Frequency Counting A Different Way

September 12, 2019 by 9 Comments

Counting frequency is one of those tasks that seems simple on the face of it, but actually has quite a bit of nuance. There are two obvious methods, of which the first is to count zero crossings for some period. If that period is one second you are done, otherwise it’s a simple enough case of doing the math. That is, if you count for half a second, multiply the result by 2, or if you count for 10 seconds, divide by 10. The other obvious method is to measure the period of a single cycle as accurately as you can. Then there’s this third method.from [WilkoL], which simultaneously counts a known reference clock alongside the frequency to be measured.  You can see the result in the video, below.

The first method is easy but the lower the frequency you want to measure, the longer you have to count to get any real resolution. Also, you need the time base to be exact. For the second method, you need to be able to make a highly precise measurement. The reason [WikolL] chose the third method is that it doesn’t require a very precise time base — a moderately accurate reference oscillator will do. The instrument gets good resolution quickly at both high and low frequencies.  Continue reading “Frequency Counting A Different Way” →

Confessions Of A Reformed Frequency Standard Nut

January 17, 2018 by 57 Comments

Do you remember your first instrument, the first device you used to measure something? Perhaps it was a ruler at primary school, and you were taught to see distance in terms of centimetres or inches. Before too long you learned that these units are only useful for the roughest of jobs, and graduated to millimetres, or sixteenths of an inch. Eventually as you grew older you would have been introduced to the Vernier caliper and the micrometer screw gauge, and suddenly fractions of a millimetre, or thousandths of an inch became your currency.  There is a seduction to measurement, something that draws you in until it becomes an obsession.

Every field has its obsessives, and maybe there are bakers seeking the perfect cup of flour somewhere out there, but those in our community will probably focus on quantities like time and frequency. You will know them by their benches surrounded by frequency standards and atomic clocks, and their constant talk of parts per billion, and of calibration. I can speak with authority on this matter, for I used to be one of them in a small way; I am a reformed frequency standard nut. Continue reading “Confessions Of A Reformed Frequency Standard Nut” →

Using A Lecher Line To Measure High Frequency

February 7, 2017 by 35 Comments

How do you test the oscillator circuit you just made that runs between 200MHz and 380MHz if all you have is a 100MHz oscilloscope, a few multimeters and a DC power supply? One answer is to put away the oscilloscope and use the rest along with a length of wire instead. Form the wire into a Lecher line.

That’s just what I did when I wanted to test my oscillator circuit based around the Mini-Circuits POS-400+ voltage controlled oscillator chip (PDF). I wasn’t going for precision, just verification that the chip works and that my circuit can adjust the frequency. And as you’ll see below, I got a fairly linear graph relating the control voltages to different frequencies.

What follows is a bit about Lecher lines, how I did it, and the results.

Continue reading “Using A Lecher Line To Measure High Frequency” →

Characterizing A Cheap 500MHz Counter Module

November 16, 2016 by 22 Comments

An exciting development over the last few years has been the arrival of extremely cheap instrumentation modules easily bought online and usually shipped from China. Some of them have extremely impressive paper specifications for their price, and it was one of these that caught the eye of [Carol Milazzo, KP4MD]. A frequency counter for under $14 on your favourite online retailer, and with a claimed range of 500 MHz. That could be a useful instrument in its own right, and with a range that significantly exceeds the capabilities of much more expensive bench test equipment from not so long ago.

Just how good is it though, does it live up to the promise? [Carol] presents the measurements she took from the device, so you can see for yourselves. She took look at sensitivity, VSWR, and input impedance over a wide range, after first checking its calibration against a GPS-disciplined standard and making a fine adjustment with its on-board trimmer.

