The quantum tunneling transmitter

February 24, 2024February 24, 2024Ashish Derhgawen 2 Comments

This has to be one of the most bizarre projects I have worked on. It is a transmitter that works without any transistors or tubes. It utilizes a strange phenomenon known as quantum tunneling. Quantum tunneling is a phenomenon wherein a particle can disappear from one side of a barrier and reappear on the other side even if it doesn’t have sufficient energy to surmount the barrier. It seems as if the particle “tunnels through” the barrier, hence the name. Quantum tunneling is a consequence of the wave nature of matter. It is nothing less than magic.

The first semiconductor devices to utilize this phenomenon were invented in the 1950s and were called tunnel diodes. These devices exhibit negative resistance, which means that the current through the semiconductor becomes inversely proportional to the voltage across it. This only happens at certain voltages. Below and above that negative resistance region, the tunnel diode exhibits a normal current-voltage relationship.

Current-voltage characteristics of tunnel diode vs conventional diode (from: RCAs Tunnel Diode Manual)

This negative resistance anomaly allows tunnel diodes to be used in very high-frequency oscillators. Tunnel diodes are an exotic breed of semiconductor devices. Commercially made ones are rare and expensive. However, recently, a genius inventor by the name of Nyle Steiner discovered an easily made substitute. He found out that he could make tunnel diodes at home by heating galvanized sheet metal!

Anyone with a propane torch and a few scraps of galvanized sheet metal laying around can easily make a negative resistance device. With this device, it is possible to make very simple RF oscillators, audio oscillators and even amplifier circuits. It is almost like making your own transistor.
Nyle Steiner

Mr. Steiner has some of the most interesting experiments I have seen, so do check out his website – http://www.sparkbangbuzz.com/.

I recreated the experiment by heating a zinc metal plate with a torch. The back side of the zinc plate (opposite from the flame) had several active spots. These black spots are where you are most likely to find negative resistance. I used a “cat whisker” style detector similar to the ones used in crystal radios to probe the zinc plate. For the “whisker”, I used #30 AWG copper wire.

I’ve used a 4 MHz crystal to set the frequency of the oscillator. The 10K pot is used to adjust the biasing. The 1K resistor was initially added to reduce short-circuit current when the pot is at its lowest setting. This resistor may no longer be necessary after I added a 1K resistor in the keying circuit (which consists of a CW straight-key, 1K resistor, and an electrolytic capacitor). The keying circuit was a later addition, so I forgot to remove the 1K from the biasing circuit. The resistor and electrolytic capacitor on the keying circuit serve an important purpose. They smooth out the keying. The charge on the capacitor prevents the voltage on the cat-whisker from changing abruptly. For some reason, the homemade tunnel diode doesn’t like abrupt keying. Without the smoothing circuit, there are many cat-whisker settings that stop working after opening and closing the key. These oscillators are quite fussy.

Adjusting the cat-whisker is time-consuming task. Be prepared to spend several minutes trying to find good spots. The easiest way to know if the circuit is oscillating is by using a radio or an oscilloscope. I tuned my Icom transceiver to the frequency of the crystal (4 MHz) and kept it nearby in CW mode. You will hear clicks and beeps as you slide the cat-whisker. If you don’t, adjust the biasing. Finding a stable spot can take a while, but once you’ve found it, it can last for a long time if you don’t disturb it. In my experiments, I was able to use a spot for an entire day. It would have lasted longer if I hadn’t disturbed it.

Output of negative resistance oscillator

Here is the “quantum tunneling transmitter” in action:

I will try injecting an AM audio signal into the oscillator. I’m sure that would also work. If only the professor on Gilligan’s Island knew about it!

WSPR-ing around the world

January 13, 2024January 13, 2024Ashish Derhgawen Leave a comment

I feel that one of the charms of amateur radio is its unpredictability. It feels like throwing a message in a bottle into the vast ocean, not knowing where the currents would take it, or who would read the message. Radio waves often take unpredictable paths when traveling. To a large extent, how these waves propagate is determined by the state of our planet’s ionosphere. The ionosphere consists of layers of charged particles that affect how RF signals travel. These layers move and shift and undergo cycles of strengthening and weakening, all under the influence of the Sun. Radio waves can bounce off the ionosphere, and essentially “skip” around the Earth.

