标签: signal

At DesignCon last year, Al Neves, the self-proclaimed signal integrity (SI) Practitioner shared that he uses airlines every week. He's also the Chief Technologist with Wild River Technology. Of course Al is referring not to taking a plane every week, but to using the specially constructed precision coaxial transmission line structure called an airline. An airline is a cylindrical coax transmission line with precision machined centre conductor and inner diameter cavity. It's designed to act as a low loss, single mode, uniform transmission line, which can be used as a reference. Its impedance is accurate to better than 1 per cent, and is often traceable to NIST (figure 1).   Figure 1: Example of the measured return and insertion loss of an airline that is designed with a stepped impedance. Courtesy of Wild River Technology. Al suggests every SI practitioner doing routine VNA or TDR measurements should have at least one 50Ω airline handy. "I use it whenever I want a quick way of verifying a VNA calibration or to compare a VNA measurement to a TDR measurement," he said. "Airlines are really about risk reduction. Measuring an airline is the start to a benchmarking strategy. You can pay a little bit to buy insurance and reduce your risk with an airline." It's easy to get a measurement from a VNA. The hard part is getting one that is free of artifact. "A VNA requires a lot of discipline to calibrate. Every now and then, it's an important sanity check o look at the measurements on a known standard like an airline." Adding an airline in series to general TDR measurements gives an immediate measure of confidence. Neves will tell you that one of the advantages of an airline is that the measurements are clean to high bandwidth. There are rarely transverse modes, so the measurements should be artifact free. An SI Practitioner's goal is to get a successful product out the door on schedule. When signal integrity problems pay a role in the design, we need to leverage all the tools available to us. Al suggests these tools are in four important areas: metrology and measurement, 3D modelling, general SI issues, and the business and economics side of things. Figure 2 illustrates the relationship among these issues.   Figure 2: A signal-integrity practitioner leverages four categories of tools. Eric Bogatin Signal Integrity Evangelist Teledyne LeCroy  

The concept of time is simple yet complex. In its simplest form, it's the counting of a regular, repeating pattern. On the opposite end, we would have to turn to Einstein. Thankfully, this story resides on the simple side. You see, before the advent of the real-time clock, electrical engineers scurried hither and yon in search of ways to calculate time accurately in their circuits. One very convenient source of a regular, repeating pattern was the 60Hz line frequency coming from the wall outlet. It was a very stable source in the US, and so long as it was stable, your machine would work fine. But if ever a crack appeared in the stability of the 60Hz source, the entire machine could be compromised. This was the case on the idle Tuesday morning our story begins. Giant pillars of white smoke soared high in the air from the large, concrete stacks that reached up into the clear blue sky. It was easy to find parking in the large lot when I arrived at the paper-making plant nestled on the outskirts of Richmond, Va. The smell of sulphur was thick, and the rumble of the large trucks carrying trees filled the air as they entered the plant. It was a short walk to the visitor's desk, where I was met by the man who had called me a few days earlier. "Damn thing doesn't work," he had said. I get that a lot. He led me to the not-working microwave solids analyser. This simple machine consists of a microwave, a balance, and an embedded computer. It heats a sample to remove all the moisture, and the computer uses the difference in weight to calculate the per cent moisture in the sample. I placed a test sample in the machine and pressed the start button. Immediately, problems were apparent. On the grey scale display, the system clock was counting down the time. The problem was it was counting way too fast. It had counted down more than two minutes, but my gut, verified by a clock on the cinderblock wall, told me less than a minute had gone by. I pressed the stop button, and another problem revealed itself. Instead of the usual single beep, I got three. It acted as if I had pressed the button a couple of times. It appeared I had debouncing as well as timing issues. It was then that I clued in on the A/C line and took a closer look. I was in for a shock.   This video and the scope screen captures above were taken directly from the 120V wall socket via a 10-to-1 stepdown wall-wart transformer using a 10x probe. As you can see, the signal is very dirty. I concluded the source of the noise was probably from one of the large paper rolling machines running elsewhere in the plant. I had identified the problem. But I still did not know how the dirty signal was causing the timing and debouncing issues and how to fix them. I had to solve a before I could even begin to approach how to fix the issues. Like any good technician, I dug into the schematic. It wasn't long until I discovered a small circuit that appeared to be converting the A/C line frequency into a 5V clock signal. The output was routed to one of the pins of the 80188 Intel processor.   I knew I had to get a scope on pin 6 of the opto-coupler (U7) to see if the noise on the A/C line was getting through.     So the noise from the A/C line was getting through the opto-coupler in the form of problematic leading and trailing edges. It was so bad that the scope had a hard time locking on to the frequency, as seen above. I did not have access to firmware, but my theory was sound. I believed that the 80188 computer was using this very signal to generate its definition of time—every 120 leading-edge counts was equal to one second in the firmware. And in this case, all the spikes on the leading edges were being counted, as well. This would cause the system's internal clock to run too fast. Also, I knew the switch debouncing was done in firmware. This code is dependent on accurate delay times. If the system clock were running too fast, that would mean the delay times would be too short, and the debouncing code would not work properly. Yessss! I had found the source of the problem, theoretically at least. Now, how to fix it. Long story short, I concluded that it would be easiest to simply replace the signal. I would remove the opto-coupler IC and use a microcontroller to generate a clean signal and route it to the processor.   Above is the schematic. (I realised I had taken the scope readings in the previous images off U6 instead of U7. U7 produces a 10 per cent duty cycle 120Hz signal, as opposed to 50 per cent.) Below is the code to generate the 10 per cent duty cycle 120Hz signal. while (true) { { output_high(PIN_A2); delay_us(833); output_low(PIN_A2); delay_us(7600); } } I made a board using toner transfer. I removed the opto-coupler, and used the empty socket to supply power to my board and route the clean signal to the processor.     I installed it in the instrument the next day, and everything worked perfectly. The timing was accurate, and no more debouncing issues. I had the board done from a board house and installed it a few weeks later. The instrument has been working well ever since.     This article was submitted by Will Sweatman, a field engineer, as part of Frankenstein's Fix, a design contest hosted by EE Times (US).

