Matching a transistor amplifier’s low output impedance with the higher impedance of an antenna (typically 50 or 75 Ohms) is just one everyday example of where an L-Network can be used. For this article, we are going to design an L-Network that matches a 75 Ohm source (function generator) with a 1 kΩ load (resistor).

To make it easier for you to understand how this network works, I will present the complete circuit first, explain how it works and then explain how the values are being calculated. Here is the finished L-Network:

L-Network matching 75 Ohms source impedance to 1000 Ohms load impedance

RS = Source Resistance (75 Ohms)
RL = Load Resistance
Ls = Series inductance (j263)
Cp = Parallel Capacitance (-j284)

Don’t be frightened by the little “j” in front of the values, in case you have never seen it. The letter “j” in electronics engineering stands for the imaginary unit. Adding “j” to a reactance is basically the so-called “vector notation.” A positive value indicates an inductive reactance and a negative value is always a capacitive reactance. If you are not used to it, you can just think of Ls as an inductor with a reactance of 263 Ohms and of Cp as a capacitor with a reactance of 284 Ohms.

Note that the reactance of a capacitor and an inductor is frequency dependent. Therefore, this circuit does not work well over a large bandwidth.

This circuit makes the 1000 Ohms load ‘look’ like a 75 Ohms load to the source. But how does it work? To answer this question, we have to look at the equivalent circuit. For that, we will reduce the overall circuit to simpler elements, step by step.

Let’s start with Cp and RL. They are in parallel. The overall impedance of two resistors is calculated as follows:

Likewise, the resulting complex impedance of a resistor and a capacitor can be calculated as follows:

In our case, we get:

75 is the real part in Ohms and -j263 is a capacitive reactance of 263 Ohms. That means the capacitor and resistor in parallel will act like a resistor with 75 Ohms and a capacitor with 263 Ohms reactance in series.

Equivalent circuit for Cp and RL

Now if you insert that back into the original circuit and have a close look at it, you may notice something. Look at the reactance of Cp and Ls.

Overall equivalent circuit after reducing Cp and RL to a series equivalent circuit

Correct, the reactance of both the capacitor and the inductor are the same, just with different vectors. If you have never worked with a complex impedance beforehand, you will now be amazed by how much easier it makes things. We can simply add the reactances together, and since 263 + -263 is zero, we only have the 75 Ohms load resistance left. Yes, it’s that simple, the source ‘sees’ the 1 kΩ load as a 75 Ω load.

Overall equivalent circuit of the L-Network

An often-welcomed side effect is that this L-Network acts as a low-pass filter. So, if it’s used to match a low transistor amplifier impedance directly to the higher antenna impedance, one suppresses harmonics at the same time.

If one wishes to have a high-pass characteristic, one can simply switch the positions of the inductor and the capacitor. It really changes nothing. The impedance values, of course, need to be properly adjusted in that case.

Let’s talk about how to design a circuit like this from scratch. We will use the above values, source impedance of 75 Ohms and load impedance of 1000 Ohms at a frequency of 16 MHz. The frequency doesn’t matter until the end when we have to pick actual values for Ls and Cp.

The first thing we have to calculate is the quality factor, or Q factor. It describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator’s bandwidth relative to its center frequency. The Q of an L-Network is calculated as follow:

R_Load = Load Resistance
R_Source = Source Resistance

With our values plugged into the equation, we get the following:

So 3.512 is our Q. The rest is just as simple. To get the needed impedance of the inductor, we use the following formula:

That means the inductor needs to have an impedance of 263 Ohms at the desired frequency. We can write this as j263 to make sure it’s apparent that it is an inductive reactance. The impedance of the capacitor needs to be the load resistance divided by the Q factor.

Again, this means that the impedance of the capacitor at the desired frequency needs to be 284 Ohms. Using the vector notation, we write -j284 to indicate that it is a capacitive impedance.

Okay, now we need to find actual values for the inductor and the capacitor. To be able to calculate either one, we need to settle on an operating frequency. For this article we will use 16 MHz. Using the following formulas, we can find the actual values for the parts at the desired frequency:

That means our inductor needs to have a value of 2.6 μH. I chose a value of 2.2 μH as it is a common value I had in stock. Since the circuitry itself in real life usually contains what’s called parasitic inductance, it won’t throw the circuit off by far. Next up is the capacitor:

Instead of 35 pF, I used a 33 pF capacitor. Not only is it a more common value, we can again assume a little bit of stray capacitance by the remaining circuit.

