WB2CBA USB Sound Card Based Digital Interface and Vox for Digital Modes

In brief: The WB2CBA design CM108AH-based USB sound card VOX circuit enables the use of FT8, FT4 and WSPR modes on QRP radios such as the uSDX that lack a CAT interface. 

This article covers the installation of the WB2CBA (Barbaros Aşuroğlu) designed USB sound card VOX circuit with a CM108AH-based external sound card, and its use with the uSDX QRP radio in WSPR mode. This low-cost and easy-to-build circuit, intended for those who want to use digital modes such as FT8, FT4 and WSPR on radios without a CAT interface, automatically handles PTT control based on the signal level at the computer's audio output. The circuit schematic, parts list, elimination of ground loop and PTT voltage problems, and a WSPR performance comparison between 200 mW and 5 W are among the main topics covered.

WB2CBA Interface for Digital Modes as I built in on a perforated board

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WB2CBA's Sound Card-Based Digital Interface VOX Circuit

There is almost nothing I can say about this circuit — other than the fact that it works very well and is very easy to build. You can find extremely detailed explanations about both the construction and operation by its designer, Mr. Barbaros Aşuroğlu (WB2CBA), on the electronics projects pages of the Antrak website. The circuit consists of a USB sound card designed to be plugged into computers via USB, along with a few components to connect it to the radio through its audio input and output connectors. It is an extremely economical solution for operating in digital modes with radios that lack CAT control — such as the uSDX or your own homebrew QRP radio.

I am not particularly fond of digital modes myself — especially modes like FT4 and FT8, which cannot be considered real two-way conversations, bore me. It is a matter of taste and preference, but these techniques feel to me like they require no operating skill, and I feel useless. As I have noted in other posts, if something doesn't require work, learning, and effort, it holds no interest for me. So I don't use these modes for actual communication — with one exception: WSPR. When I built this circuit, my intention was not to do FT8, but to operate WSPR with the uSDX.

I had actually been running my WSPR transmitter on and off for a few years, but there were two things I was curious about: what would happen if I increased the output power, and if I also received instead of just transmitting, how many stations would I be able to hear. I decided to build this circuit to connect to my uSDX and try it out. I was curious where I could reach by going from 250 mW up to 5 W. And how many of the stations that heard and decoded my signal would I in turn be able to hear?

Also designed by WB2CBA, this is my WSPR transmitter.
It connects via WiFi to my home modem, syncs the time
 from a server over the Internet, and transmits a message every 4 minutes

Components and Construction of the VOX Circuit

There isn't much to say about the construction. If you look at the schematic above, you will see that the components are extremely easy to source. The only part you need to specifically seek out is the USB sound card built around the CM108AH integrated circuit. I used the type whose photo you will see below — you can get this online for 3–4 US dollars including shipping. Mr. Barbaros has also designed a PCB to mount this sound card on, but I made the connections on perforated board in a layout similar to his. It obviously didn't turn out as elegant as a proper printed circuit board, but it does the same job.

WB2CBA Digital Interface Schematic

The CM108 USB Sound Card I used for this project

WB2CBA Interface for Digital Modes as I built in on a perforated board

Post-Build Operation, Tips, and Solutions to Some Possible Problems

What the circuit does is cause the radio to switch to transmit when a signal of a certain level is detected at the computer's audio output. It does this by amplifying and rectifying the signal and applying it to the gate of a FET transistor. The transistor, connected to the radio's PTT input, short-circuits the transmit and ground lines when it conducts. A red LED lighting up at the same time shows us that "PTT has been pulled to ground." In the other direction, the audio signal coming from the radio passes through a variable resistor before entering the computer. It sounds simple, but after assembling it I couldn't get it working right away and struggled for a while. Let me summarize the reasons:

• The positive voltage at the uSDX's PTT pin could not be pulled fully to zero for a reason I couldn't understand (it was staying around 0.2 V) and the transistor couldn't conduct. I couldn't resolve the problem until I added a diode to block the voltage coming from the radio into our circuit. Note: this diode is not in the original schematic, and when I asked Mr. Barbaros about it, he said he hadn't needed to make such an addition. So the problem may be specific to my circuit or my connections to the radio.
• After that problem was resolved, another situation began to emerge: when I plugged in the connector coming from the circuit into the radio's audio output, the transistor would start conducting and put the radio into transmit. I resolved this too by adding a diode to the end of the cable coming from the radio. I believe the problem is ground-related — probably a ground loop forming, with current flowing somewhere it shouldn't. The cleanest solution would be to use an optocoupler between the VOX circuit and the radio to break the electrical connection, but it must be remembered that this circuit was designed to be simple by nature. If we can get it working with two diodes, we'll stick with that. :)
• The third problem I encountered was on the software side. Through trial and error I realized that the level of the audio signal coming out of my computer wasn't sufficient to drive the transistor into conduction. No matter how much I adjusted the computer's audio settings, I couldn't solve the problem. I eventually realized that in the WSJT-X software I was using, it isn't possible to raise the level sufficiently using the vertical slider on the right side of the interface without adjusting it. I left that setting at the lowest point at which the radio switches to transmit. I should also mention something about the receive level setting: it should be set to 30 dB with no antenna connected to the radio.

