If you search through an electrical engineering textbook, you probably aren’t going to find the phrase “gimmick capacitor” but every old ham radio operator knows about them. They come in handy when you need a very small capacitor of unknown value. For example, if you are trying to balance the stray capacitance in a circuit, you might not know exactly what value you need, but you know it won’t be very much. That’s when you want a gimmick capacitor.
A gimmick capacitor is made by taking two strands of insulated wire and twisting them together; the length and the tightness of the twist determine the capacitance. Tightening or loosening the twist, or trimming some of the wire off, makes it tunable.
These are most commonly found in RF equipment or high-speed logic because of the small capacitance involved — usually about 1 to 2 pF per inch of twist or so. The thicker the insulation, the less capacitance you’ll get, so it is common to use magnet wire or something else with a thin insulating layer. You can take this one step further and decrease the spacing by stripping down one wire as long as it isn’t going to touch anything else.
Obviously, the insulation needs to be good enough for the voltage on them, an important consideration in tube circuits, for instance. But other than that, a gimmick capacitor is a straightforward tool to have in your box of design tricks. Can we take this further?
PC Board Gimmicks
You might wonder if the technique can be applied to PC boards. The answer is yes — sort of. Unless you use very thin boards, or thin layers in multilayer boards, it takes a lot of board real estate to get even a small capacitance. Also, typical PCB material can change over time with moisture or other effects. Practically, unless you use special board material and thicknesses, it isn’t very useful. There has been work on laying out linear capacitors on IC substrates using fractals, but we aren’t sure how that would translate into a PCB layout. We’ve seen lots of other PC trace components like antennas, shunt resistors, inductors, and transmission lines.
You can see I made a gimmick just bigger than two inches. I then went looking for something around the lab that had the ability to measure such a small capacitor. The component tester couldn’t. I have a nice digital multimeter that has a special plug-in for measuring capacitors and thermocouples, but it wouldn’t reliably read anything under 25 pF. I was thinking about building up a circuit to test when I realized I should search Hackaday first.
Hackaday Saves the Day
[Jonathan’s] capacitance meter is just what I needed and I even threw it out to an Arduino that was already hooked up using the Arduino Create web interface, so that was easy. I actually used the newer “Mark II” code but it works the same for the low values I was measuring. I calibrated with a garden variety 10 pF ceramic. It probably isn’t that accurate, but I really only wanted to see the change more than the actual value, so I thought this was sufficient.
The two inch (call it 6 cm) gimmick reads about 5.5 pF. That might not be totally accurate, but I was expecting about 4.5 pF and the magnet wire insulation is quite thin, so it’s in the right ballpark. Let’s take it as a baseline to measure change. I then cut about 1.5 cm of the capacitor away — about 25% — and the reading became 3.7 pF. Another centimeter brought it down to 2.6 pF.
Of course, hand-wound pitch isn’t very accurate, nor were my cuts or measurements, but that works out to just around 1 pF per centimeter. Obviously, your results are going to depend on your winding and the kind of wire you use. [Harry Lythall] suggests folding a single piece of wire, holding it with pliers, and twisting. Then you cut the loop when you are done.
That’s a Wrap
It is easy to forget that any two conductors near each other will have capacitance. Another common makeshift capacitor is a length of coax with connections at one end and open at the other. RG-8, for example, is about 30 pF per foot of cable. There’s even an online calculator that will tell you how much coax you need for any given value. This varies by coax type, of course, so remember to cut a little long and trim!
The next time you need a small adjustable capacitor — especially in a lab setting — don’t forget about the gimmick. Be sure to experiment with different kinds of wire if you are trying for larger values. We’ve seen this trick used in RF filters. In the case of the gimmick, you may be thinking small, but when you are really looking for high voltage capacitors, you can make those, too.
We really like when a vendor finds a great book on a topic — probably one they care about — and makes it available for free. Analog Devices does this regularly and one you should probably have a look at is Software Defined Radio for Engineers. The book goes for $100 or so on Amazon, and while a digital copy has pluses and minuses, it is hard to beat the $0 price.
The book by [Travis F. Collins], [Robin Getz], [Di Pu], and [Alexander M. Wyglinski] covers a range of topics in 11 chapters. There’s also a website with more information including video lectures and projects forthcoming that appear to use the Pluto SDR. We have a Pluto and have been meaning to write more about it including the hack to make it think it has a better RF chip inside. The hack may not result in meeting all the device specs, but it does work to increase the frequency range and bandwidth. However, the book isn’t tied to a specific piece of hardware.