75 Ohms to 1000 Ohms L-Network characterized with a return loss bridge

I used a return loss bridge, a signal generator and an oscilloscope to verify the performance of this circuit. And sure enough, it acts just like a 75 Ohm resistive terminator at about 16 MHz. The exact frequency for best operation turned out to be roughly 16.1 MHz.

Confirming the performance of the 75 Ohms to 1000 Ohms L-Network using the Teledyne LeCroy HDO4024

Update (04/06/2013): I previously wrote that one can not match a high impedance source to a low impedance load. That is incorrect. Naturally, it works perfectly fine the other way around. One only has to switch the position of the load and the source.

KJ6QBA took out the time to simulate this circuit using LTspice. He also simulated the same circuit used in reverse to match a 1000 Ohm source to a 75 Ohm load. Here’s the result:

LTspice simulation of the L-Network provided by KJ6QBA

When I saw that Abracon had reasonably priced programmable oscillators available, I knew I had to get some. There is a variety of different oscillators with different specifications available. I picked the Abracon ASDMB series MEMS (Micro Electro Mechanical System) oscillators [1] [2]. To be able to program these oscillators, one needs the MEMSpeed Pro II programmer and an adapter socket [3].

First off, a warning. MEMS oscillators aren’t the best performers when it comes to phase noise performance. So wherever phase noise plays an important role, these oscillators are probably not your best choice. But for that, MEMS oscillators or MEMS actuators / sensors in general are significantly less sensitive to vibrations and mechanical shock.

I got my MEMSpeed Pro II kit and the ASDMB adapter kit directly from Abracon. Any adapter kit comes with 50 blank oscillators. The case that the MEMSpeed Pro II comes in offers space for two adapter cards and plenty of blank ICs. There are numerous different oscillators with different parameters available [4].

Here’s a close-up of the MEMSpeed Pro II kit:

MEMSpeed Pro II kit with adapter socket and some blank oscillators

As you can see, there’s space for two socket cards and plenty of blank oscillators in the case. The USB drive contains all the software necessary for the programmer. Please make sure you install the software before you connect the programmer for the first time. The reason is that the programmer uses a proprietary USB driver that should be installed before the device talks to the PC for the first time.

Close-up of the MEMSpeed Pro II programmer and the adapter card with opened socket

The adapter card connects to the side of the MEMSpeed Pro II. Once everything is connected, one can hook up the programmer to the PC and start the software. The software is really self-explanatory. After entering the target frequency and clicking the “Program” button, the MEMS oscillator is programmed.

The software for the MEMSpeed Pro II is very easy to use

The “Measure” section of the software makes it possible to verify that the burn went correctly. However, it seems that the precision of this feature is not the best. It’s enough to make sure that the frequency is about right, but please don’t trust the last digit.

For an experiment, I programmed one of my ASDMB oscillators to the frequency 145.565, which is the national frequency for radio direction finding in the amateur radio community. I used my brand new Teledyne LeCroy HDO4024′s spectrum analyzer capabilities to obtain some data on the device’s performance.

As indicated above, the phase noise performance of MEMS oscillators is not the best. The following shot shows this clearly on the instantaneous spectrum and the historical spectrogram:

Output spectrum of a Abracon ASDMB programmed to 145.565 MHz viewed on a Teledyne LeCroy HDO4025

Next up is the harmonic output. The Teledyne LeCroy automatically marks harmonics and displays the level. Unfortunately, I entered a slightly off fundamental frequency (145.5 instead of 145.565). But the representation is still valid.

Using the harmonic marker function of the Teledyne LeCroy HDO4025 12-bit high definition oscilloscope to view the harmonic content of the output signal

The ASDMB oscillators can operate between 1 and 150 MHz. They can be operated between 1.8 and 3.3 V and work well over the entire range. There are different temperature options available ad there are 3 sub-types available, which are designed for an output load of 10 kΩ and either 15, 25 or 40 pF of capacitive load. At 3.3 volts, the rise time is better than 2 nS.

Altogether, this is a very nice product and from now, on a must have in my lab. From now on, I will have almost any frequency available whenever I need it. The only way Abracon could make me happier would be by coming out with an MEMS VCO that can be pulled by an external voltage.