To understand how waves propagate in different bands a protocol known as WSPR (pronounced “whisper”) was developed in 2008 by Joe Taylor (K1JT). It is an acronym for “Weak Signal Propagation Reporter”. It can tell us what is possible with low-power transmissions and see which radio bands have a path to which points on the globe.

A WSPR transmission conveys the sender’s call sign, station location, and power level using a compressed data format with strong forward error correction (FEC). The message is modulated using frequency-shift keying (FSK) at a very low bit rate. Sending a single WSPR message takes almost two full minutes! The WSPR protocol is effective at signal-to-noise ratios (SNR) as low as -28 dB in a 2500 Hz bandwidth, some 10 to 15 dB below the threshold of audibility.

My QCX transceiver contains an inbuilt WSPR mode. I set it up and sent a single WSPR message on the 40m band with about 4-5 watts of power. It took almost 2 minutes to send, and I was worried about heating the BS170 transistors in the power amplifier since I had never really stress-tested them in this manner. To my relief, no magic smoke was released. I monitored the transmission on a local WebSDR to make sure it was sending a decipherable message. After it was done sending, I checked the WSPRNet website to see if any stations received my feeble signal. I wasn’t expecting anything spectacular, but to my surprise, the signal had traveled much farther than I imagined!

I had reached Antarctica, Hawaii, New Zealand, Australia, the Canary Islands, and Norway (to name a few)! Well, that proves that the antenna I built is working. There are lots of tools to analyze the WSPR data. I liked the analysis tools available on WSPR Rocks. For example, you can view a SNR vs distance chart. It was interesting to see that some distant stations copied my signal better than stations which were nearby. On the website you can see the names of the stations and other details.

I also pulled all the data into a Google Sheet for analysis.

I reached 26 locations with a single WSPR transmission. Incredible! The results have encouraged me to try building a dedicated WSPR beacon using the Raspberry Pi for use on other bands. Stay tuned!

The not-so-random “random wire” antenna

January 6, 2024January 7, 2024Ashish Derhgawen 1 Comment

I live in an apartment where installing an antenna for HF use is a challenge. After evaluating various antenna options, I chose to install the so-called ‘random wire’ antenna, stretching it between my balcony and my aunt’s next door. Random wire antennas aren’t very random at all! Certain lengths perform better than others. I decided to go with a 58-ft wire based on the recommendations on this website.

A random wire antenna has an unpredictable impedance that varies with frequency. Moreover, the impedance is usually so high that most antenna tuners need additional help from a 9:1 unun transformer to bring the impedance down to a workable range. The 9:1 unun is an autotransformer with a 3:1 turns ratio, which results in a 9:1 impedance transformation.

I had an untested unun that I built a long time ago. I connected it to the antenna and used 25-ft of coax to connect it to my Emtech ZM-2 antenna tuner. Despite all my efforts to tune it on the 40m band, I failed to bring the SWR to an acceptable range.

I tried adjusting the antenna length, but it didn’t make much of a difference. The SWR was too high when I checked with the NanoVNA. What was wrong in my setup? It was time to sit on my armchair, put on my detective hat, and light a cigar.

My hunch was that the untested 9:1 unun was the culprit. To confirm this, I removed the antenna wire from the unun and replaced it with a 470-ohm resistor on the output. If the transformer was doing what it was supposed to, I expected an impedance of approximately 50 ohms (470/9) on the output.

I used the NanoVNA to plot the transformer’s frequency response. The frequency plot revealed the problem. The 9:1 transformation was happening around 30 Mhz. At 7 MHz (40m band), it was nowhere close to 9:1. No wonder the tuner was struggling!

The 9:1 unun’s terrible frequency response

In the plot, the yellow trace is the impedance. In an ideal world, a transformer would exhibit consistent performance across all frequencies. However, in reality, a transformer’s bandwidth is influenced by its inductance and various parasitic elements. The VNA trace shows that the impedance increases between 1-30 MHz, where it is approximately 50 ohms. The 9:1 transformation occurs near 30 MHz, signaling that the low cut-off frequency is higher than optimal—a clear indicator of insufficient inductance! Time to light another cigar.