Make no mistake, the book is a college-level textbook for engineers, so it isn’t going to go easy on the math. So if the equation below bugs you, this might not be the book you start with:
[Di Pu] and [Alexander Wyglinksi] have an older similar book, and it looks like the lecture videos are based on that book (see video below). The projects section on the website doesn’t appear to have any actual projects in it yet, although there are a couple of placeholders.
We’re sure all radio amateurs must have encountered the problem faced by [Alexandre Grimberg PY1AHD] frequently enough that they nod their heads sagely. There you are, relaxing in the sun on the lounger next to the crystal-blue pool, and you fancy working a bit of DX. But the sheer horror of it all, a tower, rotator, and HF Yagi would ruin the aesthetic, so what can be done?
[Alexandre]’s solution is simple and elegant: conceal a circular magnetic loop antenna beneath the rim of a circular plastic poolside table. Construction is the usual copper pipe with a co-axial coupling loop and a large air-gapped variable capacitor, and tuning comes via a long plastic rod that emerges as a discreet knob on the opposite side of the table. It has a 10 MHz to 30 MHz bandwidth, and should provide a decent antenna for such a small space. We can’t help some concern about how easy to access that capacitor is, on these antennas there is induced a surprisingly large RF voltage across its vanes, and anyone unwary enough to sit at the table to enjoy a poolside drink might suffer a nasty RF burn to the knee. Perhaps we’d go for a remotely tuned model instead, for this reason.
It used to be homebrew ham gear meant something simple. A couple of active devices that could send CW. Maybe a receiver with a VFO. But only the most advanced builders could tackle a wide range SSB transceiver. Today, that goal is still not trivial, but it is way easier due to specialty ICs, ready access to high-speed digital signal processing, and advances in software-defined radio techniques. [Charlie Morris] decided to build an SSB rig that incorporated these technologies and he shared the whole process from design to operation in a series of nine videos. You can see the first one below.
The NE612 is a child of the popular NE602 chip, which contains a Gilbert-cell mixer, and an oscillator that makes building a receiver much easier than it has been in the past. The chips are set up as direct conversion receivers and feed a Teensy which does the digital signal processing on the recovered audio.
One nice thing about the Teensy is that it has an accessory audio board that makes it easy to connect audio inputs and outputs to the device. The DSP does work on the received audio and the transmit audio. There’s also a few other stock parts like an LCD, an encoder, a speaker, a microphone, and things like that. There’s also a digital clock generator (an Si5351), but again all that is common off-the-shelf stuff these days.
The first video is a bit introductory, but by video number two he jumps right into the wiring and why all the circuits work. By the third video, the receiver is actually working and it sounds pretty good. Because the receiver needs I and Q outputs, there are actually two NE612s operating out of phase with each other.
Sometimes the best projects are the simple, quick hits. Easily designed, fast to build, and bonus points for working right the first time. Such projects very often lead to bigger and better things, which appears to be where this low-power temperature beacon is heading.
In the world of ham radio, beacon stations are transmitters that generally operate unattended from a known location, usually at limited power (QRP). Intended for use by other hams to determine propagation conditions, most beacons just transmit the operator’s call sign, sometimes at varying power levels. Any ham that can receive the signal will know there’s a propagation path between the beacon and the receiver, which helps in making contacts. The beacon that [Dave Richards (AA7EE)] built is not a ham beacon, at least not yet; operating at 13.56 MHz, it takes advantage of FCC Part 15 regulations regarding low-power transmissions rather than the Part 97 rules for amateur radio. The circuit is very simple — a one-transistor Colpitts oscillator with no power amplifier, and thus very limited range. But as an added twist, the oscillator is keyed by an ATtiny13 hooked to an LM335 temperature sensor, sending out the Celsius and Fahrenheit temperature in Morse every 30 seconds or so. The circuit is executed in Manhattan style, which looks great and leaves plenty of room for expansion. [Dave] mentions adding a power amp and a low-pass filter to get rid of harmonics and make it legal in the ham bands.
The R820T tuner IC is used in the popular Airspy software defined radio (SDR) as well as many of the inexpensive RTL SDR dongles. [TLeconte] did some experiments on intermediate frequency (IF) configuration of the chip, and you’ll find his results interesting.
Using 5 million samples per second and the device’s real mode, the tests look at a what comes out when the IC reads a noise source. There are two registers that set the IF parameters, but the tests show the effects these registers have in precise terms.
