IEEE Solid-State Circuits Magazine - Winter 2016 - 22

signals from a telephone would be
converted from an analog to a digital format, routed, switched, and
transported digitally, along with
data communications from customer to customer. This was a bold
announcement by Nortel because
the company did not have all the
pieces of the puzzle to complete this
task, and, like other manufacturers,
all of its commercial products were
primarily analog based. One of the
major missing pieces was a digital
line card comprised of analog-todigital (A/D) and digital-to-analog
(D/A) converters and anti-aliasing
voice filters. Line card electronics
sit at the customers' end of a telephone line before entering a telephone switch. Its task was to convert
analog voice to digital for switching/routing, and simultaneously
convert the return digital stream to
analog, so that the customer heard
a reconstructed analog signal. At
Nortel we developed a novel, rather
elegant, solution to integrate the
voice filters and "companded" A/D
and D/A onto a single silicon chip.
This was the world's first voice "filter codec," and it received international acclaim. The filters we
constructed using charge-coupled
device (CCD) transversal filters.
Nortel had an extensive research
program on CCDs at the time, and
Dr. Copeland, Prof. Chong Chan, and
their graduate students at Carleton
University were involved. The codec
was constructed using a charge
redistribution technique that had
been developed earlier by Prof. Paul
Gray at the University of California
at Berkeley and adapted nicely into
Nortel's mixed A/D semiconductor

W I N T E R 2 0 16

process. The technology used was
a proprietary 8-µm N-channel MOS
double-polysilicon process, which
had been developed earlier for the
CCD memory and imaging products.
The double-polysilicon structures
also adapted nicely in precision
capacitors for the A/D array and
compensation capacitors for the
high-gain operational amplifiers.
The filter-codec chip, code
named E13, was displayed in the
early 1980s on the cover of an IEEE

Figure 1: E13-the world's first filter codec.

publication with review articles on
technology that proclaimed this as
the "first system on a chip." Figure 1
shows the two, 71-tap CCD transversal finite impulse response filters clearly at the top of the chip.
The transmit filter also contained a
50/60-Hz notch filter. The A/D and
D/A structure are at the lower left,
and the digital support circuitry is
on the lower right.
These advances allowed Nortel
to open up and dominate the market for digital voice communication in North American telephone

IEEE SOLID-STATE CIRCUITS MAGAZINE

systems. Subsequently, Nortel manufactured over 1 million E13 silicon
chips in its Ottawa fab, and more
importantly, for Nortel the company, it enabled sales of the DMS100 telephone switch.
However, innovations that were
even more revolutionary from a
commercial perspective were still
to come. The generation-one filter
codec (E13) had a serious drawback.
We could not reduce the idle channel
noise in the A/D or D/A path below a
certain floor. The telephone operating companies that were buying our
DMS-100 switches were under constant pressure to reduce this parameter. If Nortel couldn't find a way
to do this, we would lose our early
lead in the emerging digital world of
communication products driven by
silicon microelectronics.
We needed another approach
to implementing the anti-aliasing
voice filters, preferably a circuit
that could be integrated in the same
silicon process technology as the
codec. Standard methods of filtering
at the time used operational amplifiers, resistors (R) and capacitors
(C ). The main parameter that determines the transfer function accuracy for such a filter is the product
of the resistor and capacitor values
(i.e., the time constant). The silicon manufacturing process used at
Nortel was great at maintaining the
relative value of similar shaped
resistors to within 0.2%. The same
control was achieved with similar
shaped capacitors. However, conventional resistors and capacitors
components on chip have an inherent absolute value variability of up
to 50%. The anti-aliasing voice filters

Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Winter 2016