplifier circuits
52
,
54
have a high ohmic value in the range of 100 Megohms to reduce the amplifiers noise for generating a cleaner output signal. The output signals from the operational amplifiers
52
,
54
are summed together at the inverting input terminal of a summing amplifier
60
. The output signal from the summing amplifier
60
is digitized and further processed to produce a display on a display device. A zero volt input to the sampling circuit
30
with no bias offset causes the relative bias level of each diode to be equal. The positive and negative strobe pulses drive the sampling diodes
34
and
36
into conduction with the resulting sampling charge from each diode being balanced. The resulting capacitor charge on each of the operational amplifiers
52
,
54
generates respective integrated voltages at the output of the amplifiers that cancel each other out. Any non-zero voltage input unbalances the instantaneous total bias between one diode and the other resulting in a measurable difference in the integrated voltages of the operational amplifiers
52
,
54
.
Current reference receivers with this type
Carlson John E.
McCormick David J.
Bucher William K.
Le Que T.
Tektronix Inc.
LandOfFree
Discrete filter-less optical reference receiver and output... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Discrete filter-less optical reference receiver and output..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Discrete filter-less optical reference receiver and output... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2862551