I wasn’t sure what toroid core I was using in the unun. I decided to replace it with an FT50-43 toroid that I had in my junk box. Mix #43 toroids can be used for wideband transformers between 3-60 MHz according to this website. The FT50-43 is a small toroid, so it was a bit difficult to fit 3 wires side by side. I used a twisted trifilar winding to save space and get about 8-9 turns on the core. The inductance increased from about 3 uH (old core) to 230 uH (new core). On the NanoVNA, the impedance response looked much more uniform across the HF band.

Uniform frequency response with the FT50-43 toroid

I connected the random wire antenna to the unun and measured the SWR across the HF band with the VNA. The impedance varies with frequency, with dips at specific frequencies. As you can see, there is a dip in the 40m band that allows the ZM-2 to easily tune the antenna.

I connected some of my QRP transmitters to the antenna, and was able to hear them on Bangalore’s webSDR!

There are still some unresolved mysteries to explore. Do I need a separate counterpoise? Would that make a difference? I haven’t observed any noticeable improvements from adding one. I believe the coax shield is the counterpoise in my setup. Does the position or orientation of the 25-ft coax cable make any difference? It does seem to affect the antenna’s impedance, so I think it does. How does the angle of my antenna affect propagation? So much to explore, so little time!

Some observations on MOSFETs

October 14, 2023October 15, 2023Ashish Derhgawen Leave a comment

I have been working on a QRP CW transmitter which I’ve described in an earlier post. The output from the buffer amplifier stage is about 50 mW. My goal is to reach about 5 watts of power. To do so, I plan to use the ubiquitous IRF510 transistor to boost power levels. The IRF510 MOSFET is a type of field-effect transistor. It was developed in the 1970s by a semiconductor manufacturing company – International Rectifier. It was originally intended to be used in the automotive industry for turn-signal blinkers and headlight dimmers to replace clunky electromechanical switches and relays. It still continues to be quite popular (and cheap). It has found its way into the hands of amateur radio experimenters who use it at frequencies way beyond what this humble transistor was intended for!

The final power amplifier in my transmitter would be a class-C amplifier using the IRF510. Before connecting the IRF510 to my circuit, I decided to investigate its input characteristics. This would help me to decide how to drive it and meet my design goals. MOSFETs typically have a very high input impedance in DC circuits since the gate is electrically insulated. However, in AC circuits, things are a little more complicated.

MOSFET basics

The IRF510 is an N-channel enhancement mode MOSFET. A MOSFET consists of an insulated gate, the voltage of which determines the conductivity of the device. The ability to regulate the flow of electricity with the amount of applied voltage can be used for amplifying or switching electrical signals.

A thin oxide layer insulates the gate from the rest of the transistor body. When a positive voltage is applied at the gate, positively charged holes are pushed away from the gate-insulator/semiconductor interface creating a depletion layer. The depletion layer is filled with negative charge carriers. With sufficient gate voltage, the accumulation of electrons forms a conducting path between the source and drain terminals, enabling current flow. The width and conductivity of the channel can be modulated by the voltage applied to the gate. The threshold voltage, commonly abbreviated as V_th of a field-effect transistor (FET) is the minimum gate-to-source voltage (V_gs) that is needed to create a conducting path between the source and drain terminals.

Gate capacitance

The gate of the MOSFET is electrically insulated because of the oxide layer, which gives it a high input impedance. When a DC voltage is applied to the gate, no current flows through the gate. The insulating oxide layer is sandwiched between two conductive layers. However, whenever two conductors are separated by an insulating layer, there will be some capacitance. Oftentimes this parasitic capacitance is an unwanted side effect. In the case of MOSFETs, this implies that the gate won’t appear high impedance to an AC input signal since capacitors allow AC signals.

The transistor datasheet should mention the gate capacitance. For the IRF510, the datasheet states a gate capacitance of about 180 pF. This could also be measured in several ways.

Observing the depletion layer

We can measure the capacitance between the gate and source pins using an LCR meter. When measuring the capacitance, I noticed that this capacitance decreases when voltage is applied to the drain and source pins (Vds). I guess this is because of the formation of the depletion layer. As the depletion layer widens, the capacitance decreases.

Analysis with a signal generator and oscilloscope

Gate capacitance could also be determined using AC analysis with a signal generator and oscilloscope. This method involves some math.