According to the post, there are three things you can set:
Coarse IF filter bandwidth : narrow/mid/large
Manual fine tuning IF filter bandwidth from 0 (large) to 15 (narrow)
High pass filter frequency from 0 (high) to 15 (low)
Some of the settings don’t make sense — at least at the 5 MHz sample rate — because of aliasing. However, it is instructive to see what each setting does. [TLeconte] uses Octave to visualize the data.
There are a multiplicity of transmission modes both new and old at the disposal of a radio amateur, but the leader of the pack is still single-sideband or SSB. An SSB transmitter emits the barest minimum of RF spectrum required to reconstitute an audio signal, only half of the mixer product between the audio and the RF carrier, and with the carrier removed. This makes SSB the most efficient of the analog voice modes, but at the expense of a complex piece of circuitry to generate it by analog means. Nevertheless, radio amateurs have produced some elegant designs for SSB transmitters, and this one for the 80m band from [VK3AJG] is a rather nice example even if it isn’t up-to-the-minute. What makes it rather special is that it relies on only one type of device, every one of its transistors is a BC547.
In design terms, it follows the lead set by other simple amateur transmitters, in that it has a 6 MHz crystal filter with a mixer at either end of it that switch roles on transmit or receive. It doesn’t use the bidirectional amplifiers popularised by VU2ESE’s Bitx design, instead, it selects transmit or receive using a set of diode switches. The power amplifier stretches the single-device ethos to the limit, by having multiple BC547s in parallel to deliver about half a watt.
While this transmitter specifies BC547s, it’s fair to say that many other devices could be substituted for this rather aged one. Radio amateurs have a tendency to stick with what they know and cling to obsolete devices, but within the appropriate specs a given bipolar transistor is very similar to any other bipolar transistor. Whatever device you use though, this design is simple enough that you don’t need to be a genius to build one.
It’s fair to say that software-defined radio represents the most significant advance in affordable radio equipment that we have seen over the last decade or so. Moving signal processing from purpose-built analogue hardware into the realm of software has opened up so many exciting possibilities in terms of what can be done both with more traditional modes of radio communication and with newer ones made possible only by the new technology.
It’s also fair to say that radio enthusiasts seeking a high-performance SDR would also have to be prepared with a hefty bank balance, as some of the components required to deliver software defined radios have been rather expensive. Thus the budget end of the market has been the preserve of radios using the limited baseband bandwidth of an existing analogue interface such as a computer sound card, or of happy accidents in driver hacking such as the discovery that the cheap and now-ubiquitous RTL2832 chipset digital TV receivers could function as an SDR receiver. Transmitting has been, and still is, more expensive.
A new generation of budget SDRs, as typified by today’s subject the LimeSDR Mini, have brought down the price of transmitting. This is the latest addition to the LimeSDR range of products, an SDR transceiver and FPGA development board in a USB stick format that uses the same Lime Microsystems LMS7002M at its heart as the existing LimeSDR USB, but with a lower specification. Chief among the changes are that there is only one receive and one transmit channel to the USB’s two each, the bandwidth of 30.72 MHz is halved, and the lower-end frequency range jumps from 100 kHz to 10 MHz. The most interesting lower figure associated with the Mini though is its price, with the early birds snapping it up for $99 — half that of its predecessor. (It’s now available on Kickstarter for $139.)
We were lucky enough to be sent a pre-production LimeSDR Mini for review by the MyriadRF folks — in fact we were sent two of them, after the first one proved to have a hardware fault suspected to involve a solder joint issue. We feel their pain, after all who hasn’t had pre-production boards springing faults at inconvenient moments!
The board itself is a PCB about 33 mm x 70 mm (1,25 ” x 2.75 “), with a USB 3 plug at one end and a pair of SMA sockets at the other, one for receive and the other for transmit. The integrated circuits are all on the top of the board, and though they have included footprints screening cans, they are not populated. There is a single multicolor status LED between the SMA sockets. It’s worth mentioning that there will be a laser-cut plastic case for the board, which is probably worth getting as it feels somewhat vulnerable as it is. Along with as the board, they supplied a pair of little rubber duck antennas for the 870 MHz band.
