From the trade magazines, selecting crystals, understanding measurement uncertainty, Maxwell’s equations

Another selection of articles from the electronics engineering trade magazines……Dan

Selecting Crystals For Stable Oscillators
Understanding how quartz-crystal resonators operate can lead to designing crystal oscillators with improved stability and better noise performance.

Tutorial on Maxwell’s Equations
There’s a lot of math here, but cracking Maxwell’s equations will give you a lot of insight on how radio works. Registration required.

Understand Uncertainty For Better Test Accuracy
How sure are you of that measurement you just made with your multimeter or wattmeter? This article might open your eyes as to the accuracy of your measurements.

 

From the trade magazines: signal generators, refurbishing ICs?

This edition of “From the trade magazines” includes items from RF&Microwaves, Radio World, and EE Times………Dan

The Fundamentals Of Signal Generation. Signal generators have become indispensable tools for producing the test signals required by today’s engineers to successfully develop and test their devices and systems.

 

Jim Charlong operates the amateur radio station at the Marconi National Historic Site of Canade.

Dedicated Ham Keeping Morse Code Alive. Operated by Parks Canada, the Marconi National Historic Site of Canada  features a museum with a model of the original transmission structure, a historical multimedia display and tour — and Jim Charlong, who keeps the site’s Morse code broadcast legacy alive and on the air. Charlong is a dedicated Morse code operator with 50 years’ experience under his “fist” — fist being a ham radio term that describes the signature speed and style of an operator’s key-tapping skills. Since the Marconi museum opened in July 1989, he has volunteered as its resident Morse code radio operator. From his “radio shack” inside the museum, Charlong regularly communicates with other Morse code operators around the world.

Smoke re-concentrator refurbishes blown electronic components. I think that perhaps they jumped the gun with this article. I’m thinking that an April 1 publication date would have been more appropriate.

How do you choose an antenna analyzer?

A reader recently e-mailed me:

In the past you told me you started with the Autek RF-1, and later moved to the Palstar ZM-30. I am finally getting around to thinking about purchasing an antenna analyzer, but I am stumped by the choices. In order of increasing purchase price this is what I’ve turned up:

How does one decide? Where does one go to find out the differences? Other than asking a fellow ham, how does one find out which one is the best antenna analyzer without paying an arm and a leg (unless the feature(s) so purchased are deemed worth the cost)?

Thanks! 73

He actually missed several other good choices:

  • Autek VA1 – $199. This is actually the antenna analyzer that I first purchased.
  • MFJ 259B - $240. This is arguably the most popular antenna analyzer on the market. MFJ has several other models with different feature sets.
  • YouKits FG-01 – $250. This is a very cute, little analyzer with a small graphical display. It seems kind of expensive, but the graphical display might be worth it.
  • Comet CAA500 – $450.

So, how do you choose just one from this list? Well, I think the first thing that you have to ask yourself is how you’re going to use the analyzer. If all you’re going to do is to check the SWR of your HF dipoles, then buy the Autek RF-1. It’s the least expensive unit, is reasonably accurate, and is small and lightweight, making it easy to use outside where your antennas are located.

Autek RF-1

The Autek RF-1 is inexpensive, and its small size makes it easy to use outdoors where your antennas are.

If you want to do some more serious frequency analysis, then you should be looking at the W4RT miniVNA or, if you have more cash, the Timewave TZ-900s. These instruments can help you do a lot more in-depth analysis of your antenna system. The figure below, for example, shows a plot generated by the miniVNA software. It shows the SWR of a multi-band vertical antenna from 3 – 33 MHz.

miniVNa display

For more sophisticated frequency analysis, consider the miniVNA. It uses a computer to generate graphs like this.

Some antenna analyzers do more than just SWR. For example, what sold me first on the Autek VA1 and then on the Palstar was that they also measured reactance. So, you can use the antenna analyzer as an LC meter as well. Palstar also says that you can use the ZM-3 as a low-level signal source.

Next, you need to consider what bands you’ll be using it on. Many antenna analyzers only cover the HF bands. That’s a bummer if you like operating 6m, or like to experiment with VHF/UHF antennas. A friend of mine bought the Palstar antenna analyzer after talking to the company at Dayton. At the time, they said that they were planning to come out with a model that covered 6m, as well as the HF bands.

Unfortunately, they never did come out with a 6m version, and he was sorely disappointed. He ended up buying a miniVNA instead.

Asking your fellow hams about the antenna analyzers they have is actually a good way to figure out what’s best for you. If you ask nicely, they might even let you borrow their analyzers or come over and show you how it works on your antennas.