The United States Army Signal Corps' first attempt to "touch" another celestial body was on January 10, 1946, when it bounced radio signals off the moon and received the reflected signals. Dubbed "Project Diana" for the Roman moon goddess, the effort led to what is today known as EME (Earth-Moon-Earth) communications, used for ham radio. Project Diana is often noted as the birth of the US space program, as well as that of radar astronomy. The project was the first demonstration that artificially created signals could penetrate the ionosphere, opening the possibility of radio communications beyond the Earth for space probes and human explorers. Project Diana also established the practice of naming space projects after Roman gods and goddesses, like Mercury and Apollo. Project Diana's first successful echo detection came on at 11:58 am by John H DeWitt and his chief scientist E King Stodola from a lab at Camp Evans, in Wall Township, NJ. A large transmitter, receiver, and antenna array were constructed at the lab for the project. The transmitter, a highly modified World War II SCR-271 radar set, provided 3,000 watts at 111.5MHz in quarter-second pulses, while the "bedspring" dipole array antenna provided 24 dB of gain. Reflected signals were received about 2.5 seconds later, with the receiver compensating for Doppler modulation of the reflected signal. Attempts could be made only as the moon passed through the 15-degree-wide beam at moonrise and moonset, as the antenna's elevation angle was horizontal. About 40 minutes of observation was available on each pass as the moon transited the various lobes of the antenna pattern. The Project Diana site is today maintained by the Infoage Science/History Learning Center.   Suzanne Deffree is EDN's executive editor. She is an award-winning journalist who manages several blogs and sections of EDN.com and EDN’s e-newsletters including its daily newsletter, EDN Today, and EDN Fun Friday. She also heads EDN’s social media and community efforts.  

At DesignCon last year, Al Neves, the self-proclaimed signal integrity (SI) Practitioner, said "I use airlines every week". He's also the Chief Technologist with Wild River Technology. Of course Al is referring not to taking a plane every week, but to using the specially constructed precision coaxial transmission line structure called an airline. An airline is a cylindrical coax transmission line with precision machined centre conductor and inner diameter cavity. It's designed to act as a low loss, single mode, uniform transmission line, which can be used as a reference. Its impedance is accurate to better than 1 per cent, and is often traceable to NIST (figure 1).   Figure 1: Example of the measured return and insertion loss of an airline that is designed with a stepped impedance. Courtesy of Wild River Technology. Al suggests every SI practitioner doing routine VNA or TDR measurements should have at least one 50Ω airline handy. "I use it whenever I want a quick way of verifying a VNA calibration or to compare a VNA measurement to a TDR measurement," he said. "Airlines are really about risk reduction. Measuring an airline is the start to a benchmarking strategy. You can pay a little bit to buy insurance and reduce your risk with an airline." It's easy to get a measurement from a VNA. The hard part is getting one that is free of artifact. "A VNA requires a lot of discipline to calibrate. Every now and then, it's an important sanity check o look at the measurements on a known standard like an airline." Adding an airline in series to general TDR measurements gives an immediate measure of confidence. Neves will tell you that one of the advantages of an airline is that the measurements are clean to high bandwidth. There are rarely transverse modes, so the measurements should be artifact free. An SI Practitioner's goal is to get a successful product out the door on schedule. When signal integrity problems pay a role in the design, we need to leverage all the tools available to us. Al suggests these tools are in four important areas: metrology and measurement, 3D modelling, general SI issues, and the business and economics side of things. Figure 2 illustrates the relationship among these issues.   Figure 2: A signal-integrity practitioner leverages four categories of tools. Eric Bogatin Signal Integrity Evangelist Teledyne LeCroy