Links and Sources:

[1] ASDMB Datasheet, Abracon: http://www.abracon.com

[2] ASDMB Blank Oscillators, Mouser: http://www.mouser.com/

[3] MEMSpeed Pro II, Mouser: http://www.mouser.com/

[4] MEMS Overview, Abracon: http://www.abracon.com/

I have no idea if that was just my perception or if the year 2012 was the year of the oscilloscope revolution. An incredible flood of new scopes was released into the market in 2012. There’s the Agilent InfiniVision series, the Tektronix MDO4000 series, the Rigol 2000, and the Teledyne LeCroy HDO4000 and HDO6000 series.

Teledyne LeCroy sent me one of their 6000 series High-Definition Oscilloscope (HDO). To be precise, it’s the HDO6054, a 12-bit, 500 MHz oscilloscope. A product like this deserves a thorough review. With my usual review style, the length of the review would exceed the length of the bible. But before I even start to write the review, let’s talk about 12-bit vs. 8-bit.

First, movie time. Let’s watch the official video LeCroy used to announce their HDO4000 and HDO6000 series scopes:

I was very surprised about the way Teledyne LeCroy presented their new scope. Aside from stating some apparent facts in a humorous and authentic way, there was no typical US marketing buzzword bingo present at all. I like it. That being said, let’s talk about 12-bit.

8-bit or 12-bit resolution, what does that even mean? A digital scope measuring an analog signal logically needs a circuit translating between the realms of the analog and digital world. An analog-to-digital converter (abbreviated ADC, A/D or A to D) is needed to allow the digital circuitry of a modern scope understand the analog input. The root of the word digital is the Latin word for ‘finger’, digitus. This is because digital systems can only indicate whether or not a bit is set (1) or not (0), kind of like counting with your fingers. There are only 256 (2⁸) different values that can be represented with an 8-bit variable. That means if the maximum permissible input range of an ADC is 0 – 5.12 V, the maximum resolution would be 20 mV. That means the ADC can’t tell the difference between two signals whose difference is smaller than 20 mV very well. The ADC rounds up and down in between full 20 mV steps.

Rise time measurement with markers on the HDO 6054

12-bit means 4096 possible values. Assuming the same input range, we now get 1.25 mV resolution. That’s obviously much better. Critics argue that one is not able to utilize the full 12-bit in a real world application due to quantization noise among other things. But let’s not get into that discussing now, it’s enough material for a whole new article.

Another term often heard in relation to 12-bit technology is “dynamic range.” To understand dynamic range, we must first understand how an oscilloscope applies a signal to the ADC. In my above example, I chose an ADC with 0 – 5.12 V input range. The number is arbitrarily picked for easy math. Now let’s a assume a saw tooth-shaped signal with a peak at 512 mV is applied. With 8-bit resolution, the ADC would turn the smooth upwards slope into 20 mV steps. The ADC could not understand that it is dealing with a linear increase but would indeed see a stepped response. To circumvent this problem, oscilloscopes include a Variable Gain Amplifier (VGA). The smart circuitry makes sure that the ADC is always driven to the max. In the case of our saw-tooth signal, an amplification factor of 10 would be just fine. The ADC could now detect steps as small as 2 mV because after amplification they are going to be 20 mV steps.

400 MHz sine wave output of a PLL synthesizer

In other words, the scope adapts. That works pretty well within certain limitations. It becomes a problem when there are two signals with large differences in amplitude present at the same time. The ADC has no problem digitizing a sine wave ranging from 0 Volts to 5.12 volts. The same way it has no problem doing the same with a sine wave ranging from 0 – 51.2 mV – the VGA will take care of it, but both at the same time causes trouble. If you think about it for a minute, this will become apparent for you.

Assume the 0 – 51.2 mV signal is noise riding on your wanted signal, the sine wave with 0 – 5.12 volts range. The ADC has to sample over its entire range in order to capture the entire 0 – 5.12 V signal. There’s nothing the VGA can do to lift the 0 – 51.2 signal in a better zone for the ADC. With the proportions being as given, the noise will barely cover more than 3 quantization steps over the whole range. In an real world application, you probably wouldn’t be able to notice anything unusual and miss the noise altogether.

Output spectrum of a 400 MHz PLL with a 200 kHz PFD frequency. Clearly visible, the spurs 200 kHz left and right from the carrier

Dynamic range is a measure for the highest and lowest level that can be detected at the same time. This number directly correlates with the resolution of the ADC. Therefore, some noise or unwanted parts of a signal can be missed with an 8-bit oscilloscope but are clearly visible with a 12-bit oscilloscope.