Reading the reviews on eHam is also a good way to gather information before making a purchase like this. You certainly have to take the reviews there with a grain of salt, but if several reviewers mention a particularly good or particularly bad feature of a product, then it’s certainly something worth taking a hard look at.

If you’re new to the hobby, starting out small and working your way up might be a good strategy. You could buy one of the less expensive models and get used to how they work,  then sell it and make the leap to a more sophisticated unit. The way things are going, you should be able to sell your first antenna analyzer for at least 80% of what you paid for it.

Whatever you do, don’t fall victim to “paralysis by analysis.” Go ahead and buy one and start using it. This is a learn by doing hobby after all.

DMM tips, anyone?

Now that I’ve published the No-Nonsense Extra Class License Study Guide, I’ve been thinking about what my next book should be. At this point I’m leaning towards The No-Nonsense Guide to Digital Multimeters. I have even started outlining the book:

  • What is a digital multimeter?
    • Compare to analog multimeter
  • Digital multimeter basics
    • Measurement types
    • Inputs
    • Range
    • Zero
    • Safety
  • Making measurements with a DMM
    • Simple DC circuit
      • voltage
      • current
    • AC measurements
      • true RMS
    • Resistance
  • Tips for Choosing a DMM
  • Hints and Kinks

I’d love to get your feedback on this.

What else should I add to this outline?

What would you like to know about DMMs?

Do you have any tips for using DMMs that you’d like to share with others? (If I use your tip in the book, I’ll send you a free copy when it’s finished.)

Extra Class question of the day: Frequency counters and markers

To measure the frequency of a signal, you use an instrument called a frequency counter. The purpose of a frequency counter is to provide a digital representation of the frequency of a signal.(E7F09) A frequency counter counts the number of input pulses occurring within a specific period of time. (E7F08)

To accurately measure high-frequency signals digitally, you need a highly stable and accurate frequency source, called the time base. The time base provides an accurate and repeatable time period, over which you count the number of pulses of the test signal. The accuracy of the time base determines the accuracy of a frequency counter. (E7F07)

An alternate method of determining frequency, other than by directly counting input pulses, that is used by some counters is period measurement plus mathematical computation. (E7F10) An advantage of a period-measuring frequency counter over a direct-count type is that it provides improved resolution of low-frequency signals within a comparable time period. (E7F11)

You also need an accurate and stable time base to generate and receive microwave signals. All of these choices are correct when talking about techniques for providing high stability oscillators needed for microwave transmission and reception: (E7F05)

  • Use a GPS signal reference
  • Use a rubidium stabilized reference oscillator
  • Use a temperature-controlled high Q dielectric resonator

If you want to measure a signal whose frequency is higher than the maximum frequency of your counter, you might use a prescaler. The purpose of a prescaler circuit is to divide a higher frequency signal so a low-frequency counter can display the input frequency. (E7F01) A prescaler would, for example, be used to reduce a signal’s frequency by a factor of ten. (E7F02)

You might use a decade counter digital IC in a prescaler circuit. The function of a decade counter digital IC is to produce one output pulse for every ten input pulses. (E7F03)

In some cases, you might use a flip-flop. Two flip-flops must be added to a 100-kHz crystal-controlled marker generator so as to provide markers at 50 and 25 kHz. (E7F04) The purpose of a marker generator is to provide a means of calibrating a receiver’s frequency settings. (E7F06) You mostly find marker generators in older, analog receivers.

On the Internet: W2AEW videos, Raspberry Pi programming, classic radio

Here are a couple of Internet resources to start off the week:

W2AEW on YouTube. Alan, W2AEW, has a great selection of cool videos on YouTube. Some of the latest cover the basics of phase-locked loops, how to zero-beat WWV to check out a frequency counter’s accuracy, and a tutorial on resonant circuits. Good stuff!

Baking Pi – Operating Systems Development. This course, published by the University of Cambridge Computer laboratory, is a free online course that takes you through the basics of operating system development. The Web page notes, “[This course]  is aimed at people aged 16 and upwards, although younger readers may still find some of it accessible, particularly with assistance….I have tried not to assume any prior knowledge of operating systems development or assembly code. It may be helpful to have some programming experience, but the course should be accessible without.”

Classic Exchange. Mac, WQ8U, wrote to the Glowbugs mailing list, “The Classic Exchange (CX) is a low-key, on-air celebration of rigs of days gone by – particularly boat anchors. The latest CX Newsletter is available on the CX web site, as well as details for the next CX on September 16th (for AM and SSB) and September 23nd (for CW). Please enjoy the newsletter, spread the word and join in the fun during the next CX.”

