Cable model for high speed data transmission

Looking at cable models, an interesting expression was found. Though approximate and using " typical cable parameters" it can be used to at least get a feel for the challenges of transmitting high speed data down a UTP cable. The expression is: f3db = 2.25E4/(d*d). Here d is the length of the cable in Km. From this an interesting conclusion can be drawn. The bandwidth of this "typical" cable of length 100 Meters is only 2.25 Mhz. At the other extreme, a 1 Meter cable will have a 22.5 GHz of bandwidth. A 10 Meter cable will have a bandwidth of 225 Mhz and so on. So if we need to transmit data at high MHz or low GHz speeds, on a longer cable, then there is no option except to design and use an equalizer. This is of course a whole new ball game with many issues. From an initial assessment it appears that an analog equalizer is the best option here. The design is understandable and the technology is available to implement it in a reasonable amount of cost, time and space on an ASIC as needed.

Analog and wireless design: Re - visiting average and effective values, voltage, current and power.

Having become accustomed to clicking icons on a simulator to get fast readings of average and rms values, power values etc we were startled to find that we could not easily compute effective and/or average values of arbitrary waveforms to crosscheck simulation results. So we decided it was time to re-visit the so called " text book " definitions and see what we come up with and re-educate ourselves. It is often the bare essentials that seem to trip us up sometimes. So after a fairly close look we documented, in a workbook fashion, our findings, just in case we need them from time to time. This little report is available on our website under engineer's corner/engineering pages. For interested readers it is located at www.signalpro.biz.

Analog and mixed signal IC design: DMOS transistors

We are all familiar with the MOSFET. Some of us are also very familiar with JFETs.
However, there are a number of transistor types that are not so common. One of these is the DMOS transistor or double diffused MOS transistor. In recent years the DMOS transistor has been used more and more to provide high voltage capability to analog and mixed signal IC designers. It is very popular in the design of MEMs interfaces where higher voltages are required. Currents are usually not high. DMOS transistors can deliver higher currents but need a larger size. The tradeoff is obvious. The DMOS structure is an interesting one. For further detailed information please go to our website, www.signalpro.biz and take a look at the DMOS tutorial article in the engineering pages.

Analog and mixed signal and RF ASIC success factors

An analysis of several success factors in analog/mixed signal/RF ASIC design and manufacture turned up a number of interesting facts. There were many reasons for success that have been already described elsewhere in this blog. However, the interplay of relationships and their impact on the success of ASIC design and development was not touched. Much to our surprise the analysis of over 100 ASIC projects executed in SPG indicated that when significant success was achieved, not only were the obvious success factors present ( see the blog entry) but a key factor was the customer interface. (1) The customer interface was a technical person who was really closely involved in the design from the system side; (2) the technology that was being used to implement the ASIC was an excellent fit to the requirements; (3) the fabrication vendor relationship was strong and close with SPG; then the probability of clear success was over 99% (conservatively). We did not find a single failure in our list of 100 projects when these conditions were also met. ( In addition to the success factors quoted elsewhere in this blog. The very first entry in the blog since its inception). Thus the objective of this entry is to add this success factor to the list. The search for success in the analog/mixed signal/RF ASIC design and development is critical for our success.

Analog and mixed signal design: FIR filters

FIR filters are strictly not analog or even mixed signal in nature. They are in fact, digital circuits. However, it seems that more and more of these filters are being used in mixed signal designs, specially in fine line semiconductor processes, where analog processing is used to convert to digital and then circuit blocks such as FIR filters, Comb filters, multipliers etc take on the task of further signal processing within a chip. A perfect example is a sigma delta A/D converter. Here there is a minimum of analog circuitry, followed by significant amounts of digital ciruitry. Among these are digital filters. ( Usually sinc filters). A brief note on the practicalities of FIR filter design are presented and can be found in the engineering pages of the our website: www.signalpro.biz.

Analog system design: Second order system analysis and design

Second order systems appear frequently in the design of analog systems as well as digital systems. In most cases these types of systems are difficult to understand analytically and designers must resort to simulations and empirical assessments.
Examples of these types of systems ( or circuits) are PLLs, switching power supplies, analog equalizers, mechanical servomechanisms, filters etc. There are some expressions available to do approximate analysis and design before resorting to long simulations or empirical data gathering. These are mainly based on the second order characteristic equation. Solution of this equation yields at least two very useful quantities. The natural damped frequency and the damping ratio. Use of these parameters can greatly facilitate the analysis and design of second order systems. For a brief cookbook style treatment of this analysis please read the article in our website www.signalpro.biz under engineering pages.

Cable modeling for high speed data communications

I have not been blogging for a while. The reasons are many, but mostly because of some interesting work that came up. This post is a result of grappling with cable modeling and cable parameter information extraction. The simple comment is, that this is a very tedious affair as very little data is available in the literature. High speed data communication cables come in various disguises. Some of the more popular types are the CATX types and STP ( shielded twisted pair) types. Various gauges are being used. We looked at 24 and 26 AWG types.

The issue is, that if you are designing a cable equalizer for example, you need a cable model. There are a number of ways to do this. The most expensive way is to either buy or rent a laboratory piece of equipment which can perform up to at least 12 Ghz, buy or make connectors, and then make measurements. The other way is to use information available in the literature to build a cable model. The latter is very sparse and difficult to obtain. Anyway both approaches should be tried.

The modeling parameter W in some SPICE based programs is a useful one and with proper manipulation can yield fairly accurate models. It is quite complicated to understand. Simple transmission line models in PSPICE can be used but will only offer very basic models and may not be accurate for design.

There are also cable model ( analytical) parameters available in some text books for coaxial, parallel wire cables, microstrip etc. which can also be used. These can be used to obtain the so-called RLGC SPICE model. Using the RLGC circuit is not enough to model transient response since the cable delay cannot be modeled. A transmission line model is required for this. These too are only first order estimates.

So what is required is more empirical data of cables and cable models. One or two researchers have actually done this. However, the need to model parameters such as attenuation, crosstalk, ISI etc is still an open field, ready for someone to step in and do the needful!