This entire 12-bit thing isn’t new at all, though. Agilent has 12-bit scopes and Teledyne LeCroy had some for a while, as well. A 12-bit ADC alone won’t do you any good, though. Because we can see signals so much clearer, there are now very stringent requirement to be met in regards of the oscilloscope’s front-end. The VGA needs to be super low-noise and the entire input stage needs to be even more linear than in an 8-bit scope. And that is something that LeCroy claims to have perfected in their HDO series.

Okay, that is all I am going to cover about this subject for now. It is a pretty large subject, though, and one could write books about it. As a matter of fact, I am sure someone has written books about it. As a matter of fact, this article is an extreme reduction of a rather complex matter.

Pictures are always better than words. Therefore, I will write another article on this matter. I will then add noise to a desired signal and show screenshots of an 8-bit scope and then on the HDO (12-bit) for comparison. There’s a lot of talk on the web on why 12-bit is really not worth it and such, but I will not engage in that discussion and let the pictures speak. So stay tuned!

Not too long ago I reviewed ADI’s wideband PLL synthesizer ADF4351 with integrated low phase noise VCO. The ADF4360 series family of synthesizer chips is very similar. The primary difference is their restricted frequency coverage and thus much lower price.

The ADF4360 is an integrated integer-N synthesizer with an integrated voltage controlled oscillator (VCO). The center frequency is set by external inductors. There are 9 chips in the family with 8 different frequency ranges. The frequency range are as follows:

ADF4360-0: 2400 to 2725 MHz
ADF4360-1: 2050 to 2450 MHz
ADF4360-2: 1850 to 2170 MHz
ADF4360-3: 1600 to 1950 MHz
ADF4360-4: 1450 to 1750 MHz
ADF4360-5: 1200 to 1400 MHz
ADF4360-6: 1050 to 1250 MHz
ADF4360-7: 350 to 1800 MHz
ADF4360-8: 65 to 400 MHz
ADF4360-9: 65 to 400 MHz

Even though the ADF4360-8 and ADF4360-9 have the same frequency range, they are a bit different. The ADF4360-9 has an auxiliary divider with division ranges from 2 to 31 on board. The ADF4360-8 has – just like all other ADF4360 – a hardware power down input (CE).

Analog Devices kindly sent me one of their Evaluation Kits for the ADF4360-9 (EV-ADF4360-9EB1Z) for review and evaluation [1].

EV-ADF4360-9EB1Z evaluation board from Analog Devices

What I intend to use this chip in is called a Fox-Hunt transmitter. It has very little to do with hunting actual foxes and actually relates to a common radio direction finding exercise. The way this works is that a small transmitter (the “fox”) is hidden somewhere and a group of people will attempt to locate the transmitter. Whoever finds the transmitter first, wins the game.

I want to build a very small transmitter for the 2m amateur radio band (~145 MHz) with just a few miliwatts of output power. With such a low power VHF transmitter, a radio direction finding exercise could be conducted in a small area like a park, or even more challenging, with several transmitters at the same time. A microcontroller is supposed to be in charge of setting the frequency and keying the required station identification (call-sign of the control operator) as required by the FCC.

But now back to the ADF4360. The chip has an SPI compatible 3-wire interface, operates between 3 – 3.6 Volts and its inputs are 1.8 V logic compatible. In other words: this chip will interface with pretty much any microcontroller out there. My project will probably be Atmel AVR or MSP430 based and I program in C. However, I will write example code for Arduino (AVR) / Energia (MSP430) for folks who would like to experiment with it more easily.

I looked at the output spectrum of the ADF4360-9 set to 400 MHz on a Teledyne LeCroy HDO6054. The phase frequency detector (PFD) frequency is 200 kHz and you can clearly see spurs 200 kHz spaced to both sides of the carrier. The spurs are smaller than -70 dBc and, to be fair, the ADF4360-9 is not correctly terminated. The IC has a differential output and the datasheet warns that the performance of the output signal may be degraded if not both ports are properly terminated with 50 Ohms. In my case, only one port is fed into the 50 Ohm port of the scope. The other port is open.

Output spectrum of a ADF4360-9 at 400 MHz with a 200 kHz PFD frequency. Clearly visible, the spurs 200 kHz left and right from the carrier

In any case, -70 dBc is a lot of attenuation. As a matter of fact, the output signal could be transmitted the way it is over the air. The FCC demands in 74 CFR 97.307 (e) that “the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency between 30 – 225 MHz must be at least 60 dB below the mean power of the fundamental.” This is clearly the case.

