Archive for April 2011
I recently came across some souvenir pictures and artwork of work I had done early in my career at Bell Northern Research. For those of you who are old enough to remember, it will bring back some fond memories of the technology back in the day; and for you young designers, this is how we did things back in the late ‘70s, early ‘80s.
Figure1 Northern Telecom T1C line repeater circa 1980.
Figure 1 is a T1C line repeater I helped to design early in my career circa 1980. Line repeaters were used to regenerate digital signals along a span between two central offices. There were two regenerators per line repeater, and one repeater for every 4-pairs (2Tx, 2Rx) in the cable. They were housed in apparatus cases mounted on telephone poles or pedestals every mile or so. In the city they were usually installed in manhole vaults buried underground.
T1 digital transmission was introduced in 1961 as a way to replace older analog voice frequency technology, and is still in use today. T1 data rate is 1.544 Mb/s and carries 24 channels of DS0 at 64Kb/s. As digital technology exploded through the 1970’s, it became more affordable, allowing T1 to become more popular. By the early 1980’s, the installed base was reaching capacity especially in large cities, and the industry was looking for ways to increase its bandwidth. Sound familiar? To address this issue, a new T1C standard was developed to double the bandwidth. T1C stands for T1-concatenated, and doubles the data rate to 3.152 Mb/s allowing it to carry 48 DS0 channels.
As part of the T1C project team, my primary responsibility was to package the design and lay out the printed circuit board. Because of the limited real estate available and because through-hole component technology was the only choice for PCBs, we needed to use thick-film technology for the receiver equalization circuitry.
Thick-film technology was quite popular at the time, and was the predecessor to today’s surface-mount technology on PCB’s. It allowed for the miniaturization of circuitry by screen printing conductive traces and resistive ink onto a ceramic substrate, then firing it to a high temperature. Surface mount components were limited to capacitors, SOT transistors and diodes.
At the time, Northern Telecom (NT) had their own in-house thick-film design and manufacturing facility located in Aylmer, Quebec. All of the thick-film designs used in NT’s products prior to the T1C project were single in-line packages (SIPs). Because of the height restriction, and the amount of circuitry needed to be integrated onto the substrate, SIPs were impractical, so we had to develop dual in-line manufacturing capability at the same time we were developing the product.
The final dual in-line thick-film packages are shown near the faceplate. Since the packaging of the repeater was so dense, I needed to place components under the thick-film substrates. This was all well and good until I was testing a bunch of repeaters for a field trial in California coming up in December of that year. I accidentally dropped one and it happened to land flat with component side up. After I picked it up, I had noticed both thick-film substrates were cracked. How could this be? There was enough clearance from the highest component underneath, and enough pins to support the ceramic substrate, so why did it break?
Fortunately, we had a state of the art photography lab in the building with high-speed camera equipment. So we set up a controlled experiment to capture what went wrong. We built up some test samples and dropped them while capturing it all at high-speed. Well it wasn’t a fluke. Every one that we dropped and filmed showed the same result. It turns out there was enough flex in the long right angle pins, that the momentum of the substrate caused it to hit the radial capacitor underneath, then spring back as if nothing had ever happened. Under other circumstances, this would have been cool to see, but not when the project was in jeopardy.
To make a long story shorter, I eventually came up with an elegant solution for a plastic carrier that would support the substrate and keep it at a fixed height above the board. Not only did it solve the reliability problem, but it also solved the shipping and handling protective packaging issue for the thick-film assembly at the same time.
Figure 2 shows the actual artwork for the repeater’s PCB. Back then, all our boards were double-sided and all layouts were done by hand; first in colored pencil, then using red/blue tape and pads on mylar film for final artwork. Red usually represented the solder (bottom) side of the board and blue was the component (top) side. The artwork was usually done at 2:1 scale and later photo reduced to produce the 1:1 photo-masks. Red and blue filters were used during the photo reduction process to separate individual layer masks. A red filter generated the component side and blue filter produced the solder side photo-masks respectively. All drilled holes were manually specified on a separate drawing with various symbols for the drill sizes. Line widths and space were typically 25 mils and components were on 100 mil pitch. All components were through-hole mounted on one side only and passed through a solder wave.
Figure 2 Example of double-sided artwork for the T1C line repeater. Red is solder side, blue is component side.
The T1C line repeater project from its inception, to designing, testing, building 50 prototypes by hand and completing a successful field trial in California, took about 6 months; all with a team of three plus our manager, and mechanical design support staff. Finding these pictures truly was a blast from the past. Looking back, I sometimes wonder if we could have done it any faster with today’s modern technology, CAD tools and outsourcing business model. What do you think?
Born in Hamilton, Ontario, Canada, Bert graduated in 1976 from Mohawk College of Applied Arts and Technology in Hamilton, Ontario, Canada as an Electronic Engineering Technologist. Over a 32 year career at Bell Northern Research and Nortel, he helped pioneer several advanced technology solutions into products and has held a variety of R&D positions, eventually specializing in high-speed signal integrity and backplane design. He is the founder of Lamsim Enterprises Inc. providing innovative signal integrity and backplane solutions. He is currently engaged in signal integrity, characterization and modeling of high-speed serial links associated with backplane interconnects. With three patent applications and two patent grants to his name, he has also (co)authored several publications, including an award-winning DesignCon2009 paper related to PCB via modeling. His current research interests include signal integrity, high-speed characterization, and modeling of high-speed serial links associated with backplane interconnects. To contact Bert, email him at: firstname.lastname@example.org