Dear community,
We are currently developing a system based on the ST7580 FSK modem, designed to communicate over a shared power and data line. The architecture consists of a Master device communicating with more than 100 Slaves over a single pair of wires, used simultaneously for 24V DC power delivery and FSK communication.
Our objective is to support communication over distances of up to 10 km in a free topology (e.g. star, tree, or mixed). We're sharing the schematics of the Master and Slave power supply and ST7580 interface circuits, and would appreciate your feedback on key design decisions and areas that may benefit from improvement.
System Overview
Modulation: FSK using 120 kHz and 84 kHz, with a deviation factor of 1.
Power Supply: ST7580 is powered at 12V.
Transmission Gain: Increasing the TX_GAIN parameter has proven effective at raising the signal amplitude beyond VCC, although some waveform distortion is observed.
Design Goal: Achieve reliable communication with a high number of nodes across long cable distances, while minimising interference and maintaining waveform integrity.
Current Status & Known Issues
We encountered communication failures at approximately 1 km, which we traced to two factors in the Slave circuit:
Clipping of the FSK signal caused by the bridge rectifier diodes.
Signal attenuation due to the value of the L5 inductor.
These issues have since been mitigated, and signal integrity has improved significantly after adjustments. However, we are still looking to:
Determine the optimal value for the inductor (L5).
Validate or refine the overall filtering approach for separating FSK and DC components, particularly on the Slave side.
Feedback Requested
We would greatly appreciate input from the community on the following points:
FSK Frequency Selection
Are 120 kHz and 84 kHz appropriate for long-distance, high-node-count applications?
Would alternative frequencies reduce line attenuation or improve immunity to noise?
How do frequency choices affect the analog front-end and line filter design?
Filtering and Isolation Best Practices
Are there recommended strategies or topologies for cleanly separating DC and FSK signals?
Do the filter values used in our design (shown in the attached schematics) appear well-suited for this application?
Powering the ST7580
Would increasing the modem supply to ~18V help improve performance when using high TX_GAIN values?
Is there a known threshold beyond which waveform distortion becomes problematic?
General Recommendations
What precautions should we take in terms of impedance matching, reflections, or signal degradation over long cables?
Are there any layout guidelines, protection strategies, or tuning practices specific to ST7580-based systems in this kind of topology?
Attached Schematics
You will find images showing the Master and Slave circuits, focusing on power input and the ST7580 analog interface. We welcome any observations — whether detailed suggestions or general best-practice advice.
Thank you in advance for your time and insights. We look forward to learning from your experience and improving the robustness of our system.
Best regards,
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