The eval board comes with a very comfortable software, just like the ADF4351 did. It is very nice to be able to manipulate all registers and parameters and watch what happens right away.

So what’s next? I will design the circuit, design a PCB, and write the necessary software code for the little VHF tracker (“fox”). The project will be an open hardware project. That means you will be able to use my project free of charge for personal use. As soon as that is done, I will post a new article with the entire project in it. Stay tuned!

Links and Sources:

[1] ADF4360-9, ADI: http://www.analog.com/

To make the evaluation of the CC2531 a bit easier, TI put together the CC2531EMK. The CC2531EMK is a an evaluation kit containing a USB dongle built around the CC2531 [1]. Element 14 sent me a CC2531 USB dongle development kit as part of a RoadTest. RoadTest is a program sponsored by Element 14 that promotes real product reviews by real people like us.

CC2531 USB Dongle plugged into my Netbook

Overview and Tech Specs

The CC2531 is a USB-Enabled System-On-Chip Solution for 2.4-GHz IEEE 802.15.4 and ZigBee applications in a tiny 6-mm × 6-mm QFN40 package. The chip is 8051-based, has 8 kB of RAM and is available with either 128 kB (CC2531F128) or 256 kB (CC2531F256) of In-System-Programmable (ISP) flash [2].

Since the chip is compliant with various national and international standards, namely ETSI EN 300 328 and EN 300 440 (Europe), FCC CFR47 Part 15 (US), and ARIB STD-T-66 (Japan), products built around this chip do not need to worry about national and international compliance. With a maximum output power of 4.5 dBm (2.82 mW), the chip is perfect for small range communication.

The CC2531 provides extensive hardware support for packet handling, data buffering, burst transmissions, data encryption (AES Security Coprocessor), data authentication, clear channel assessment, link quality indication (Accurate Digital RSSI/LQI) and packet timing information.

With 21 general-purpose I/O Pins and a 12-Bit ADC with 8 channels and configurable resolution, the chip supplies plenty of peripherals for external circuitry.

Unboxing

The box contains the CC2531EMK USB dongle and a quick start guide. The quick start guide is straight to the point and easy to understand.

CC2531EMK on top of the quick start quide

While reading the quick start guide, I realized that there is no way to program the MCU’s flash memory without the use of an external debugger. Such a debugger needs to be purchased individually. In other words, one can’t program the CC2531EMK without buying a debugger like the CC Debugger, for instance [3].

Software Installation

To get started with the CC2531 dongle, one needs to install the appropriate drivers. The driver package comes bundled with a packet sniffer application which TI provides. Be careful to use the correct URL for the download. It is not – as falsely indicated in the quick start manual – http://www.ti.com/packetsniffer! The correct URL is: http://www.ti.com/tool/packet-sniffer

Download and install the software package from the previously mentioned URL. After the installation is complete, your PC should automatically recognize your CC2531 dongle and apply the pre-installed drivers.

Upon start-up of the packet sniffer software, one has to select the protocol one intends to use. For the CC2531, the only possible options are “IEEE 802.15.4/ZigBee” and “ZigBee RF4CE”.

TI Packet Sniffer Application

The next thing that needs to be selected is, of course, the correct RF channel.

SmartRF Packet Sniffer Channel Selection

One can specify which fields of the captured traffic that the SmartRF packet sniffer software displays.

SmartRF Packet Sniffer filter selection

Summary

This is a nice piece of kit to evaluate the CC2531. However, the kit alone is not of much use. To actually accomplish anything useful, one needs at least one additional IEE 802.14 / ZigBee device. And in order to program it, an extra programmer / debugger is required.

I will go ahead and buy a CC Debugger and post again soon with my experience of the CC2531EMK in an actual (demo) application.

Links and Sources:

[1] CC2531EMK, Newark: http://www.newark.com/

[2] CC2531 Datasheet, TI: http://www.ti.com/

[3] CC Debugger, Newark: http://www.newark.com/

Before I write any further, let’s agree on some nomenclature. For the sake of this article, I will use 220 Volts and 240 Volts interchangeably. In Europe, the correct supply voltage is 240 Volts AC. However, actual supply voltages range between 220 Volts and 240 Volts. Americans usually speak of “220″ when referring to European voltages. Similarly, many Americans refer to their own supply voltage as 110 Volts even though it is actually rated at 120 Volts nominal. So again, for the sake of this article I will use 110 and 120 interchangeably.

