The View from Signal Hill

Semi-Informed Speculation on the
Radio Art

By David M. Upton, WB1CMG

NE Div. Cabinet meeting

The summer edition of the NE Division Cabinet meeting was held on July 15th at the Andover Marriott hotel just off of I-93. Speaker Chris Imlay, N3AKD, ARRL Counsel spoke at length on many of the legal issues connected with amateur radio. The writer was quite impressed with the broad grasp and understanding evinced by Mr. Imlay in both broad national and international issues as well as more local ones. There was quite a bit of discussion about any possibilities for new spectrum including some mentioned in VfSH for last month. Rich Palm, K1CE, ARRL Field Service Director, reported and exchanged viewpoints with club and section representatives on various issues of concern. Tom Frenaye, K1KI, detailed the recent ARRL restructuring strategy as well as what has been accomplished so far. Everyone from HQ emphasized that changes are being made at all levels of the organization. One hot potato being seriously considered is further fragmentation of QST technical content in order to attract newcomers. Most all of the parties in attendance were glad that the vanity call sign issue was largely resolved since the last meeting. Equally well received were the positive outcomes of the spectrum decisions by the FCC at 2400 and 900 MHz. The ARRL has identified as a major goal to help focus FCC attention on enforcement within the next few years. Success of the amateur auxiliary’s role in pursuing and documenting offenders is critical to maintain morale but ARRL feels that the FCC has not done very well in bringing the cases to a conclusion. NALs (Notices of Apparent Liability— bureaucratese for fines assessed) were excoriated by all concerned as being ineffective in addition to helping to create disrespect for the authority. Club officials urged greater communication with the League at lower levels of the Field Service hierarchy. Next meeting will be in January of 1996 at the same QTH .

Engineering Software for Experimenters

Most people that are remotely associated with electronics know that the industry has adopted Computer-Aided-Design (CAD) methods almost 100%. There is very little remaining of the traditional cut-and-try methods anywhere. The home experimenter wishing to try some of these methods though is a little bit stymied by the difficulties in acquiring the necessary software. There are several programs or ensembles of programs out there that aim at the advanced experimenter or small technical business/ consultant. Many of these are affordable - some are downright cheap. I am going to highlight some of the more common ones as well as new entries in the field from a serious RF experimenter’s viewpoint.

SPICE in all of its incarnations is the most commonly used program for analysis in electronics. Slightly dumbed-down or limited versions are marketed at a reduced price. SPICE is geared to the unique requirements of the silicon IC design world which makes the program seem awkward when used with RF circuits. SPICE excels at ferreting out problems with tricky bias circuits comprised of many matched bipolar transistors connected as repeaters or mirrors. Optimizing the tuning of a GaAs FET for low noise with SPICE though would be a very frustrating exercise. SPICE does not understand S-parameters nor can it input files as data. The concept of handing it packets of pre-measured data does not exist. SPICE does not have any optimization or tuning functions although variables or values may be swept over a range.

Noise calculations are handled in an exceedingly inconvenient form as total output voltage and or current noise contributions in units of nV/(Hz.)**-1/2 . In order to see the effect of tuning a parameter, these units must be input-referred to the stage in question by dividing by the true voltage gain. SPICE can be made to understand S-parameters by defining sub- circuits in connection with sources that mimic how RF sources behave which is constant power-as opposed to constant voltage or current. In order to calculate common RF measurements such as stability factor or available gain, SPICE either requires additions to interpret S-parameters or iterative sweeps of all possible impedances at the input and output of the circuit. SPICE will, by generating voltage and current waveforms against time, give insight into circuit behavior that is extractable from nowhere else. The modifications to SPICE circuits required to use it effectively at RF are petty intense so lets just say that the user needs a very firm understanding of network theory applied to RF circuitry.