We connect a known resistor (R) in series with the gate to create a series RC circuit. Vs is the peak-to-peak from the signal generator, and Vc is the peak-to-peak voltage measured across the capacitor on the scope. The first equation is the formula for capacitive reactance since a capacitor behaves as a frequency-dependent resistor in an AC circuit. The second equation calculates the total impedance by taking the Pythagorean sum of R and Xc. You can read more about impedance in RLC circuits here. The third equation is Ohm’s law: V = IR. The voltage across the capacitor (Vc) is equal to the total current (Vs / Z) multiplied by the reactance of the capacitor (Xc). I derived the formula for capacitance from the first three equations. If you use this method, make sure you consider the output impedance of the signal generator in your calculations.

An interesting phenomenon

While trying to determine the gate capacitance, I tried measuring the RC time constant by charging the gate capacitor through a known resistor. The RC time constant equals R x C (R is in ohms, C is in farads). The time constant is the time it takes for a capacitor to acquire 63.2% of the difference between its initial voltage charge and the voltage applied to it. When I tried measuring this, I noticed something unusual.

The graph doesn’t look like a typical capacitor charge graph. There is a region where it plateaus for a while before it resumes charging again.

This effect was first discovered in the 1920s by John Milton Miler when working with triode vacuum tube amplifiers. The parasitic capacitance between the grid and anode affected the performance of the triode amplifier. This effect is called “Miller effect”. According to Wikipedia:

“In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals.“

Later on, we invented solid-state transistors that replaced vacuum tubes, but the Miller effect didn’t go away. There’s no getting around physics! To observe the Miller effect on a scope, I added a 10K resistor in series with the MOSFET’s gate. This resistor slows the charging time of the parasitic capacitors and allows us to see the Miller effect in all its glory.

In a MOSFET, there is parasitic capacitance between the gate and source (C_gs) and between the gate and drain (C_gd). There is also parasitic capacitance between the drain and the source, but we will ignore that for now. Initially, when there is no voltage at the gate, the C_gs capacitor is at 0V. The C_gd capacitor is charged to the supply voltage since the voltage at the drain is whatever the supply voltage is.

When the gate voltage rises, the C_gs capacitor charges until the MOSFET’s V_th threshold is reached. This is the voltage at which a channel is formed between the drain and the source. As the MOSFET switches on, the drain voltage drops, and eventually approaches the source voltage. When it drops, the C_gd capacitor sucks in current from the gate, preventing its voltage from rising. This is why we see the plateau in the graph. When the drain voltage settles, and the C_gd capacitor is fully saturated, the gate voltage continues rising.

How do we counter this effect? Of course, the higher the input drive current, the quicker it would pass the Miller plateau. There are other more complicated techniques to counter this effect. I don’t fully understand them yet.

The reality is that I can’t just drive the IRF510 with the 50 mW output from the buffer amplifier and expect 5 watts of output power. According to “Handiman’s guide to MOSFET amplifiers”, the input impedance is about 130 ohms at 70 MHz (40-meter band). To provide an 8 Vpp at that impedance, we’d require about a half-watt of drive. I would have to add another amplifier stage to drive the IRF510.

Archery and electronics

September 20, 2023October 3, 2023Ashish Derhgawen Leave a comment

I enjoy making conceptual connections across different disciplines. When I learned about “impedance matching” in electronics, I realized that this concept is not limited to the world of electronics. Impedance matching is an interesting and tricky topic in RF circuits. Since I practice archery, I’ll share an archery analogy of impedance matching that can help develop an intuitive understanding.

In archery, arrows are tuned to match the bow. Many factors determine a good match, but we will only focus on the weight of the arrow for our example. Suppose you shoot an arrow that weighs 26 grams (~400 grains) from a bow with a 40 lb draw weight. In archery terminology, we would say that the arrow is 10 GPP (grains per pound), since 400 / 40 = 10. The energy stored in the limbs of the bow would get transferred to this arrow. If you use a lighter arrow (< 10 GPP), the arrows will certainly fly faster. However, if it is too light, it won’t be able to effectively penetrate the target. You will also feel significant hand shock when shooting arrows that are too light for your bow. Shooting very light arrows could potentially damage the bow as it would be akin to dry firing. Now, think of the opposite case, where we shoot very heavy arrows (>10 GPP). Imagine shooting a thick branch! The arrow would be too slow to do any real damage to the target.