I have a bunch of European equipment that works on 240 Volts only. Here in the US, I naturally only have 110 V available. While it is technically possible to combine two 110 Volt lines to create a 220 Volt line, I do not currently have that option to do so as my lab is fed by a single 110 V line only.

The solution to my problem is as simple as finding a 110 to 240 Volts transformer. If you have a power requirement of more than 1000 Watts, this can be a pretty expensive solution, though. Therefore, iIwas very surprised to find this 2000 W transformer for less than $100 [1]. For that price, I decided I’d give it a try.

Goldsource STU-2000 front view

There’s no actual magic inside the box. Inside the case is a standard transformer with a set of different primary taps.

According to the tech specs, this transformer will accept 110, 120, 220, and 240 Volts as input voltage. It supplies 110 Volts and 220 Volts on its output at the same time over two different receptacles.

I did measure 220 Volts output in actual working conditions with 110 Volts input. Accordingly, I could measure 240 Volts out with 120 Volts input while keeping the input selector set to “110 Volts.”

Goldsource STU-2000 input voltage selector

The transformer was pretty smelly at first. I assume this was the smell of the fresh paint. After airing it out for a few hours, it stopped smelling funny. The operation noise is barely audible and depends on the actual load.

Goldsource STU-2000 opened up.

The overall look of this transformer is pretty solid. The wiring has been done pretty well and the wires seem to have adequate thickness. The device is fused twice. There is a 20 A fuse on the primary side and another resettable 20 A fuse on the secondary side (220 V only). There are 2 spare primary fuses supplied with the transformer.

Goldsource STU-2000 secondary side

Just when I was about to close the transformer back up, I noticed a wire that looked a little bit suspicious. Have a look, do you see it?

Faulty wire in the 200 V primary hook-up

The transformer works very well for my purpose. I have several SMD soldering tools and other scientific devices powered through this device. Most devices do not mind about the fact that the mains frequency is not being translated. European mains frequency is 50 Hz and in the USA it’s 60 Hz. Only devices which derive timing information from the mains frequency, like cheap alarm clocks for instance, will have difficulties dealing with the wrong frequency.

Links and Sources:

[1] Goldsource STU-2000, Amazon: http://www.amazon.com

Today, I tried out a 300 Watt 12 V DC to 220 V AC inverter I bought off eBay not too long ago. For just $26.50 including shipping, I got it from the eBay seller hi-autopia [1]. I was already suspicious because of the low price, but decided to give it a try anyway. So here it is:

Label of the One-Hung-Low brand 300 W inverter

I could not find any information on this manufacturer at all. A tag on the side of the inverter indicates it was manufactured somewhere in Asia. So how well would this One-Hung-Low brand inverter perform? Well, let’s start with some visible results of my test:

Molten 4mm probes (Hirschmann Kleps 30)

Yes, those were perfect 4mm test clips at some point. So what happened? The inverter was connected to a 12 V battery through the above test clips and a pair of 4mm test leads. Probably not the best for continuous current at the maximum 300 Watts, but okay for the 60 Watts I used as a test load. And just for the record, the DC hook-up wire that was supplied with the inverter was about half as thick as my test leads.

Input side of the 300 W inverter.

The load I used for the test was a 60 W 220 Volt soldering iron. A soldering iron is an entirely resistive load and shouldn’t cause any hardship on an inverter. I measured the output voltage across the output of the inverter, a steep 420 volts while idle. Once I connected the soldering iron, it fell to 230 Volts. The voltage seemed to drop every second or so. It looked as if the circuitry had difficulties regulating the output voltage. But before I could hook up my oscilloscope and take a screenshot, I started smelling smoke. And shortly after that, the smoke started coming out of the case of the inverter. My test clips started glowing bright orange before I finally got to disconnect the inverter from the battery.

Output side of the 300 W inverter.

I thought this was awfully weird. Not only was the behaviour of the inverter strange, it seemed like the inverter had no protection circuitry whatsoever. While thinking about it, I noticed that this inverter does not have a fuse on the input side like most professional inverter do. So the fuse had to be on the inside. I mean, it couldn’t be that the inverter isn’t fused, right?