Time is a simple yet complex concept. In its simplest form, it's the counting of a regular, repeating pattern. On the opposite end, we would have to turn to Einstein. Thankfully, this story resides on the simple side. You see, before the advent of the real-time clock, electrical engineers scurried hither and yon in search of ways to calculate time accurately in their circuits. One very convenient source of a regular, repeating pattern was the 60Hz line frequency coming from the wall outlet. It was a very stable source in the US, and so long as it was stable, your machine would work fine. But if ever a crack appeared in the stability of the 60Hz source, the entire machine could be compromised. This was the case on the idle Tuesday morning our story begins. Giant pillars of white smoke soared high in the air from the large, concrete stacks that reached up into the clear blue sky. It was easy to find parking in the large lot when I arrived at the paper-making plant nestled on the outskirts of Richmond, Va. The smell of sulphur was thick, and the rumble of the large trucks carrying trees filled the air as they entered the plant. It was a short walk to the visitor's desk, where I was met by the man who had called me a few days earlier. "Damn thing doesn't work," he had said. I get that a lot. He led me to the not-working microwave solids analyser. This simple machine consists of a microwave, a balance, and an embedded computer. It heats a sample to remove all the moisture, and the computer uses the difference in weight to calculate the per cent moisture in the sample. I placed a test sample in the machine and pressed the start button. Immediately, problems were apparent. On the grey scale display, the system clock was counting down the time. The problem was it was counting way too fast. It had counted down more than two minutes, but my gut, verified by a clock on the cinderblock wall, told me less than a minute had gone by. I pressed the stop button, and another problem revealed itself. Instead of the usual single beep, I got three. It acted as if I had pressed the button a couple of times. It appeared I had debouncing as well as timing issues. It was then that I clued in on the A/C line and took a closer look. I was in for a shock.   This video and the scope screen captures above were taken directly from the 120V wall socket via a 10-to-1 stepdown wall-wart transformer using a 10x probe. As you can see, the signal is very dirty. I concluded the source of the noise was probably from one of the large paper rolling machines running elsewhere in the plant. I had identified the problem. But I still did not know how the dirty signal was causing the timing and debouncing issues and how to fix them. I had to solve a before I could even begin to approach how to fix the issues. Like any good technician, I dug into the schematic. It wasn't long until I discovered a small circuit that appeared to be converting the A/C line frequency into a 5V clock signal. The output was routed to one of the pins of the 80188 Intel processor.   I knew I had to get a scope on pin 6 of the opto-coupler (U7) to see if the noise on the A/C line was getting through.     So the noise from the A/C line was getting through the opto-coupler in the form of problematic leading and trailing edges. It was so bad that the scope had a hard time locking on to the frequency, as seen above. I did not have access to firmware, but my theory was sound. I believed that the 80188 computer was using this very signal to generate its definition of time—every 120 leading-edge counts was equal to one second in the firmware. And in this case, all the spikes on the leading edges were being counted, as well. This would cause the system's internal clock to run too fast. Also, I knew the switch debouncing was done in firmware. This code is dependent on accurate delay times. If the system clock were running too fast, that would mean the delay times would be too short, and the debouncing code would not work properly. Yessss! I had found the source of the problem, theoretically at least. Now, how to fix it. Long story short, I concluded that it would be easiest to simply replace the signal. I would remove the opto-coupler IC and use a microcontroller to generate a clean signal and route it to the processor.   Above is the schematic. (I realised I had taken the scope readings in the previous images off U6 instead of U7. U7 produces a 10 per cent duty cycle 120Hz signal, as opposed to 50 per cent.) Below is the code to generate the 10 per cent duty cycle 120Hz signal. while (true) { { output_high(PIN_A2); delay_us(833); output_low(PIN_A2); delay_us(7600); } } I made a board using toner transfer. I removed the opto-coupler, and used the empty socket to supply power to my board and route the clean signal to the processor.     I installed it in the instrument the next day, and everything worked perfectly. The timing was accurate, and no more debouncing issues. I had the board done from a board house and installed it a few weeks later. The instrument has been working well ever since.     This article was submitted by Will Sweatman, a field engineer, as part of Frankenstein's Fix, a design contest hosted by EE Times (US).