The diode I had to insert between the transistor's drain pin and the radio

Finally in operation - stable now

The input level on the left needs to be set to 30–40 dB, and the output level
on the right should be left at the lowest point at which the radio switches to
 transmit when pressing the 'tune' button just above it.

Conclusions and Some Observations

I used the circuit I built this way in WSPR mode for a while (and even did a bit of FT8 and FT65). What I found:

• Despite the large 14 dB difference between 200 mW and 5 W output power, I didn't see a very large difference in the distance of the stations that heard me. To give an example from the eastern direction: at 200 mW I was heard at most from Austria. At 5 W I was heard from Italy, the Balkans, and once from Turkey.
• The number of stations I could hear was, as I expected, far fewer than those I could reach. I wasn't expecting more than that anyway, with a 10-meter wire strung from a first-floor window to the garage awning.

These results once again confirm what is already known:

• Unlike modes such as SSB and AM, with CW or — if you like them — digital modes, you are always heard from farther away.
• Instead of higher power (100 W and above), it is better to invest in a better antenna.

One final point: to operate in the digital modes mentioned above, your computer's clock must be accurate. If you use WSJT-X as I do, you will also need software that periodically connects to a server over the Internet and re-synchronizes your computer's clock. I used BktTimeSync for this.

A wonderful circuit for those who want to get into digital modes with an inexpensive and enjoyable build. I wish success to the amateurs who try it, and once again extend my thanks to Mr. Barbaros.

FAQ — Frequently Asked Questions

1. What does the WB2CBA USB Sound Card VOX circuit do? 

Simply put, it allows the computer to communicate with the radio. When the audio signal from the computer reaches a certain level, the circuit automatically switches the radio to transmit. It stands out as one of the most practical and inexpensive ways to use digital modes such as FT8, FT4, or WSPR on radios without a CAT interface — such as the uSDX or a homebrew QRP radio.

2. Which USB sound card should be used? Is the CM108 mandatory? 

The design is based on the CM108AH chip, and cards with this chip can be easily found online at very low cost.

3. The PTT voltage won't go to zero and the radio won't switch to transmit — what should be done? 

This problem is encountered particularly with the uSDX: a residual voltage of around 0.2 V remains at the PTT pin and the transistor cannot conduct. The solution is surprisingly simple — adding a single diode between the radio and the transistor's drain pin is sufficient. This diode is not in the original schematic, but it has been found to work when this voltage problem is encountered.

4. The radio switches to transmit as soon as the audio cable is plugged in — what causes this? 

Most likely a ground loop problem is occurring. Current is flowing somewhere between the radio and the computer where it shouldn't be. Adding a diode to the end of the cable coming from the radio resolves the problem. For a more permanent and cleaner solution, using an optocoupler could be considered; however, since simplicity is at the heart of the circuit's design, sticking with two diodes is reasonable if that gets the job done.

5. The audio level in WSJT-X is never sufficient — what should be done? 

Opening the horizontal slider in the interface all the way usually isn't enough. The vertical slider on the right should be left at the lowest point at which the radio switches to transmit. For the receive level, 30 dB with no antenna connected is a good starting point. It requires some trial and error, but once found it stays stable.

6. Which radios does this circuit work with? 

It is expected to work with any HF radio that has a PTT input. The primary target audience is radios without CAT control — uSDX, homebrew QRP radios, and similar models. If a commercial radio with a CAT interface is being used, controlling PTT through software is a more practical approach.

Links

WB2CBA's original article: Click here

BktTimeSync download: Click here

WSJT-X download: Click here

Kent Iambic Paddle Kit Review: Build Quality, Feel, and CW Experience

Kent dual-lever paddle is a classic iambic key designed for CW (Morse code) operators in the amateur radio world. In this article, the structure of the kit, the assembly process, and its usage features are discussed. In particular, the advantages it offers in terms of precision, ergonomics, and response time in CW communication are examined, explaining why it is preferred by both beginners and experienced amateur radio operators.

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Why did I start looking for a dual-lever (iambic) paddle?

When I first started amateur radio, I was very interested in learning radio telegraphy (CW), something I had admired for years but never had the opportunity to seriously pursue. This year, I finally found the chance to do so. Over three months, I attended the “CW Academy,” offered online for free by CW Ops club volunteer instructors, and was able to bring my telegraphy skills to a beginner level.