Extra Class question of the day: Measurement techniques: Instrument accuracy and performance limitations; probes; techniques to minimize errors; measurement of Q; instrument calibration

One thing about test instruments is that you need to take the readings with a grain of salt. By that, I mean that chances are that the instrument reading is not exactly the value of the parameter you’re measuring. The reason for this is that no instrument is 100% accurate.

Let’s consider frequency counters. Frequency counters are useful instruments for measuring the output frequency of amateur radio transceivers. While a number of different factors can affect the accuracy of an instrument, time base accuracy is the factor that most affects the accuracy of a frequency counter. (E4B01) The time base accuracy of most inexpensive frequency counters is about 1 part per million, or 1 ppm.

Now, let’s see how that affects the accuracy of a frequency measurement. If a frequency counter with a specified accuracy of +/- 1.0 ppm reads 146,520,000 Hz, 146.52 Hz is the most the actual frequency being measured could differ from the reading. (E4B03) Practically, what this means is that while the frequency counter reads 146,520,000 Hz, or 146.52 MHz, the actual frequency of the signal might be as low as 146.519853 Mhz or as high as 146.520147 MHz.

More accurate—and therefore more expensive—frequency counters might have a specified accuracy of .1 ppm. If a frequency counter with a specified accuracy of +/- 0.1 ppm reads 146,520,000 Hz, 14.652 Hz is the most the actual frequency being measured could differ from the reading. (E4B04) This is very accurate for amateur radio work.

Very inexpensive frequency counters might have an accuracy of only 10 ppm. If a frequency counter with a specified accuracy of +/- 10 ppm reads 146,520,000 Hz, 1465.20 Hz is the most the actual frequency being measured could differ from the reading. (E4B05) This might be adequate for amateur radio work, but as you can see, the difference between the frequency counter’s reading and the signal’s actual frequency can be up to ten times as much as with the frequency counter with a 1 ppm accuracy.

In the previous section, we talked about using oscilloscopes to make measurements. One of the factors that affects the accuracy of oscilloscope measurements is the probe being used. You not only have to use a good probe, but you have to know how to use it properly.

When making measurements at RF frequencies, it’s important to connect the probe’s ground connection as close to the location of the measurement as possible. Keeping the signal ground connection of the probe as short as possible is good practice when using an oscilloscope probe. (E4B07) Keeping this connection as short as possible reduces the inductance of the connection, which in turn, makes the measurement more accurate.

Good quality passive oscilloscope probes have an adjustable capacitor in them that needs to be adjusted so that  the probe capacitive reactance is at least nine times  the scope input capacitive reactance. When this capacitor is adjusted properly, we say that the probe is properly compensated, and the scope will display the waveform with as little distortion as possible.

How is the compensation of an oscilloscope probe typically adjusted? A square wave is displayed and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible. (E4B13) High-quality oscilloscopes will have a special square-wave output specifically for the purpose of compensating probes.

Probably the most common test instrument in an amateur radio station is a voltmeter. The voltmeter may be part of a digital multimeter (DMM) or volt-ohm meter (VOM). DMMs have the advantage of high input impedance, and high impedance input is a characteristic of a good DC voltmeter. (E4B08) The higher the input impedance, the less effect the meter will have on the measurement.

The quality of a VOM is given by the VOM’s sensitivity expressed in ohms per volt. The full scale reading of the voltmeter multiplied by its ohms per volt rating will provide the input impedance of the voltmeter. (E4B12) A higher ohms per volt rating means that it will have a higher input impedance than a meter with a lower ohms per volt rating.

Directional power meters and RF ammeters are two instruments that you can use to make antenna measurements. With a directional power meter, you could measure the forward power and reflected power and then figure out how much power is being delivered to the load and calculate the SWR of the antenna system. For example, 75 watts is the power is being absorbed by the load when a directional power meter connected between a transmitter and a terminating load reads 100 watts forward power and 25 watts reflected power? (E4B06)

With an RF ammeter, you measure the RF current flowing in the antenna system. If the current reading on an RF ammeter placed in series with the antenna feed line of a transmitter increases as the transmitter is tuned to resonance it means there is more power going into the antenna. (E4B09)

There are a number of instruments that you can use to measure the impedance of a circuit. An antenna analyzer is one. Some sort of bridge circuit is another. An advantage of using a bridge circuit to measure impedance is that the measurement is based on obtaining a signal null, which can be done very precisely. (E4B02) 

That’s the principle behind the dip meter. You adjust the meter’s controls so that the reading “dips” to a minimum value. The controls then indicate the resonant frequency. When using a dip meter, don’t couple it too tightly to the circuit under test. A less accurate reading results if a dip meter is too tightly coupled to a tuned circuit being checked. (E4B14)

For some experiments, you’ll want to know not only the resonant frequency of a circuit but also the quality factor, or Q, of the circuit. The bandwidth of the circuit’s frequency response can be used as a relative measurement of the Q for a series-tuned circuit. (E4B15)

Finally, a method to measure intermodulation distortion in an SSB transmitter is to modulate the transmitter with two non-harmonically related audio frequencies and observe the RF output with a spectrum analyzer. (E4B10) The instrument we use to do this is called, oddly enough, a two-tone generator. Typically, these generators provide tones of 700 Hz and 1,900 Hz simultaneously.