It is evident that the LimeSDR Mini is an extremely capable board that in the hands of a real expert in SDR and FPGA programming could have the potential for great things. It is also evident that as your Hackaday scribe I am not an SDR extreme power user. Despite holding an amateur radio licence for over three decades I have been a relative late comer to the world of SDRs, and have not progressed beyond RTL-SDRs or simple devices using a PC soundcard for baseband. But it’s probable that while many SDR programming experts will indeed buy this board, the majority of its customers will be similarly newcomers to the art. Therefore this review will be biased towards the SDR non-guru, the long-time radio enthusiast considering the LimeSDR Mini as a first transceiver.
The first task with any SDR will always be to install whatever software is required on the host machine. Here that means a copy of the latest Ubuntu distribution, but Windows and MacOS machines are also supported. There is a handy page of instructions, which in the case of Ubuntu require you to add a PPA repository for the drivers, then install the Lime Suite software and the SoapySDR abstraction layer. It is this final package that makes the LimeSDR an interesting prospect, by offloading software compatibility onto the widely used abstraction layer they hope to avoid some of the pain seen with other products.
Testing it Out
Once the drivers have been installed, it is time to decide which software to run first. The Lime Suite GUI supplied with the driver packages will be the first port of call to test the board, but I am told that the version in the PPA at the time of writing with the Mini not having been released is written with the LimeSDR USB in mind and therefore I should use GQRX. In the case of Ubuntu this can be installed through the graphical software installer, but as luck would have it I already had it on my machine. Selecting “other” as my SDR and pasting driver=lime,soapy=0 as my device string soon had the familiar interface in front of me, and with a suitable antenna in no time I was listening to my local BBC Radio 4 FM transmitter.
Two things are immediately apparent to an owner of an RTL-SDR, gone are the huge number of spurious peaks, and the noise floor is much lower. Reading GQRX with different front ends is an inexact and even slightly meaningless way to take measurements, but with all-automatic AGC, the RTL has a -60 dB noise floor and the LimeSDR has one below -90 dB. Just looking at the FM band, there are stations poking out of the noise that simply don’t exist with the RTL. It’s unsurprising that a piece of dedicated SDR hardware would outperform a $10 TV stick running a hack to make it an SDR, but if you are an RTL-SDR user then you will be pleasantly surprised by the Mini when you see it in action.
To have a board like the Mini and simply use it for GQRX is to waste so much of its potential. We are promised a library of tailor-made applications via Snappy Ubuntu Core, but this isn’t yet available pre-release. Your next stop would then probably follow ours with GNU Radio, and in particular its drag-and-drop GUI application GNU Radio Companion. This is nuts-and-bolts homebrew radio for the SDR age, just as analogue radio amateur homebrewers would solder their own radios while others bought shiny transceivers, so the SDR homebrewer can build their own devices using GNU Radio. It’s a package that’s beyond the scope of this review, but as an example when playing with the Mini it was fairly easy to cobble together a little GNU Radio receiver to pull in and extract the signals from a 433 MHz remote control transmitter we have in the house, and then regurgitate them through a 433MHz Baofeng antenna for the satisfying sight of a table lamp at the other end of the bench turning itself on. The Mini itself doesn’t intrude into this process beyond simply doing what it’s told once its communication with GNU Radio has been achieved, so experimenting further into the mechanics of decoding the bitstream itself became a matter of working through a set of tutorials and burning the midnight oil. The steep learning curve is amply offset by the satisfaction of playing with the instant gratification of radio building blocks without the pain of reworking any intricate soldering.
In transmission terms the maximum 100 mW output power is fairly modest for anyone used to amateur radio. But given that many applications for this board will involve the likes of sniffing for and responding to more local devices rather than seeking contacts from other continents this is something likely only to trouble radio amateurs without the wherewithal for a power amplifier. The twin antenna connectors will be somewhat annoying if you are used to a single one on a simplex transceiver, unless you are transmitting and receiving on different frequencies, of course.
A review of an SDR over a short period can not hope to cover all its many capabilities, so this one has been an impression of the Mini as a platform for experimentation and learning about how to use an SDR transceiver. But in just the time that it has been on the bench here, the Mini has opened a significant new vista over an RTL-SDR, and given a few months in which to play with GNU Radio will I am sure provide both some useful radio applications and a seriously interesting learning process.
Previous SDRs at the budget end of the market such as the HackRF have all remained somewhat expensive purchases, ones a typical radio amateur might have had difficulty concealing from their partner in the family accounts. With a price point that is almost edging into the realm of an impulse purchase, the Mini has the potential to become an SDR transceiver for everyone. If you have been holding back because of the price, maybe it’s time you gave it a look.
