R/C Varioprop 6 Signal Decode_a

Introduction

I have always been interested in model aircraft and while on a trip to Europe in 1970, I purchased a Graupner R/C remote control plane kit. I took my time building it so as not to make a mistake. Several years later I bought the remote control gear. I never flew the plane since I put in all that work and was scared of crashing it! However flash forward to 2023 and my interest has been renewed. I thought the first step would be to analyze the R/C signal to see how it works, since I finally have the test equipment to do the job.

Test Setup

Fig.1 Varioprop 6 Test Equipment Setup
Fig.2 Varioprop 6 Test Equipment Schematic

Figure 1 shows a picture of the test setup and Figure 2 a connection schematic. I use both a scope and spectrum analyzer to study the Varioprop 6 signal. The original transmitter had a compact cylindrical DEAC 12VDC nicad battery. However, after 50 years I have to replace it. So for now I am using an external 12VDC Gel Cell battery. The scope probe is clipped on the transmitter antenna with the ground clip connected to the negative battery terminal. The spectrum analyzer just uses a whip antenna with 20dB pad. The transmitter uses a crystal in the 27MHz CB Citizens Band.

Signal Capture

Fig.3 Varioprop 6 Tx Carrier 27.145MHz Span=5MHz
Fig.4 Varioprop 6 Tx Carrier Span=50KHz
Fig.5 Varioprop 6 Tx Carrier Time Domain Showing PWM

Figure 3 shows the transmitter spectral line at a span of 5MHz and Figure 4 shows it at a span of 50KHz. Note the sinx/x shape with nulls every 4KHz approx. Figure 5 shows the time domain as captured on the Picoscope with a single shot trigger.

Signal Analysis

Fig.6 All Controls Trimmed
Control StatusBurst #msec duration
All controls trimmedB11.7msec
B21.4msec
B31.3msec
B44.2msec
Rudder Full 1B11.0msec
B2/3/4Same as all trimmed
Rudder Full 2B12.2msec
B2/3/4Same as all trimmed
Elevator Full 3B22.0msec
B1/3/4Same as all trimmed
Elevator Full 4B20.8msec
B1/3/4Same as all trimmed
Speed Full 5B32msec
B1/2/4Same as all trimmed
Speed Full 6B30.7msec
B1/2/4Same as all trimmed
Fig.10 Pulse Widths for CH1-6

Figure 6 shows the transmitter signal with all controls trimmed in neutral position. Note it consists of 4 bursts of carrier repeated with a period of 11.6 msec. Rudder control works by pulse width modulation of the first burst with width of 1msec for full port and 2.2msec for full starboard. Elevator control similarly works on Burst 2 and finally Speed control on Burst 3. Figure 10 gives the PWM pulse width modulation values.

Fig.11 YouTube Video R/C Varioprop 6 Signal Decode_a

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References

#1. – “Learn Telecommunications by Simulation”
https://www.clarktelecommunications.com/simulation.htm

#2. – “HF Radio Telecommunications Learn by Simulation”
https://www.clarktelecommunications.com/simulation.htm

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.