Using crystal filters at VHF high band
The following review covers a couple of previous column topics and responses from the readers.
Power supplies The January “Technically Speaking” column discussed dc power supplies for serv-icing and operating communications equipment. One of the topics of that discussion included duty cycle. The following is an excerpt from the January column covering the duty cycle topic:
There are two specification figures concerning duty cycle-continuous duty and intermittent duty. Continuous duty is easy to understand. It is virtually all the time. For example, if a power supply is rated at 20A continuous duty, it can be operated at a load current of 20A 24 hours per day, seven days per week.
On the other hand, if a power supply is rated at 30A intermittent duty, what does “intermittent” mean? This must be spelled out in the manufacturer’s specifications. For example, one manufacturer states “transmit duty 20 minutes, 20% duty.” This indicates that the power supply can deliver at the intermittent level for four minutes out of a 20-minute period.
There is also the ICS duty. This stands for intermittent communications service where the intermittent service is on a 50% duty cycle or five minutes on and five minutes off. For example, a power supply may be rated for 15A continuous duty or 20A ICS.
The statement “transmit duty 20 minutes, 20% duty” was based on specifications for a power supply found in Newmar’s catalog. This reference prompted a response from Jeff Patrick, manager of product applications and engineering for Newmar. His response is printed in full here.
Although this letter is in regards to a column which ran in your January edition, a misstatement within the column has just been brought to our attention, and we feel it is worthy of correction or clarification.
Harold Kinley’s “Technically Speaking” column for that issue focused on the important parameters in selecting a power supply. The article was very informative and addressed issues which are of importance not just to the selection of bench test supplies, but of power supplies in many applications, large and small.
However, it contained an error of interpretation that must be clarified. Kinley states, with regard to duty cycle, “… one manufacturer states ‘transmit duty 20 minutes, 20% duty.’ This indicates that the supply can deliver [current] at the intermittent level for four minutes out of a 20-minute period.”
While he never cites us specifically as that manufacturer, that is the spec that we use in our product literature. Therefore, we must emphatically point out that is not what our spec indicates. Rather, it means that the power supply can deliver current at that level for as long as 20 minutes at a time, with the overall duty at that level being 20%.
Mr. Kinley goes on to describe the ICS duty cycle as “… 50% duty cycle or five minutes on and five minutes off …,” the implication being that this is a much more stringent standard, when, in fact, the opposite is true. When a power supply is rated this way, it is an indication that the heat rise at that current is so rapid the load must be removed after only five minutes to allow the components to cool. When a power supply is constructed with a heat sink large enough to allow continuous operation at the intermittent rating for 20 minutes, the heat rise after five minutes is only a fraction of that of the “ICS” rated supply.
There are other current-rating considerations not mentioned by Mr. Kinley which are also an important factor in power supply selection, particularly if the investment is large or the application critical. For instance, a manufacturer may claim longer operational periods at the intermittent rating by tolerating higher component temperatures, sacrificing long-term reliability. Semiconductor life can be shortened by as much as 50% for an increase of 108C operating temp.
While we understand the scope of Mr. Kinley’s article may not have included these more elusive parameters, it is important for your readers to understand that any manufacturer’s performance claims can be skewed in their favor by their chosen specification method. Finally, perhaps the most important selection criterion should be the earned reputation of the product.
By the way, in Mr. Kinley’s defense, we realize in retrospect that it is perfectly understandable that he misinterpreted how the duty cycle rating is expressed, and for that reason we are going to clarify the meaning of our duty cycle ratings in all future product literature.
There are a couple of statements in Mr. Patrick’s letter to which I would like to respond. First, he mentions an error of “misinterpretation” of the duty cycle specification.
It has always been my position that specifications should be clear and concise with no ambiguity such that they are not subject to misinterpretation. Furthermore, it is my position that when specifications are a bit ambiguous and can be interpreted in more than one way, I always take the most conservative interpretation. As Mr. Patrick stated himself, manufacturers can skew a particular specification in their favor simply by the wording used to describe the particular specification.
In this particular instance, the manufacturer’s wording, or the interpretation thereof, led in the wrong direction. Not to belabor the point, but the wording “transmit duty 20 minutes, 20% duty” is misleading. In the communications service, I don’t know anyone who would transmit for 20 minutes continuous, but to transmit four minutes out of a 20-minute period makes sense. So, the word “transmit” here had misleading connotations. Apparently, the specification was meant to indicate that the supply could deliver at the intermittent level for 20 minutes followed by a period of 100 minutes in which the load current does not exceed the continuous rating.
On the bright side, Mr. Patrick indicated that the terminology used in writing these specifications for Newmar power supplies was going to be changed to eliminate any ambiguity. It is refreshing to find companies that are responsive and who strive to eliminate any misunderstanding of the literature describing their products. I would hasten to add that this type of problem is not limited to power supplies or manufacturers of power supplies. The problem is very broad in scope and can include every type of equipment that is used in the communications industry or, for that matter, any kind of industry. Hats off to Newmar!
An email from Michael Leamer at the Tarrant County, TX, radio shop concerning the March column on RF measurements stated:
I am having trouble understanding how it is that some power density measurements are in watts per square meter and others are in volts per square meter.
I responded directly to his email, and there was a correction printed in a later issue of MRT. However, many of you might have also been confused by this and not have seen the correction. The answer is that the V/m2 is in error. It should read W/m2. That was an error on my part- pure and simple. I can’t lay that one on MRT or the printers! Thanks for catching that one Mike. I’ll try to do better!
Online conversion calculations
Also, I received an email from Robert Perelman, Eupen Cable USA, to let the readers of this column know about an online calculator that can be used to convert between VSWR, return loss and reflection coefficient. Any of the three can be input to convert to the other two. You can find this on the Internet at www.eupen.com. Select VSWR Wizard. Also, their Cable Wizard calculates attenuation for various cable sizes by inputting the frequency and length of cable.
Thanks to all of you who respond through email either to get a clarification, to ask a question or just to set me straight! Your input is valuable to the MRT staff and to me as a contributing writer.
Until next time-stay tuned!