AM, or amplitude modulation, was the earliest way of sending voice over radio waves. That makes sense because it is easy to modulate a signal and easy to demodulate it, as well. A carbon microphone is sufficient to crudely modulate an AM signal and diode — even a piece of natural crystal — will suffice to demodulate it. Outside of broadcast radio, most AM users migrated to single side band or SSB. On an AM receiver that sounds like Donald Duck, but with a little work, it will sound almost as good as AM, and in many cases better. If you want a better understanding of how SSB carries audio, have a look at [Radio Physics and Electronics] video on the subject.
The video covers the math of what you probably already know: AM has a carrier and two identical side bands. SSB suppresses the carrier and one redundant side band. But the math behind it is elegant, although you probably ought to know some trigonometry. Don’t worry though. At the end of the video, there’s a practical demonstration that will help even if you are math challenged.
Dealing with signals as math equations are a staple if you seriously study electrical engineering. For example, you can model a pure sine wave with the equation: S(t) = A sin(2*pi*F*t) where A is amplitude, F is the frequency in Hertz, and t is time. Try it. In that example, the frequency is 1 kHz and the amplitude is 20, but you can tweak the values and see what happens. If you add something to the argument to the sin function, you’ll change the phase of the wave. If you followed that, you should have no problem with the video.
This math isn’t just good for working homework problems. It is the same math you need to do synthesis with a computer or digital signal processor. If you want to dig in, we talked about phase angle mathematics, earlier.
But wait, what’s with that screwy title? Have you ever heard an SSB signal on an AM radio? Most people describe it as sounding like Donald Duck. Listen at about the 20 second mark of this mp3 file and just after that, too where it is Upper Side Band (the other kind of USB) but off frequency and you’ll hear that famous voice. If all of this is unfamiliar, you need to explore the speech transmission origins of AM.
Air Traffic Controllers use Automatic Dependent Surveillance-Broadcast (ADS-B) as an alternative to secondary radar to track aircraft. The ADS-B is transmitted by the aircraft and contains information such as GPS position, pressure, altitude, and callsign among other things at a 1090 MHz frequency, which can be decoded using any of a number of software tools.
[Mike Field] lives near an airport, and decided he wanted to peek into the tracking signals for fun. He turned to an RTL-based TV Dongle. Since the stock antenna was not cutting it, he decided to make one specifically for the 1090 MHz signal. His design is based on Coaxial Collinear Antenna for ADS-B Receiver by [Dusan Balara] which uses pieces of the coaxial cable cut to the right length. There are a number of calculations involved in determining the size of the cable, however, the hack in this design is the way he uses a USB based oscilloscope to measure the speed of RF waves inside the line in question.
We reached out to [Mike], and this is what he had to say. The idea is to use a cable of half the size of the wavelength which is calculated as
lambda = c/f
For the best reception, the sections of coax need to be half a wavelength long – but the wavelength of the signal inside the coax, which is shorter than the wavelength in free space. As this was a generic cable he had no idea of the dielectric that separates the core from the shield, so the ‘velocity factor’ could be anything depending on the exact composition.
To determine the speed of the signal in the cable, his approach omits the more expensive equipment. A length of coax acts as a stub – any energy that is sent into the cable reaches the far end of the transmission line and is then reflected back to the source. When the cable is 1/4th of the wavelength long, the reflected signal arrives back at the start of the signal 180 degrees out of phase – in a perfect world it would completely null out the input signal.
[Mike] starts his experiment with a 10m cable as he needs a test signal with a wavelength of 40m. In order to get the test signal into the cable, just two resistors at the back of a connector are all that was needed. The diagram shows the 330 ohms and 100 ohms in series with the center point at around 75 ohm which is a match for the cable.
Using the Digilent Analog Discovery 2’s Network Analyzer the connector is swept from 1kHz to 10Mhz without the cable attached, and then with the cable attached. The dip at 5.666 MHz, caused by the reflected signal down the coax is very clearly seen. From there it is simple math – 40m/cycle * 5,666,000 cycles per second = 226,640,000 meters per second or 75.6% of the speed of light.
So the wavelength of the ADS-B signal is (226,640,000 m/s) / (1090,000,000 Hz) = 0.208m, and the desired length to cut is 104mm 1/2 wave elements and 52mm 1/4 wave elements and get soldering!
This is a great example of how a little bit of math and human ingenuity can be better than expensive test equipment and if you looking to get into software defined radios from scratch, start with Scratch.