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| WB2CBA Interface for Digital Modes as I built in on a perforated board |
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?
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| 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.
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| WB2CBA Digital Interface Schematic |
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The CM108 USB Sound Card I used for this project
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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.
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| The diode I had to insert between the transistor's drain pin and the radio |
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| Finally in operation - stable now |
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| 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
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