Handheld instrument analyzes antennas and cables, measures power

I came across this product announcement in the daily e-mail I get from EE Times. Wouldn’t it be great to have this analyzer? There is one drawback. It costs almost $7k. 

Site Master Handheld Cable & Antenna Analyzer S331L

 Features
  • 2 MHz to 4 GHz Handheld Cable and Antenna Analyzer, impact, dust, and splash resistant
  • Measures power from 50 MHz to 4 GHz
  • More than 8 hours of continuous battery operation
  • Standard built-in InstaCal™ module and Power Meter
  • FlexCal™ maintains calibration with frequency changes
  • Built-in one button Help function
  • 800×480 7” TFT touch screen display and multiple USB ports
  • Internally store >1000 files with fast preview of stored traces
  • Industry standard *.dat format compatible with Line Sweep Tools (LST) and HHST

You can download the product brochure to get an even better idea of what a cool meter this is.

21 Things to Do: Buy a DMM

21 Things to Do After Getting Your Amateur Radio LicenseA digital multimeter, or DMM for short, is the most basic piece of test equipment you can own, and every ham should have one. With a digital multimeter (DMM), you can make voltage, current, and resistance measurements. Some multimeters do even more, but that’s a topic for another book.

Why do you need a multimeter? Well, the multimeter is the first thing you’ll reach for when you have problems with your equipment. For example, let’s say you go down to your shack, switch on your radio, and nothing. It doesn’t turn on. The first thing you should check in this case is that the power supply is supplying the proper input voltage. To do this, you pull out your DMM, set it to measure voltage, place the probes on the + and – outputs, and verify that the power supply is working.

Klein DMM

This multimeter costs about $50 and features a rugged case that helps prevent damage should you accidentally drop it.

OK, now we’re sure that the power supply is working OK, but the radio still doesn’t power up. The next thing to check is the power cable from the supply to the radio. It’s possible that the cable has an open connection. To check that, you first disconnect the cable from the power supply and from the radio.

This multimeter costs about $50 and features a rugged case that helps prevent damage should you accidentally drop it.Then, set your DMM to measure resistance. Set it on the lowest resistance scale. Connect one test probe to one end of the cable and the other test probe to the other end. The resistance you measure should be very low—less than 2 or 3 ohms. An open connection will register an infinite resistance.

I think you get the picture. Without a DMM, you’re dead in the water. With a DMM, you can figure out what’s wrong and fix it.

There are a wide range of DMMs available. On the low end, you’ll find DMMs at Harbor Freight for $5 or less. On the high end, you could spend $300 or more for a Fluke multimeter. I would advise against both. The $5 multimeters are not very well-made and can be inaccurate. They tend to quit working just when you need them.

The $300 DMMs are great, but you needn’t spend that much. A DMM costing between $30 and $100 will do pretty much all you need to do at this point in your amateur radio adventure, and you can use the money you have left over for other things. You can buy them at any Lowe’s or Home Depot. Ask your friends or Elmer what kind of meter they own and whether or not they would recommend that you buy something similar.

New QRP kit measures power, SWR

From Terry, WA0ITP, via the qrp-l mailing list:

QRP-o-meterThe Four State QRP Group is pleased to announce a new kit, the QRPometer, a sensitive and accurate power/swr meter designed by David Cripe, NMØS.   Complete specifications, assembly manual, and ordering information can be found online PayPal is accepted.

The range of accurate power measurement extends down to a low 100 milliwatts.  This kit was conceived to fill a need within the hobby for an inexpensive, highly accurate RF power and VSWR meter for QRP power levels.  With it’s large digital display it makes a very useful addition to your shack.

The QRPometer uses simple analog signal-processing circuitry to provide a set of essential measurement features not previously available in a single unit. High quality, double sided, printed circuit board construction is used, with solder mask and silk screened component reference designators.

All components are  through-hole for easy assembly. NO toroids are required, and all controls  and jacks are PCB mounted. The QRPometer can be constructed by beginners as well as experienced builders. Construction time is approximately 3 hours, depending on experience level. The only equipment required for calibration is a digital voltmeter, and a QRP transmitter..

All proceeds  go to fund OzarkCon.  As always, thank you for supporting the Four State QRP Group.