Closer look at the input section of the inverter

Well, guess again, there’s no fuse or any other means of protection in the input circuitry. To skip ahead: There’s none in the output section either. But more about that later. Just by glancing at the circuitry, I suspect this is a modified sine wave inverter. And with that in mind, I am very surprised about the size of the transformer (yellow). For 300 W output, I’d expect a transformer to be capable of handling at least 500 VA. The transformer used here looks more like 12 VA…

Close up view of the circuitry revealing several safety issues

The first thing that caught my eye was the lack of a proper routing for the 220 V traces. At high voltages, traces must be spaced out as far as possible to avoid arcing to ground planes or other traces. Ideally, the design should implement an actual physical separation by utilizing milled notches. In the above pictures, the 220 V wires are the two thin black wires in the bottom right corner.

General overview of the 300 W inverter

The low spacing between the PCB and the case is concerning, as well. There are absolutely no precautions taken in order to prevent the solder joints to touch the case. As a matter of fact, the two bend transistors in the bottom left corner may have been bent over the way they are for some degree of “protection.”

There seems to be a temperature sensor as fault prevention. But just by looking at the discolored casing of one of the two driver MOSFETs, I suspect this fault detection is not very effective, to say the least.

By the way, the device is, of course, not certified under Part 15 of the FCC rules.

I did send a message to the eBay seller hi-autopia about this and will supply him with a link to this article. His reaction should be rather interesting. While I doubt he knew about the quality (or the lack thereof), this is a great opportunity for him to show how well he responds to customer complaints and how he resolves them.

Links and Sources:

[1] hi-autopia, eBay: http://www.ebay.com/

Every passive probe and every piece of coax cable has a specific capacitance. This fact is simply given by the laws of physics. A Teledyne LeCroy PP016 (300 MHz BW) probe, for instance, has a maximum input capacitance of 12 pF in the 1:10 position. Probes rated for smaller bandwidths often have an even higher input capacitance.

12 pF doesn’t sound like much at all. At 1 GHz, though, this capacitance acts like a load with a resistance of 13.26 Ohms. Correctly, this would be referred to as a reactance and not as a resistance, though. Needless to say, 13.26 Ohms is way too low and, in most cases, will put too much weight on whatever it is we measure. Remember, our goal is usually to have the highest possible impedance to put the least possible load on the stage we are testing.

Of course there are commercial solutions for this problem available. Look at the Teledyne LeCroy PP066 ‘High Bandwidth Passive Probe’ for up to 7.5 GHz, for instance [1]:

Teledyne LeCroy PP066 transmission line for up to 7.5 GHz

The probe really looks like a modified SMA connector with a tip attached to the center pin and a grounding pin connected to the shield. A probe like this costs more than an inexpensive entry-level oscilloscope itself. So can we built something like this at home? Obviously I wouldn’t write this article if we couldn’t, so here is what I did.

While looking for a good starting point, I stumbled across my box of small semi-rigid coax jumpers [2]. Semi-rigid cable has a very low specific capacitance of around 0.1 pf/mm. That would mean a short piece of about 2.5 cm or 1″ would barely have 2.5 pF of capacitance. At 1 GHz, that would mean a reactive load of 63.66 Ohms. Since most circuit stages in that frequency range work with a impedance of 50 Ohms anyway, the line impedance is high compared to the 50 Ohms. So how do we transform the coax into a cheap probe?

Semi-rigid coax cable with SMA connectors

The first thing I did was carefully stripping a piece of the copper shield off. I used a normal pocket knife for this purpose. To my surprise, it’s fairly easy to cut the shield without damaging the dielectric too much. Next, I stripped some of the dielectric to expose a few mm of the center conductor.

Cut-off semi-rigid coax cable

The only thing that was missing now was a ground lead. Thanks to the solid copper shield, that was an easy task. I simply soldered a piece of silver plated copper wire to the shield.

Cut-off semi-rigid coax cable with ground lead as improvised GHz probe

The result doesn’t look nearly as fancy as the commercial solution, but it’s cheap and easy to make. However, I’d like to point out that I was not doing anything fancy with this makeshift probe and cannot really give a performance feedback. Directly connected to a frequency counter, I could successfully probe a 1.4 GHz and a 2.8 GHz signal. I will try to probe the LOs of C- and Ku-Band LNBs shortly and post back how well that worked. But for less than $1 investment, this is certainly a nice thing to try, though.

Links and Sources:

[1] PP066 High Bandwidth
Passive Probe, Teledyne LeCroy: http://teledynelecroy.com/

[2] Semi Rigid Cable, eBay: http://www.ebay.com/

Dieter has a huge selection of toroids and magnet wire for an extremely low price. I was really surprised by the prices he offers as I used to pay a fortune for the same things from other distributors. At first, I thought the prices were a little too good to be true but I tried out his store, paid with PayPal and received the goods within 2 business days, no problems or catches at all!

