Some Interesting Article References with Comments
This is a selection of some of my favourite and most frequently used non-book references. Some of them are quite old but that is simply because they are classic papers and have stood the test of time. Perhaps some of the more recent ones will also become classics in due course. For fuller details and some of my personal comments, click on the chosen title.
This Agilent application note is essential reading to properly understand how to do accurate vector network analyzer (VNA) measurements. It explains concisely the types and sources of errors and how they are corrected during calibration. It describes the error models used and how they relate to practical measurements requiring various adaptor arrangements. I have used the equations given with Matlab to calculate the errors for my very old HP 8505A network analyser which of course did not have processing capability you now find in the latest VNAs.
When you are trying to write well presented and professional documents you need to get the fundamentals right such as units and symbols. Sometimes unit symbols are mis-used, for example talking about a waveform having a period of 64 micro-siemens (64 µS) when they presumably mean 64 micro-seconds (µs). Another favourite is not putting a space between the number quantity and the unit symbol, such as using 12V and not 12 V.
This is a very interesting paper about oscillators and, in particular, how they are designed to minimise phase noise ie. free running and not phase locked so we are talking about far from carrier phase noise. In particular I have used the varactor diodes in anti-parallel configuration as Dr. Rohde described. Even in a phase locked oscillator the PLL only reduces the phase noise close to carrier inside the loop bandwidth superposed onto the carrier. Wikipedia tells me that Dr. Ulrich L. Rohde was one of the co-founders of Rohde & Schwarz no less so it must be him.
I built and tested one of these some time ago and it worked brilliantly at the narrow band of millimetre wave frequencies concerned. It produced very good quality Gaussian beams in both the E and H fields so it was very effective in coupling efficiently in and out of a very low loss quasi-optic 'beam' waveguide. It was used as a sort of transition between fundamental mode rectangular waveguide and the lens or reflector type transmission line. The Potter horn itself is fed from circular waveguide so my version also included a rectangular (TE10) to circular (TE10) transition, a fairly simple taper. Of course the only drawback is that it is fairly narrow band. If you needed Gaussian beams across a wider band you would need something like a scalar or corrugated horn.
I was first given a copy of this paper when I was a student with Cable and Wireles a long time ago. We had to use the various formulas and nomographs in it to predict the behavoir of some VHF terrestrial propagation paths across the local countryside. Afterwards we took out the suitably equipped van to measure them. I think my predictions were something like 20 dB out on the low side which was not impressive. In fact when we are talking about fading 20 dB is very modest; deep fades can easily reach 40 dB to 60 dB notably due to multipath fading at these frequencies. I think I had overestimated the multipath.
This is another one of those classic papers, which basically suggested designing an antenna based on the cassegrain optical reflecting telescope. With light having a wavelength of around 600 nm, even a small 4 inch telescope would have a diameter (D) of about 160,000 wavelengths. That compares to D being something like 1000 to 1500 wavelengths across a very large satellite groundstation antenna at Ku band, but it probably still qualifies as a 'large number of wavelengths' which we so often need. I have used this paper to design a cassegrain antenna to be fed with the Potter horn at millimetre wave frequencies which worked well. In this case D was only around 30 to 40. Certainly the cassegrain is used widely for satellite ground station antennas. I guess it is quite expensive to build, having the more complicated reflectors which have to be well aligned. That is probably why it does not seem to be used much for the mass market satellite TVRO type antennas. They seem to be mostly offset types. One disadvantage of the cassegrain is of course that the subreflector blocks a portion of the main beam and typically couses some scattering (diffraction).
Although you can still get Hewlett Packard printers and other computer peripherals which I will not comment on, you probably know that Agilent inherited several of their former businesses including semiconductors and test equipment. Agilent were very quick to disassociate themselves with HP, judging by how quickly they changed the annotation of their highly respected application notes from HP to Agilent. This is one of the early HP ones and I have not as yet found the Agilent update, perhaps because it is based on an early silicon bipolar device. If is a barely legible photocopy cubed and is actually referred to by Gonzalez and probably many other authors. It describes how to design a noise matched low noise amplifier extremely well, step by step and if you need some revision in that area well worth reading.
I photocopied this paper from a bound volume of the Proceedings of the IRE (a predecessor of the IEEE) some years ago to help me with a project I was doing on thermal noise at University College London. The librarian had to unlock the basement for me and I had to search around the stale smelling volumes to find it. This looks like Friis's original paper in which he arrives at the famous cascaded noise figure equation which is often referred to using his name. I am sure he wrote many others and there may well be subsequent corrections or clarifications. In his conclusion he says "...it is hoped that the definitions and symbols suggested will come into general usage." Well I am pleased to say that they did, perhaps with the exception that he used an upper case 'k' for Boltzmann's constant.
This short paper (it is more of a 'letter' actually) includes lots of useful information on balanced amplifier configurations. These are popular and offer many advantages for cancelling harmonic distortion, increasing bandwidth and power handling in the case of PAs. However they are not so common in the low power stages where thermal noise performance is usually important. That is where this paper is useful.
This is more of a datasheet really based on the ATF-38143 low noise pseudomorphic high electron mobility transistor (PHEMT) which are very popular for LNAs these days. It is a basic cheap plastic package and is optimised for typical G2 and G3 frequency bands probably aimed at mass markets and yet has very good low noise performance. There is useful information on noise figure matching. It is probably a good example of the level of today's technology with small signal devices.
Whilst looking around on the Web for freely downloadable papers you have to wade through dozens of them which are not really worth spending much time on before finding odd gems like this. Superbly written and entertaining, really holds your attention describing the mechanics of quadrature modulation and demodulation using the complex exponential notation instead of the more traditional real trigonometry. Once you memorise Euler's identity, get to grips with his 3 dimensional vector diagrams and think in terms of the negative frequency it really will become much easier than trying to remember and apply all of those trig identities that we all learnt at school.
I was a bit skeptical at first when somebody suggested I design an absorptive notch filter from these articles previously published in Wireless World. As many engineers in the UK will remember, Wireless World was a semi-professional magazine aimed at radio amateurs and home constructors. It was most famous for Arthur C. Clarke's article in 1945 suggesting that radio communications might be possible via artificial satellites positioned around the Earth in geosynchronous orbits. Despite this, Wireless World was not taken too seriously by some established professional journals. I thought that an article from Wireless World might be popularist, for want of a better expression. I was so wrong (again)! The theory and equations are all explained in great detail with several example configurations. I built one of these (absorptive) notch filters for around 30 MHz, strictly observing all the good RF design practices, and it worked superbly. These type of filters come into their own when you wish to attenuate some discrete spurious and simply attenuating it by reflection, as a traditional notch filter would do, would cause problems due to the reflected spurious in the other direction. With good low loss RF design techniques there is no reason why it should not be effective up into VHF.
This is such an interesting set of readable articles on PAs and general high power technologies. They are in tutorial style and it does not get bogged down with excessive theory or derivations. However all the references are there, some 120 for the whole set of articles, for anybody who needs to concentrate on any particular aspect. Subjects covered include: history, efficiency, devices, classes, matching, architectures, linearisation and new modulation methods.
Another very good application note from Agilent. This one is very strong on all aspects of S-parameters: theory, properties, conversions to and from other parameters such as the transfer type, etc. Other topics include: signal flow graphs, Smith chart properties, stability circles, measurements, high frequency design, constant noise figure and gain circles, broadband design and matching.
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