Once upon a time, at a company where I was working, we had problems with the performance of our prototype. One of the major contributors to the system’s performance was the clock generation and distribution.
We had to drive four RF boards from our FPGA design with the exact same clock, and this clock should be stable with low jitter. Our FPGA board had a clock generator, which was something like a DLL (Delay Locked Loop). This generator generated a clock with some jitter in it. Our design performed worse than expected by the simulations, so we started digging. As we observed the clock jitter, we decided that for our application this was simply not acceptable. We needed a clean 40MHz clock signal that would drive the FPGA board and the four RF boards (up/down-converters from baseband to 5GHz for WiFi). As time was very important, we considered the options of how to produce a clock generator and distribution circuit:
- Design a PCB and send it for a prototype service
- Build a circuit on standard general construction PCB
The first option had the drawback that the delay from PCB design to have the actual boards at our disposal would be minimum one week. The second option had the problem that we needed a good signal quality and some components were SMD.
I was thinking of how we could build our own PCB in a day or so. Usually with through-hole components, I used one board with full copper (used as a ground plane) and I attached components using their leads to the circuit. I learned this technique when I was junior engineer reading Bob’s (Pease) book and articles and seeing other engineers practicing this method. The following image is from his excellent book Troubleshooting Analog Circuits
cover illustrates the concept.
In this case, however, I had a circuit that was far faster than anything I had built till then using this technique. As I had to solve high frequency problems, I knew that this technique may not be the best thing to do. Also the mechanical stability was an issue as well. If I could only build a PCB prototype….but how? I never was a master of the PCB process. In fact, I quit building PCBs very early as I wasn’t very successful.
At about this same time, we had purchased a very nice and versatile tool called a Dremel:
We had bought this in order to make our prototype boxes. There are many tools you can attach to it. I’ve seen videos created by handicraft people achieving almost the impossible with this kind of tool. I had also the provision to buy a milling tip tool for it; I always wished I had a milling machine in house. Then somehow, something hit me. There are tools that actually mill the copper from the PCB in order to generate prototypes! Why not do this manually (it was a small PCB)?
As this was very challenging I was up to the task immediately. I needed to have a good plan. First the components. I needed a 40MHz crystal oscillator. Easy; I de-soldered one from our RF boards. The clock would be external in this case so we did not need it on board.
At that time our lab stock was almost non-existent. However, we had recently purchased some SMD buffers. These high-speed ICs were almost our entire component stock! Then I had to provide series termination for the output clocks. We had to drive five outputs. We had 50 Ohm coaxial cables for distribution. Their lengths would be from a few centimeters to 30cm for the RF section.
I made a small hand-made schematic. Then I made a sketch of the board layout: External shape, mechanical mounting holes, signal routing etc. As the SMD components cannot be stressed mechanically, I had to provide a small PCB extension section where I would attach the hard coaxial cables and secure them with tie-wrap. I used one layer of the PCB as ground for shielding and the top layer as routing layer.
My CAD was a piece of paper and my mechanical pencil. Wow, exciting; and cheap. How I would make small lines with accuracy? The SMD SOIC package it is not the easier thing to do routing. I know, I know, nowadays SOIC are huge in respect to other packages. I had to secure the PCB somewhere. Ha! A vise is a must for your electronic lab:
I secured the empty PCB at the vise. It was placed vertically, so with my Dremel, I could create either horizontal or vertical lines. As I started to mill the lines, I discovered that my hand was more stable when I did vertical motions. My horizontal lines were not so good. So instead, I decided to turn the PCB at the proper direction in order to do only vertical routing. In order to reduce routing work (and risks), I planned to pull the unused IC outputs up from the copper or for common inputs to keep the pads as one.
After the PCB was ready I soldered the components. I tested for shorts and opens and then I came to the hard job of soldering the coaxial cables. I soldered the ground jacket to the PCB first and then each tip to the output termination resistor. This provided adequate mechanical stability. After soldering all the cables I used a tie-wrap to extra-secure the cables. You can see the result in the picture below:
Finished clock generator and distribution handmade PCB
If you looked at this PCB from a distance it seemed awesome. A close-up revealed all its ugly handmade routing.
This was a good solution to our problem, but we still had others to resolve…About the author
Fascinated by computers seen in advertisements in the start of 80’s Ilias Alexopoulos knew just what he wanted to do. He started playing with early computers for fun and both the hardware and software was his everyday joy.
Later, at the university, Ilias entered the world of embedded computing, DSP, and FPGAs. His passion for high-end systems has lead him to solve many challenges from production testing equipment to high-tech startups.
Commitment to engineering excellence and enjoying the engineering journey is always his aim. Recently, Ilias has started to contribute to the open source community with articles, code, etc. You can find more information at www.ilialex.grClick Here
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series...Editor's Note: It would be great if you took the time to write down short stories of your own. I can help in the copy editing department, so you don’t need to worry about being “word perfect”. All you have to do is to email your offering to me at max@CliveMaxfield.com with
“How it was” in the subject line.I can post your article as “anonymous” if you wish. On the other hand, what would be really cool would be if you wanted to add a few words about yourself – and maybe even provide a couple of
“Then and Now” pictures showing yourself as a young engineer
("Then") and as the hero you've grown into
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