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all channel servos centered

channel 1 servo full CCW

channel 1 servo full CW

channel 1 decode output to servo 1

channel 1 decode output to servo 1

channel 1 decode output to servo 1

Proportional Radio Control Encoder and DecoderRick Campbell January 2020

out

Figure 1 is a classic pulse modulated proportional radio control encoder. The output is connected to the on-off input of a CW transmitter, and sends a timed sequence as shown in figure 2. The two-transistor circuit on the left is a multi-vibrator with two time constants. The negative pulse in the output waveform is always the same width, set by the 100k and 10nF R and C on Q2. The positive pulse is variable, a long pulse set by the 750k sync resistor, and 4 variable 100k resistors that set the width of the positive pulse.

Fig. 1 Proportional Radio Control Encoder

Fig. 2 Encoder output

Fig. 3 Channels 1 and 2 information

Fig. 4 Timing for Channel 1 Full Clockwise to Full Counterclockwise

The multivibrator sends a clock pulse to the CD4017 decade counter IC, which sequentially sets outputs 0 through 9 to logic 1, 6v in this case. In this example 4 channel encoder, out-puts 0 through 3 are connected through diodes to 100k variable resistors that encode the timing of the positive pulses in the output. Note that the clock pulses from the multivibrator to the IC clock input are the inverse of the output shown in figure 2. Q3 inverts the clock to obtain the desired output waveform

The sync pulse length is set by a 750k resistor in this example. It may be shorter or longer, just long enough to enable the reset pin on the decoder shown on the next page.

Figure 2 shows the complete encoded sequence at the encoder circuit output. The repeated sequence, called a “frame,” starts with a long sync pulse to reset the decoder, and then four pulses containing the position information of the four 100k variable resistors. Channel 1 information sent to the servo is the rise time of the first pulse after the sync pulse to the rise time of the second pulse. By using rise time instead of pulse width, variations in pulse width with signal strength are avoided.

Figure 4 shows the precise timing sent to the servos to determine rotation. Servos are centered when they receive a 1.5ms pulse, repeated at the frame rate. For full counter clockwise rotation, the servo pulse is shortened to 1.0ms, and for full clockwise rotation the servo pulse is 2.0ms. Note that all of the information needed to set a servo position is set by the rise times of a pair of pulses in the encoded sequence. No information on precise transmitted or received pulse widths, the precise length of the sync pulse, or the frame rate is used. This scheme is robust to many variables.

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Figure 6 illustrates how logic reset is obtained from the sequence of pulses in each frame. Note that the reset pin on the CD4017 has a 390k resistor to the plus 6v supply, and a 22nF capacitor to ground. A diode is connected from the reset pin to the clock input. When the clock pulse is high, the 22nF capacitor charges through the 390k resistor, and when the clock pulse is low, the charge is dumped through the diode to ground. As long as the pulses are close together, the voltage on the 22nF capacitor never rises to the reset pin threshold voltage.

Figure 5 is the complete decoder circuitry, with low level audio input from an envelope modulation receiver and pulse width modulated outputs to 4 servos. Only the channel one servo connection is shown in figure 5, but the connections to highlighted channels 2, 3 and 4 are identical. With no changes to the decoder, the number of channels may be easily changed from 2 to 6, simply by adding the necessary 3 pin connector and 5.1k logic line resistor.

Proportional Radio Control Decoder

The three transistor circuit on the left is a low-level analog input to logic output amplifier. Its overall gain is set so that a 5mV peak input sine wave results in a logic clock pulse into the CD4017 decade counter IC. The three transistor circuit gracefully saturates, so that signals from 5 mV to several volts on the input result in a clean logic output. Since timing of the proportional encoder-decoder only depends on the rise times of sequential pulses, there is no penalty for changes in pulse width or shape. The resistor values were chosen for a 5v to 6v supply. 6v is shown in figure 5, but some servos may be limited to 5v maximum logic and supply voltages.

During the long sync clock pulse at the beginning of each frame, the charge on the 22nF capacitor increases to the threshold voltage, and the CD4017 resets to zero. The next rise time then begins the count sequence from 0 to 3, and then the next sync pulse. It does not matter where in the sync pulse the reset happens, so sync pulse length is not important.

Input from the radio control receiver is through a 220n capacitor. It is necessary to have the correct polarity of the clock pulses into the decoder. The receiver used for this development needs an inverter between the receiver audio output and clock input, so a 3 stage input amplifier was used.

Fig. 5 Decoder to Drive RC Servos

Fig. 6 Decoder Reset Timing

This encoder and decoder has been built and tested using a 20mW 50.800 MHz on-off keyed CW transmitter and an image-reject single conversion 50.800 MHz receiver with 455 kHz IF. Photographs and schematics of the Perma-Proto encoder and decoder boards, receiver and transmitter are on the following pages. The receiver provides reliable servo outputs with input RF signal levels below 1uV. Based on early measurements, this system is capable of reliable radio control of small boats out to at least several hundred meters. Operation on 50.800 MHz requires an Amateur Radio License, and is regulated by FCC Part 97 in the United States. Part 97 suggests a maximum transmitter power of 1w for radio control operation, which would permit operation beyond the visible horizon.

This encoder and decoder use classic basic timing circuits, and are minor variations on recent designs published on the web, for example the Proportional Radio Control description by Harry Lythall Amateur Radio call sign SM0VPO. There is some confusion as to whether Proportional Radio Control uses pulse width modulation (PWM) or pulse position modulation (PPM). It is exactly both. If you look at the waveforms shown in figure 4, clearly the signal sent to the servo has the information coded in the width of the pulse. But all of the negative pulses from the transmitter, for example in figure 2, are exactly the same width. So they represent a sequence if identical pulses with information in their position. The PPM description is particularly apt for the inverted train of identical pulses at the CD4017 encoder clock.

Proportional Radio Control Transmitter and Receiver

Fig. 7 Proportional Radio Control Transmitter with Encoder on Top

Fig. 8 Proportional Radio Control Receiver with Decoder on Top

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Bottom photo, new 50MHz RC receiver designed May 2019 using a recent IEEE published “IQ Mixer with Single SPDT Switch” image reject front end and TA7842 455kHz IF

Fig. 11 Prototype Construction 50MHz Transmitter used here

Fig. 12 Commercial Printed Circuit Version of the 50 MHz Transmitter

Fig. 12 New 50 MHz Receiver using IQ Mixer and 455 kHz IF