In sensitivity terms it’s a bit deaf, requiring 0.11 Vrms for a lock at 10 MHz. Meanwhile its input impedance decreases from 600 ohms at the bottom of its range to 80 ohms at 200 MHz, with a corresponding shift in VSWR. So it’s never going to match a high-end bench instrument from which you’d expect much more sensitivity and a more stable impedance, but for the price we’re sure that’s something you can all work around. Meanwhile it’s worth noting from the pictures she’s posted that the board has unpopulated space for an SPI interface header, which leaves the potential for it to be used as a logging instrument.

We think it’s worth having as much information as possible about components like this one, both in terms of knowing about new entrants to the market and in knowing their true performance. So if you were curious about those cheap frequency counter modules, now thanks to [Carol] you have some idea of what they can do.

While it’s convenient to buy a counter module like this one, of course there is nothing to stop you building your own. We’ve featured many over the years, this 100MHz one using a 74-series prescaler or this ATtiny offering for example, or how about this very accomplished one with an Android UI?

The Three Dollar Frequency Standard

July 26, 2015 by 19 Comments

[Paulie] over on the EEVBlog forums picked up an inexpensive frequency counter on eBay and realized it was just a little bit off. As a result, he decided to build a frequency standard. His build wound up costing him about $3 and he shared the design and the software for it.

The hardware design is very simple: a TCXO (also from eBay), an ATMega8, a pushbutton, and a AA battery with DC to DC converter to power the whole thing. The software does all the work, providing frequencies from 10MHz down to a few hundred hertz (including some common audio test frequencies).

If you haven’t worked with a TCXO before, it is a crystal oscillator that includes a temperature compensation circuit to pull the crystal frequency up or down depending on temperature. Although crystal oscillators are pretty accurate already, adding this temperature compensation improves accuracy over the design temperature dramatically (typically, 10 to 40 times better than a naked crystal oscillator). If you want to learn more about TCXOs, here’s a good write-up.
A TCXO isn’t as good as an OCXO (where the first O stands for Oven). However, OCXOs cost more, are larger, and drain batteries (after all, it is running an oven). You can even hack your own OCXO, but it is going to cost more than $3.

If you want to see the real guts of one TCXO, check out the video.

Continue reading “The Three Dollar Frequency Standard” →

Hacking A Telecoms Frequency Standard For Your Lab

July 23, 2015 by 18 Comments

[Shane Burrell] came across a Nortel GPSTM and re-purposed it as a 10MHz reference for his lab. The GPSTM is designed to slot into a backplane, most likely for telecoms applications. So [Shane] needed to hack the board to run from a 48v PSU. Once powered up, it was relatively easy to interface as the card appears to contain the well known Trimble Thunderbolt module and is compatible with its software.

We’ve covered frequency references before and they can be a valuable addition to a lab. On the back of most scopes, spectrum analyzers and function generators you’ll find a 10MHz reference input allowing the user to supply a reference more accurate than that generated internally. Not only is an external reference often more accurate, it also allows you to keep all your equipment in sync with a common reference, which can be particularly important in some measurements. While some hackers opt for Rubidium sources, the GPS disciplined temperature-controlled oscillator in the Nortel unit should provide a nice stable reference.

A word of warning to [Shane] though, get sucked into hacking frequency references and you may become a time nut finding yourself climbing mountains to test the theory of relativity.

Continue reading “Hacking A Telecoms Frequency Standard For Your Lab” →

Counterfeit Apple Charger

More Counterfeit Apple Chargers Than You Can Shake An IPod At

May 10, 2014 by 68 Comments

Phones, MP3 players, designer bags, artwork, money…. anything with value will bring out the counterfeiters looking to make a quick buck. Sometimes the product being counterfeited isn’t even necessarily expensive. For example, an Apple iPad Charger. [Ken Shirriff] got a hold of a counterfeit iPad Charger, took it apart, and did some testing.

So why would someone buy a counterfeit product? To save some money! The counterfeits are usually cheaper to reel the potential buyer in thinking they are getting a deal. In this case, the Apple product costs $19 and the knock-off is $3, that’s a huge difference.

Continue reading “More Counterfeit Apple Chargers Than You Can Shake An IPod At” →