The 817/817ND/818 doesn't need an introduction; it's a classic QRP radio. The XPA-125B also doesn't require further explanation; it's a 100W amplifier with built-in antenna tuner, released by XieGu. Having an antenna tuner makes the amplifier much easier to use. To combine these two devices, XieGu offered an accessory called CN-20. I didn't buy it because I was originally using the X6100 with the XPA-125B. Recently, I thought about connecting the 818 and the 125B together, and after some research, I found that the CN-20 has two potential drawbacks:
1: CN-20 occupies the original ACC port, which is not suitable for situations where CAT commands need to be input via the ACC port.
2: The CN-20 device does not yet support the functionality of linking with the 817/818 band and 125B, possibly because it has lower priority or because XieGu is focused on developing the GPA100.
Therefore, my idea is to design an interface box that can enable the 817/818 and 125B to work together, while also exposing the original ACC/DATA ports for CAT applications. Furthermore, it should be able to automatically switch between the bands of the 817/818.
First, display the schematic diagram and briefly introduce the circuit principle:
1: The DATA port doesn't require much work; I made a clean connection for aesthetics.
2: ACC mouth, primarily concerned with three signals: PTT, BAND, and ALC, where:
1) PTT Signal: When the FT-817/818 is transmitting, the PTT_GND port on the ACC connector provides a ground connection, and this port has an open-collector output. Therefore, it can be directly connected to the PTT input of the XPA-125B ACC connector.
2) BAND Signal: This signal is provided by 817/818, and represents an analog voltage value. The specific voltage value and the corresponding frequency band are shown in Table 1.
However, reviewing the XPA-125B manual reveals that while the XPA-125B's automatic band switching also uses simulated voltage indication, the value is different from the standard specified in 817, as shown in Table 2.
Therefore, we need to perform level conversion within the interface box so that signals from the 817/818 can be converted into the correct voltage signal values recognized by the 125B. Here, we chose an inexpensive 8-pin microcontroller, STC8G1K08A, and used its internal ADC to read the voltage values from the 817/818, and drove a low-cost single-channel DAC, MCP4725A0T, to output a voltage signal that conforms to the XPA-125B standard. Both have the same reference voltage (Vcc), so for the power supply part, I used an adjustable linear regulator, LM317, and adjusted the voltage to approximately 5V after assembly.
Since this signal is only used for transmitting analog information and does not carry a load, connecting the DAC output directly to an amplifier should not be a problem. If you are particularly concerned, you can add an additional level following circuit yourself.
3) ALC: Normally, when connected to an amplifier, the amplifier provides an ALC voltage of 0 to -4V to the radio to control the radio's output power and prevent excessive drive power from exceeding the amplifier's design limits. However, based on my testing: ① Using the 6W power of the FT-818 to drive the XPA-125B does not cause it to exceed its limit. ② The ALC output voltage of the XPA-125B appears to be +4V.Using the standard of 0V, if you want to return to 817./818, it also needs to subtract 4V. This increases the complexity of the circuit. Given the above two points and the KISS (Keep It Simple, Stupid) design philosophy, we have decided not to design an ALC return circuit.
Considering the above points, the entire circuit is quite simple. The project used Creação EDA for design and JiaLiChuang for prototyping. After returning, we performed soldering and testing, and everything worked as expected. CAT can control the radio, and the radio can automatically control the amplifier. The Creação open-source page will be attached below after approval. GitHub project address:https://github.com/bg2kaj/FT817toXPA125B
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