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.

Extra Class question of the day: spectrum analyzers and oscilloscopes

Two instruments that amateur radio operators frequently use when experimenting or when debugging equipment are the oscilloscope and the spectrum analyzer. How does a spectrum analyzer differ from an oscilloscope? A spectrum analyzer displays signals in the frequency domain; an oscilloscope displays signals in the time domain. (E4A01) What this means is that an oscilloscope will show you have the amplitude of a signal changes with time, while a spectrum analyzer shows you how the amplitude of a signal changes with frequency. The drawing below shows typical displays from an oscilloscope and a spectrum analyzer.

Spectrum analyzers and oscilloscopes

The oscilloscope shows how a signal's amplitude changes with time, while a spectrum analyzer shows how a signal's amplitude changes with frequency.

Because the spectrum analyzer shows how the amplitude of a signal changes with frequency, amplitude is the parameter a spectrum analyzer would display on the vertical axis. (E4A03) Frequency is the parameter a spectrum analyzer would display on the horizontal axis. (E4A02)

Spectrum analyzers are very useful for troubleshooting problems. For example, a spectrum analyzer is used to display spurious signals from a radio transmitter. (E4A04) A spectrum analyzer is also used to display intermodulation distortion products in an SSB transmission. (E4A05) The reason for this is that in both of these cases we are looking for signals that are being erroneously generated.

Whenever frequency is an important part of the measurement, you want to use a spectrum analyzer, if one is available. All of these choices are correct when talking about parameters than can be determined with a spectrum analyzer (E4A06):

  • The degree of isolation between the input and output ports of a 2 meter duplexer
  • Whether a crystal is operating on its fundamental or overtone frequency
  • The spectral output of a transmitter

Because spectrum analyzers are sensitive instruments, you need to be cautious when using them. For example, an important precaution to follow when connecting a spectrum analyzer to a transmitter output is to attenuate the transmitter output going to the spectrum analyzer. (E4A12) Not doing so could damage the spectrum analyzer because its input circuits are not designed to handle high power.

Despite all this talk about spectrum analyzers, the oscilloscope is actually the more versatile instrument,  and will be more useful than the spectrum analyzer for most radio amateurs. For example, the oscilloscope is the instrument used for detailed analysis of digital signals. (E4A11) You can make a number of digital-signal measurements with a scope, including rise time and fall time, as well as analyze how two or more digital signals change in time with regard to one another.

What’s your favorite test instrument?

Simpson Model 60 VOM

The venerable Simpson Model 60 VOM has been in production for more than 60 years.

In the Test&Measurement World group on LinkedIn, editor Martin Rowe asks, “What’s your favorite test instrument”? He gets some interesting answers, but the two top vote-getters are the Bird Model 43 Wattmeter and the Simpson Electric Model 60 VOM (see right).

About the Bird wattmeter, one engineer said, “That’s easy – The Bird Model 43 Wattmeter. No other instrument has been in production, in its original design, for so long. Since 1952, which makes next year 2012 its 60th anniversary. Also, it must be the simplest, most rugged instrument ever produced. We regularly see Model 43 Wattmeters returned to us for calibration that are 30-40 years old, still working and, when they leave our facility, as accurate as the day they first came of the production line in Solon, Ohio.”

About the Simpson VOM, another replied, “My favorite test instrument of all time is the Simpson 260. I used it back in the 1970s while serving in the US Navy. It is rugged, easy to use, and does the job!”

Those are really two classics, and either is a great choice. But, what do  you think? What’s your favorite test instrument?

From the Twittersphere

The Twittersphere is kind of like the ionosphere. It helps you make contact with other hams and brings you news from far and wide. Here are a few interesting links that I found on Twitter in the last day or so:

Global Pirate HF Weekend 14-15.1.2012.  This station lists pirate SW radio stations that it expects to be on the air this weekend. They include one using the callsign WEMP. Look for it between 15.005 – 15.095 MHz. They’ll be broadcasting with 100 W to Europe: 12.00 – 16.00 UTC – (check 15.010 or 15.040 or 15.090 MHz).

The Evil Mad Scientist Laboratories Zener Diode Tutorial. Confused about zener diodes and how they work? Read this.

Monitor your Ham Radio transmitter with an oscilloscope. In this video, Alan, W2AEW builds a little adapter that lets you connect your transmitter output to a scope input so that you can see how clean its output is.

Hack a Day: Use an Arduino to Measure Inductance

It’s been over a year since I built an Arduino microcontroller at our club’s annual construction night, and I still haven’t done anything with it. I’ve had a couple of ideas, including a keyer that would actuate accept paddle inputs and actuate a solenoid that would open and close a straight key, but just haven’t had the time or inclination to actually put something together.