Assortment of common toroid types

Some other things I found in his store were Intermediate Frequency (IF) transformers. IF transformers are also referred to as IF cans because of the metal can shield around the actual inductor. The formerly big players in the market for IF cans, Sumida and Toko, discontinued their production a long time ago. Since then, it can be a hassle finding a reliable and inexpensive source for these products. I asked about the reliability of his stock and Dieter indicated: “All parts listed on my website are expected to last as long as I do.”

455 kHz IF filters

Links and Sources:

[1] Dieter Gentzow, W8DIZ: http://www.kitsandparts.com/

The ADF4351 from Analog Devices (ADI) is a modern wideband PLL synthesizer with integrated low phase noise VCO. It is capable of generating signals between 35 MHz to 4400 MHz with a very low jitter of typical 0.3 ps. ADI agreed to send me the EVAL-ADF4351EB1Z, an evaluation board for the ADF4351, for review purposes. Let’s check it out!

The ADF4351 has an integrated voltage-controlled oscillator (VCO) with a fundamental output frequency ranging from 2200 MHz to 4400 MHz. In addition, divide-by-1/-2/-4/-8/-16/-32/-64 circuits allow the user to generate RF output frequencies as low as 35 MHz. For applications that require isolation, the RF output stage can be muted. The mute function is both pin- and software-controllable. An auxiliary RF output is also available, which can be powered down when not in use. Control of all on-chip registers is through a simple wire interface. The device operates with a power supply ranging from 3.0 to 3.6 V and can be powered down when not in use.

ADF4351 evaluation board

ADI ships the EVAL-ADF4351EB1Z with a USB cable and a CD. The CD contains all software necessary to get started right away. The board itself makes a very clean impression. Despite the surface mount technology, all case styles of the components used can comfortably be handled with appropriate hand soldering tools. This allows for easy application specific modifications.

There are two 4 mm jacks for power (3.75 V to 5.5 V), a small USB connector, and 3 SMA connectors on the board. The first SMA connector serves as reference input for an external reference signal. Alternatively, it can be used as an output for the on-board reference (25 MHz). The other two SMAs are the primary RF output of the ADF4351. It is a differential pair. For best performance, make sure to terminate both outputs correctly (50 Ohms) even if just one output is being used.

Power can either be supplied through the 4 mm banana jacks or the USB port. Switches S1 and S2 are used to select the power source. Switch S1 is used to power the ADF4351 from the external DC connector and switch S2 to power from the USB port. The USB clock can cause spurs in the RF signal if power is derived from the USB port.

Plenty of test points on the board allow easy troubleshooting. An additional 100 mil / 2.54 mm header can be populated to gain easier access to the most important logic signals.

The external loop filter on the eval board has a bandwidth of 35 kHz. This value can easily be changed by changing the value of the corresponding components. The software package ‘ADIsimPLL’ from Analog Devices is a great tool for designing an application specific loop filter.

Make sure to install the software from the CD before connecting the evaluation board to the PC for the first time. Once the software is installed properly, connect the board to the PC and start the ADF4351 software package. If the two power LEDs on the board (D5 and D6) do not light up upon connection, verify S1 and S2 for proper selection of the desired power source.

ADF4351 Application Software

The software is self-explanatory. It allows accessing and manipulating of all functions and registers of the ADF4351. Additionally, the software allows you to use the evaluation board as a sweep generator and it can do frequency hopping between two frequencies.

After trying this board out for a while, I highly recommend this chip. It is small, inexpensive ($8.25, 100 QTY) and extremely simple to integrate.

I can think of many applications for the ADF4351 in the amateur radio community. The ADF4351 a perfect 21st century alternative for older SP5055 based designs. The SP5055 was a very popular synthesizer chip used in many older amateur radio projects.

The chip is predestined to be used as a flexible Local Oscillator (LO) in amateur radio transverters. A flexible LO frequency allows to cover more bandwidth in the target frequency range than the IF transceiver itself offers. Paired with a baseband processor and a power amplifier, this chip easily transform into an inexpensive amateur television (ATV) transmitter. I will show some practical designs and applications in future articles. Stay tuned!

Links and Sources:

[1] ADF4351, ADI: http://www.analog.com/

[2] ADF4351 Datasheet, ADI: http://www.analog.com/

[3] Evaluation Board User Guide, ADI: ftp://ftp.analog.com/