CTDAB has been originally designed for pulse transmission, but has
already been tested in operation with the GMT 500KHz DC-balanced
CTDPR has been originally designed for pulse reception, but we
and in the
on the existing 3U boards that it can achieve reception of a
DC-balanced signal up to >10MHz
Optical components selection
Regarding the transmitter, note that we need high power bandwidth
(around 1mW) due to the expected high attenuation and the
sensitivity of the receiver
The HFBR-2316T receiver has been successfully used in CTDPR since
years. According to the company Broadcom
> _"There are currently no foreseen plans to obsolete HFBR-1312TZ nor
> The above transmitter/receiver combination is designed for
signal/data transmission over graded index multimode fibers e.g. 50 or
> Only the receiver can also be used with single mode fibers.
> The transmitter is a 1300nm LED with optical power coupled in
multimode fiber between about 10-100µW.
> A 1mW version is currently not available."_
Optical transmitter and receiver design
The design is based on the following existing modules:
Due to the increased price of the laser transmitter used in the CTDAB,
it was decided to switch to a different transmitter (Appointech/Municom
The new transmitter comes in a sugarcube package, similar to the one
used for the receiver, which allows to mount both at the edge of the RTM
PCB and avoid any fiber patch cables on the PCB, like it was done in the
The new transmitter is more powerful, a fact which has also led us to
change the laser driver IC, from the Micrel SY88822V to the more
powerful MAX3766EEP+ from Maxim. The Micrel SY88822V was also much
harder to procure, so this modification should make it easier to produce
In place of the jumper which provided two power settings for CTDAB, we
introduced a resistor, in parallel with a (non-mounted) trimmer. By
default, the power will be fixed with a specific resistor value, but a
trimmer could be used instead for experimentation and/or fine-tuning for
very special cases.
On the receiver side, the circuit has remained largely the same, with a
The two transistors were replaced by a single matching dual
transistor pack, for better tracking/mirroring under varying
The pull-up/down resistors at the inputs of the comparator were
changed in order to increase the detection threshold. In the
original CTDRP design, which was conceived for very weak
transmitters, the threshold was chosen such as to allow detection of
pulses of few mV. With CTDAB (and our new optical transmitter which
is even more powerful), one can safely expect pulses of more than
100-200mV, therefore the threshold can be increased in order to make
the receiver more robust and immune to noise.
The size of the capacitor on the emitter of the second transistor
was increased. This was done to allow longer pulse widths, since the
RC circuit formed on the emitter of the second transistor controls
the "filtering" of the input signal in order to generate the
threshold; if the capacitor value is too small (or, equivalently,
the pulse width of the incoming signal too long), the threshold will
start rising towards a point where eventually there will be no
positive output on the output of the comparator (thus creating
shorter pulses than those on the input).
The above modifications were verified with a SPICE
using the SPICE models provided by the manufacturers for the transistors
and the comparator.
On our setup with CTDAB and CTDPR we noticed that when the optical power
at the receiver CTDPR was ~500uW (CTDAB, without attenuation), the
signal in the output was ~16ns longer.
The following pictures zoom in a rising and a falling edge of a 5MHz
signal (200ns pulse width) at the input of the CTDAB transmitter
(yellow) and at the output of the CTDPR receiver (pink).
The rising edges with or without attenuation show ~40 ns skew.
The falling edges with attenuation (that means optical power at the
receiver ~10uW) show also ~40 ns skew, as the rising ones.
The falling edges without attenuation show 40+16ns= ~56 ns skew, as we
are exceeding the Peak Input Optical
As CTDPR and CTDAB have been used together in operational installations
for pulse transmission, we believe that this pulse extension when the
optical signal is very strong, is not a problem.
Note that for the GMT signal, these 16ns are a very small fraction of
the 1us-long pulses.
CTDAB-CTDPR system latency
In the setup we connected the CTDAB to the CTDPR with just few meters of
We measure in yellow the GMT input of the CTDAB and in pink the output
of the CTDPR.
The system latency as the following figure shows is is