SHF Communication Technologies AGWilhelm-von-Siemens-Str. 23D • 12277 Berlin • Germany
Phone ++49 30 / 772 05 10 • Fax ++49 30 / 753 10 78
E-Mail: sales@shf.de • Web: http://www.shf.deApplication NoteMulti-Level Signal Generation for QAM
using theMulti-Channel SHF 12103 A

SHF reserves the right to change specifications and design without notice – App-note Multi-Level, V003 – Nov. 08, 2011 - Page 1/7

Introduction
As the development of 100 Gbps system products based on Dual Polarisation (DP) QPSK andcoherent detection matures, and system deployment anticipated shortly, QAM could become a key
technology for future developments towards even higher spectral efficiency. The SHF 12103 A
multi-channel bit pattern generator is developed specifically, though not exclusively, having in mind
for such advanced optical system developments.
The objective of this note is to provide a simple guide on how to make the best use of the SHF
12103 A Quad-32, 4-channel 32 Gbps BPG, as a multi-bit data source, to realize 4-level electricalsignals when used in conjunction with an external broadband power combiner. The resultant multilevel analogue electrical signal could be used to modulate, when used in conjunction with asuitable drive amplifier, a commercial I-Q optical modulator for the generation of 16QAM signal.
This basic implementation may conceptually be further expanded to at least a three-bit structure for
8-level signal generation for 64QAM applications.

The Basic Idea
Figure 1 illustrates how a 4-level signal can be generated using the SHF 12103 A Quad-32 BPG
and minimal external components. The basic external components are: two RF cables to connect
the sub-rate outputs of the SHF 12103 A to a broad band power combiner. Two important features
within the BPG are used. These are the output amplitude adjustment control, and the skew control
which allows fine adjustment of the timing delay of each output data stream, with a step size of 0.1
ps and a range of ± 25 ps, as illustrated in figure 2. Additionally, for de-correlation purpose, thesub-rate outputs can be set to different patterns (prbs or user-defined). If the same pattern ispreferred, the integer bit delay function can be activated to set the number of integer bits of delay.
Figure 3 shows how these are set in the control software.

Signal Generation Concept• This is the basic building block• Basic external components– Matched cables– Power divider• Timing and amplitude adjustment performedinternally with the BPG• Scalable concept towards higher number ofeffective bits (3,perhaps 4)
32 Gbps

Figure 1

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Control of Binary Signal for Performace
BCC Software control of the 4 Level parameters

BPG‘s amplitude control toadjust the eye opening
symmetry

BPG‘s skew control
to minimize the jitter& dead zone

Figure 2

Signal De-correlation

Same PRBS pattern lengths
Bit delays activatedDifferent PRBS pattern lengths

Figure 3

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Optimization for Signal Quality
Figure 4 shows how the 4-level signal quality may be further improved by using additional
attenuation to minimise the manifestation of RF reflections within the hybrid power combining
structure. However, a small reduction in the over-all amplitude of the resultant 4-level signal isincurred due to the additional attenuation. However, the resultant 400 mV amplitude, inconjunction with a suitable linear driver, such as the SHF 807, should be sufficient to drive acommercial I-Q modulator.
By choosing the right amplitude ratio between the two sub-rate outputs from the SHF 12103 A, it isalso possible to define (within a limited range) the amplitudes of the inner and the two outer ‘eye’ of
the 4-level signal. This is illustrated ion figure 5.

Managing Reflection
Amplitude adjust via BPG
onlyExtra 3 and 10 dB attenuation
Plus amplitude adjust via BPGCA3 dBA

10 dB

C

Figure 4

Eye Amplitude Adjustment (Pre-emphasis)

Equal eye amplitude

Outer eye pre-emphasis

Inner eye pre-emphasis

32 GBaud

Figure 5
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Scaling for 3-bit, 8-levelTo further illustrate the versatility of combining the SHF 12103 A and the passive combiner
approach for multi-level signal generation, figure 6 shows how using three binary data channels ofthe SHF 12103 A, and a suitable augmentation of the power combining structure, an 8-level signal
could be generated for 64QAM. In this case, an additional 6 dB attenuator is added to the 3rd bit
before the 2nd power combiner. Again, the amplitude and individual channel skew control are
crucial in achieving the right delay and amplitude combinations in order to realise a good quality
multi-level signal.

3-Bit 8-Level for 64QAM
20 GBaud

25 GBaud

AC

3 dB10 dBD6 dB

Figure 6

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Two-channel implementation for 4-level I-Q modulation
Figure 7 shows the actual implementation of two 4-level signals suitable I-Q modulation. Here, allfour sub-rate channels of the SHF 12103 A are used.
In the event that only two sub-rate channels are available (Dual-32 option), the inverted data
outputs (data_bar) may be used to form the second 4-level signal. However, care should be
exercised to ensure correct timing delay(s) is realised. For de-correlation purpose, a suitable
external delay, which could be a suitable piece of high quality coaxial cable (with pre-determineddelay value) or an adjustable delay line, should be used.

Dual 4-level Implementation for 32 Gbaud 16QAM

3 dB

3 dB

10 dB

Potential choice of components:• Coaxial cables
10 dB– E.g. CA 185 or 292 series
Combiner• Attenuators– 1.85, 2.92 mm or equivalentCombiner
I• Combiners
Q– 1.85, 2.92 mm or equivalent Linear amplifiers• Linear amplifiers
I-Q Modulator– SHF 807

Figure 7

A practical realization of one attenuator-combiner structure to match the sub-rate outputs of theSHF 12103 A BPG is shown in Figure 8. The structure comprises the 1.85 mm (or equivalent)components as described in Figure 7 to maximize the over-all bandwidth. The two semi-rigid
cables are designed to match the physical spacing of the sub-rate outputs A & C, and B & D. Two
additional 1.85 to 2.92 mm adaptors are used to interface to the SHF 12103 A sub-rate outputs.

Figure 8 1.85 mm realization of attenuator-combiner structure
The complete structure or specifically the custom-designed semi-rigid cables are available onrequest.
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It should also be pointed out that by removing the 1.85 to 2.92 mm adaptors, the remainder can beused for generating high baud rate (>32 GBaud) 4-level signals if the dual-56G configuration of theSHF 12103 A is also available. Figure 9 shows 4-level signals at 44 and 50 GBaud generated byusing the same 1.85 mm attenuator-combiner structure. However, two additional delay lines may
be required for skew control adjustment.

Beyond 32 GBaud

Matchingdelay lines

50 mV/div

3 dB

44 GBaud

10 dB• Scaling to higher baud rates• Additional timing delay adjustment would benecessary• Requires linear driver development for highbaud rate

Figure 950 GBaud

11Matching linear driver amplifiers
Since the peak-to-peak amplitude level of the 4-level signal is typically less than 1V, a suitablechoice of linear amplifier with the right bandwidth, phase linearity and linear output power level
amicable for commercial I-Q modulators is a pre-requisite. Please refer to the SHF home page for
New Amplifiers, link http://www.shf.de/en/.
For baud rates up to 32 GBaud, the SHF 807 could be considered for I-Q modulators with 2 V ofup to 6 V. However, for baud rates in excess of 40 GBaud, the ultra-broad band amplifiers SHF
824 potentially offers the least distortion. Although a moderate small signal gain of 11 dB will limit
the maximum available output drive level, thus restricting the modulation excursion to the linear
regime of the modulator and increased optical attenuation.

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