The beautiful Gera Fox Max bow with matching arrows

The ideal arrow weight is somewhere in between the two extremes. When the arrow is close to the ideal weight, it would carry the most energy. So, what are we doing when we are finding this ideal weight? In electronics, this is called impedance matching.

In the context of RF transmitters, we are trying to maximize the energy going into the antenna. This can only happen if the impedances are matched. If there is a mismatch, there will be power loss and energy will be reflected. In extreme cases, the reflected energy could damage the transmitting circuit (similar to dry firing a bow).

This is easy to understand with a simple resistive circuit:

In the above circuit, power transfer would be optimal when Zₛ equals Zₗ. This can be verified with simple math. If we make Zₗ < Zₛ, the current (I) would certainly increase. However, the voltage across the load would drop. Similarly, if we made Zₗ > Zₛ, the voltage across the load would increase, but at the cost of current. So, optimal power (V*I) would be transferred when Zₛ = Zₗ.

Matching source and load impedance is not always desirable in electronics. For example, if we were designing a “stiff” voltage source that provides a steady voltage, we would want the source impedance (Zₛ) to be significantly lower than the load impedance (Zₗ). Otherwise, the load overload the voltage source and pull the voltage down. However, when driving a speaker from an amplifier, or when sending radio waves with an antenna, we want optimal power transfer. In these cases, impedance matching is desirable.

Waves in the ether

September 17, 2023September 20, 2023Ashish Derhgawen 2 Comments

I’ve always been fascinated by electronics and radio waves. It’s something we take for granted these days. But think about it – you can push electrons back and forth in a wire, and the effects of this swashing could be sensed thousands of miles away. Isn’t that magical? I didn’t study electronics in college – my major was CS. We learned how to write code and design algorithms. I learned electronics through self-education and experimentation. I feel that this style of learning is often better than a formal education. You can take things at your own pace and be driven by your curiosity and passion. My interest in electronics began with me trying to control things in my house with my computer (see my old blog). One of my first projects was connecting an LED to my computer’s parallel port. Later on I figured out how to connect all kinds of things, such as a floppy drive camera panner, RC cars, etc.

I watched a documentary that left a strong impression – “Shock and Awe: The Story of Electricity” (by Jim Al-Khalili). This film inspired me to learn more about electronics and go beyond controlling LEDs and relays. Soon, I became obsessed with radio circuits. My first transmitter/receiver was a primitive spark-gap transmitter and coherer receiver that I built. My interest in radios eventually pushed me to get an amateur radio license (callsign N6ASD) in 2015.

The person who has inspired me the most in my electronics journey is Frank Harris (K0IYE). He is the author of the book “From Crystal Sets to SSB”. I couldn’t put this book down once I began reading it. His approach and passion for learning were something I could relate to. It wasn’t long before I contacted the author. After exchanging emails for about a year, I met him in person when I visited Colorado in 2017. For me, it was like a dream come true to meet my electronics hero in real life! He showed me his basement lab, with all his radios and electronics creations. Over the years, we’ve stayed in touch and Frank continues to inspire me. If you read Frank’s book, you will find my name mentioned in a few places (particularly in the sections about regenerative receivers and homebrew electrolytic capacitors).

In 2020, I moved from San Francisco to Bangalore. Many things changed in my life, and electronics took a backseat. I focussed my time on other non-technical hobbies. Fast forward to 2023, I found myself back in the world of electronics. My wife (Aditi) encouraged me to set up a little workstation in a corner of my apartment. Having a space in your house/apartment dedicated to something you enjoy is important, and makes it easier to pursue your hobby.

Oil lamps and electronics – a great combination!

These days, I am building a 40-meter QRP (low power) transmitter from Frank’s book (chapter 6). The circuit consists of multiple stages – the oscillator, buffer amplifier, driver, and final power amplifier. So far, I have completed the first two stages – the oscillator and buffer amplifier. I don’t have the transistors that Frank is using in his circuit in the final stages. So, I plan to design my own driver/power amplifier stages. I’m planning to use transistors that are readily available in India.

The joy of successful oscillations

Stay tuned for updates!