Well, here’s a cool idea as to how to use an Arduino to measure inductance.

Measure inductance with an ArduinoA signal in from the Arduino excites an LC circuit. The L in the LC circuit is our unknown inductor. The output of the LM339 is a square wave whose frequency is proportional to the L of the LC circuit. Measure that frequency with the Arduino and you know the unknown inductance. Of course, calibrating this thing could be a bit tricky, but it might be fun to play with.

Device Tests PowerPole Outlets

PowerPole Voltage and Polarity CheckerI just found a link to this unique little device and had to blog about it right away.  The PowerPole Polarity and Voltage Tester is similar to one of those little devices that you plug into an AC outlet that tells you if the outlet is wired correctly. Except that you plug this device into an unknown PowerPole outlet.

What  surprised me is that this little device actually uses a microcontroller to measure the voltage. I guess that I really shouldn’t be surprised, though. Microcontrollers come in very small packages now, and programming them is very simple to do .

Because microcontroller are programmable, you could extend the functionality of this device. For example, you could add a small alpha-numeric display or more LEDs to indicate different voltage ranges.

While the circuit is very simple, and is easily fabricated on some perfboard, the author of this Instructables project, has indicated that he intends to make a PC board and kit available. I’ve e-mailed him about this, and will report on what he has to say about price and availability.

Another thought that occurs to me is that I could make one of these with the Arduino that’s been languishing beneath my workbench for the last year or so. Basically, I’d just take the front end circuitry from this project and connect it up to my Arduino. I’d have to do some programming, but I think I can handle that. :)


Multimeter Links to iPhone or iPad

Redfish iDVMHere’s an interesting new product—the iDVM Multimeter—a product the company claims is the first digital phone-enabled multimeter. As you can see, the instrument itself has no display or controls. All display and control is done through an iPhone or iPad app.

The connection to the iPhone or iPad is a Bluetooth link. The app allows users to acquire, visualize, and share electrical measurement data on their Apple devices. Data can also be exported and e-mailed for further analysis.

The kicker is that the instrument won’t ship until June, and it will cost $220! A Fluke 177 multimeter costs only $240, and undoubtedly has much better analog specs. (The “Technical Specs” page on the Redfish website is currently not available.) It does do some things that the Fluke doesn’t do, such as speak the measurements and use a rechargeable battery, but I’m not sure those features are worth the price.

I like the iDVM concept a lot, but I’m afraid that they’re going to have to reduce the cost of this thing by at least 50% to be successful.

Is Now the Time to Buy a Tek Scope?

Having your own scope is a real pleasure. I’ve had one myself for the last 20 years or more. When you really need to look at a signal, you just hook up the scope and you can see the waveform. It’s beautiful.

There are many places that you can be a used oscilloscope for a reasonable price. Ham radio operators have been getting by with industry castoffs for years. Now, however, it might be time to think about buying your first, new oscilloscope. Tektronix is holding what they’re calling their 2011 Scope Sales Event. The sale runs through July 1, 2011.

New scopes, like this TDS2000C, are amazingly versatile when compared to scopes that amateurs typically use.

You can save 10% on TDS2000C series scopes (see right), 25% on MSO/DPO2000 Series scopes, and 10% on the MSO/DPO3000 Series scopes. For example, you can get a 200-MHz TDS2022C with two analog inputs for only $1,854. A two-input 100-MHz TD2012C is only $1,308.

Not only are these scopes (relatively) inexpensive, they’re much easier to use and fully-featured than older models. For example, the TDS2000C series uses an LCD display, meaning the scope is more like a small, flat-panel TV, and it takes up a lot less bench space than the older scopes. They also have a whole range of analysis features that the older scopes don’t have.

A scope is one of those things that you never knew you needed until you have one. And, once you have one, you wonder how you ever did without it. Now, with the cost of a new scope about the same price as a mid-level transceiver, perhaps it’s time that you got one of your own.

NIST Conducting Time and Frequency Survey

NISTDo you use NIST radio station WWV or WWVH? Do you have a radio-controlled clock or set your computer clock using NIST? Do you get NIST time via telephone or Internet?

Please take a moment to complete the NIST Time and Frequency Services survey. Your input will be greatly appreciated.

By the way, you might also want to visit the NIST Time and Frequency Division website. It has a bunch of interesting info for time and frequency geeks.

For example, currently there’s a piece on the world’s most precise clock. It says:

NIST scientists have built a second “quantum logic clock,” using quantum information processing techniques on a single ion of aluminum to make a clock that would not gain or lose more than one second in about 3.7 billion years.