At the opposite end of the spectrum is PUFF. PUFF is available for $10 from the Electrical Engineering Department of Cal Tech. Written as a student project, it was designed to be ultimately user-friendly to make microwave design fun. PUFF handles moderate size circuits and allows hierarchical structures. Hierarchy is the term used when a circuit is defined, named, and then used as a part of another circuit. It supports S-parameter analysis and will also generate artwork for simple circuits. It has one really neat feature in that it will perform Fourier Transforms to generate time-domain equivalents of the frequency response. As a consequence of its hierarchical structure, PUFF also allows S-parameter data files so manufacturer’s data can be readily used. The manual is also helpful in illustrating how to use the program if a bit . . enthusiastic. The downside of PUFF is that accuracy of some of the transmission line models is not guaranteed and there is no provision for DC analysis. In fact, there is no provision for other than 2-port input/output analysis and all elements are assumed to be linear. These are not overly restrictive provisions but do exclude some common circuits. Workarounds are possible but may ultimately consume more time than using some other program. It certainly can be defined as the biggest bang for the buck and will handle the majority of circuits the amateur is likely to encounter.

ARRL Radio Designer for about $150 is sort of the next logical step after PUFF. ARD is basically a re-packaging job of SUPER- COMPACT-a professional simulation/analysis program selling for thousands of dollars. What got left out of SUPER-COMPACT to make ARD is the transmission line models which would be used in professional work. What got left in was WINDOWS ™ compatibility and the S/C syntax which was a joy to use compared to later competitors. What also got left in was relatively easy access to S-parameter measurements and files. ARD speaks S- parameters like a native but also understands other forms of circuit dialects such as Y, Z, and chain(or ABCD). The hierarchical structure supports multi-port linear circuits and associated file-handling. The biggest improvement of ARD over PUFF though is that ARD includes the S/C optimizer which is a way of stating specific goals to the program and then letting it “tune” variables to match those goals. QST had an example of doing this for an antenna matching network. One way of using the optimizer is to calculate a circuit down to several decimal places then place real values and parasitics in the circuit file along with the performance goals and say “sic ‘em” or words to that effect. The optimizer routine will then try to “fit” those real components to the “calculated” goals. Needless to say, this takes some experience to use effectively. It is possible to do real damage to the circuit file with the optimizer or “bomb” the program but this single feature along with the increased calculating ability make the package worthwhile.

There are additional programs which are available to address certain unique problems or groups of related calculations such as antenna design and analysis. receiver design, or propagation prediction. Another issue is the graphics capabilities - both internal and external - of the program. Here, SPICE ranks about dead last with only raw data or maybe a raster plot or line-printer plot provided without extensive post-processing capability by a program like PSI-PLOT. PUFF is next with readable graphics on- screen and some ability to communicate with the outside world through S-parameter files and artwork generation. ARD leads the pack with its WINDOWS-compatible graphics and file formats. These files are readily imported into other programs without difficulty. Unfortunately, except for SPICE, system simulation and non-linear analysis remain the difference between the big boys and these programs discussed here. SPICE is cumbersome enough to use but can do anything given time and a big enough computer with a knowledgeable programmer. If the reader wishes to develop skills to help in industry-learn SPICE well. Then using any other approach will be a cakewalk.

The Manchester ARRL Convention

Thanks to all NARC members who attended the talk given by the author at the New England Division convention at the end of July. Overall, the convention was a success except for somewhat disappointing attendance. Hot and unpredictable weather is blamed for some of the lack of big crowds. The effect though was to make it more cozy and enjoyable instead of the people-crusher that Boxborough tends to be. The organizing committee seemed to have everything under control and to have anticipated most all requests. Programs were varied-appealing to many different interests in both operating and technical subjects. Alternative programs were also well-planned with some added horsepower provided by the appearance of KA7ITT. My own talk seemed to go OK-once it started. The previous speaker ran over so in order to not hold up WB4HFN, the participatory part of the hour got slashed. I hope some people came away with ideas on how to build things for 900 and 2.4 GHz. More hand- outs could be made available to interested parties. If I had one peeve for the Manchester ‘97 steering committee it would be to expand the flea and open it up to tailgating. The present highly structured approach isn’t working very well.

Don’t forget about the Eastern States VHF/UHF Conference in Hartford, CT on August 25th through the 27th. Technical and band-specific sessions will run concurrently on Saturday whereas antenna measuring and a specialized flea market occupy Sunday. Mohawk ARC will hold a flea in Gardner, MA at the drive-in on the 27th as well. Wellesley ARC just held a flea on August 6th which will probably repeat about the same time next year.

73, Dave

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