When I started this course, my intention was to use a single-lever “straight key” paddle that I had built myself. However, after seeing one of the recommendations—“if you are seriously going to work CW, get used to a dual-lever paddle from the very beginning”—I borrowed a Bencher BY-2 paddle from a club friend and continued my lessons with it. As a side note, this brand has been around for 45 years but continues its life under the Vibroplex brand. In North America, this is probably the most commonly seen paddle; you can often find them at flea markets for around 50–60 USD.

QST, May 1978: Bencher paddles have been manufactured 

since 1977, based on the design by W8FYO.

Bencher: the most established fixture of all flea markets

My opinion about this paddle, which I used almost every day for three months, is that it is a very difficult paddle to adjust. I don’t want to go into a long explanation, but as you can also see on IW5EDI's page, the contact points are not on the sides like in other dual-lever paddles; instead, they are at the front, working through sharp-pointed screws at the ends of spring-loaded “elbow” shaped arms that move slightly back and forth into corresponding slots. To adjust this mechanism, you need to tighten or loosen the screws that control both the spring tension and the position of these elbow-shaped parts. To set the contact gap, you also have to rotate two screws clockwise or counterclockwise. No matter how much I tried, I could not achieve a configuration that felt comfortable, and overall I did not find this paddle very usable.

On the other hand, the three months I spent with the Bencher also helped me clarify what I expect from this type of paddle. I decided that the following four features are my priorities:

• Smoothness in the mechanism: being able to translate finger pressure into contact without requiring excessive force or abrupt movements, and having fast return of the levers
• Quick and easy adjustability: being adjustable even during a contact if necessary is a plus
• Durability: since it is a relatively expensive piece of equipment, it should have reasonable build quality and the ability to find spare parts
• “Stability” / footprint: when using the paddle, it should be heavy enough not to move on the desk, but not so large that it takes up too much space

Kent Paddles Versus Other Alternatives

As I was nearing the completion of the course, I started looking for a paddle online as a kind of reward to myself. I ruled out some brands despite their quality (such as Begali) because they were above my budget, and I set aside smaller “field-type” paddles (for example Bamatech). In the field, a simple paddle made from 3D-printed parts would have worked perfectly well. When I ranked the remaining candidates by user count and positive review ratio, not many options were left. I first leaned toward an American paddle that looked attractive in terms of price (AMM), but the shipping cost was almost as high as the paddle itself, so I slightly increased my budget and decided on Kent Engineers’ dual-lever iambic paddle.

Although Kent is originally a British manufacturer, after the United Kingdom left the EU, it moved to Germany in order to maintain tax and logistics advantages. As a result, the paddles are now produced in Germany and shipped to you via DHL. All parts of the paddle are brass, while the base is steel. Since the levers sit on bearings, friction is very low. Although springs are used for return action, the response is fast, and overall the mechanism feels smooth and comfortable to use. The contact points are silver-plated. There are no adjustments other than contact spacing and lever tension, and these are made easy to adjust using large knurled screws, so you don’t need any tools like screwdrivers or Allen keys. Additionally, the left and right sides can be adjusted independently.

Another advantage of purchasing this paddle is that you can choose to buy it as a kit and assemble it yourself. I preferred the kit version because I was curious about it, it reduced the risk of customs issues, and it also lowered the cost slightly. The package arrived about three weeks after placing the order. Since no special adjustment was required, I completed the assembly in about an hour. I have been using the paddle with my own keyer now and I am very satisfied with it; I will most likely continue using it for many years.

The completed assembly; I later installed the bakelite parts
on the levers with their tips facing downward.

FAQ

1. What is an iambic paddle?
An iambic paddle is a dual-lever key used in CW (Morse code) communication. It allows dots and dashes to be generated in a fast and rhythmic way.

2. What does “iambic” mean?
The word “iambic” comes from a poetic rhythm term and refers to a weak-strong stress pattern. In Morse usage, it represents rhythmic signal generation.

3. How does an iambic paddle work?
One lever produces dots (dits), the other produces dashes (dahs). When both levers are pressed at the same time, the device automatically alternates dots and dashes.

4. What is the difference between an iambic paddle and a straight key?
A straight key produces signals manually with a single lever. An iambic paddle works with an electronic keyer, enabling faster and more consistent sending.

5. What are the advantages of iambic keying?
It provides less hand fatigue, higher speed, better timing accuracy, and more efficient Morse sending.

6. Is an iambic paddle suitable for beginners?
Yes, but it requires some practice to get used to. Once learned properly, it offers more efficient operation in the long term.

7. Does an iambic paddle require additional circuitry?
Yes. An electronic keyer (Morse keyer) is usually required for proper iambic operation.

Links

Kent Morse Keys - Manufacturer's Web Page: click

GW6ITJ's review: click

Another review: click

Adding a 3.5 mm jack to Kent Iambic Key's base: click

IW5EDI's excellent page on Morse key adjustments including Bencher: click

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