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Radio Controlled Systems | Back to Article Index

By Stan Yeo

Often when talking to modellers it becomes apparent that a surprising number do not understand the basics of a radio control system, how it works or what is compatible with what. Whilst I readily accept that this is not essential to the enjoyment of the hobby a lack of knowledge can and does restrict the modeller when buying supplementary airborne equipment. Fortunately though this lack of knowledge means the modeller errs on the safe side when making these purchases. The purpose of this article is to explain in simple terms the basics of our current radio control systems and the meaning of some of the most common buzz words.

The System

As we all know the system comprises of a transmitter which encodes and transmits the control information. A receiver which receives the transmitted signal, decodes it and allocates the information to the respective servos and the servos which move the control surface (throttle) to the desired position.

1. The Transmitter

The main modules to the transmitter (Tx) are the Encoder and the Radio Frequency module (RF section) which includes a Power Amplifier (PA) and a 'Mixer'.

The Encoder

The Encoder takes the position of each channel control stick or switch in turn and converts it into 'digital' information. This can be a pulse of a certain length (time) (PPM) or a binary number (PCM). It does this approximately 50 times a second (50Hz). So that the receiver (Rx) knows when one cycle of information is complete and another cycle starts the Encoder inserts a synchronisation pulse which 2 to 3 times longer than a normal pulse between each sequence of pulses (PPM). PCM uses a different system which varies between manufacturers.

The RF Section

The purpose of the RF section is to transmit the encoded information to the receiver but to do this it must first generate a radio signal at the correct frequency and mix it with the encoded information from the Encoder. It must then amplify the signal so it is strong enough to reach the Rx. Consequently the RF section has three sections, a Crystal Oscillator to produce a signal at the desired frequency. A Mixer to mix it with the signal from the Encoder and a PA (Power Amplifier) to increase the signal strength to the desired level for transmission.

2. The Receiver

Like the transmitter the receiver has a number of sections and is almost the mirror image of the Tx. There is an RF section sometimes known as the 'Front End' to receive the incoming signal from the Tx, a Crystal Oscillator, a Mixer, an IF (Intermediate Frequency) Strip, a detector and a decoder. The RF section is tuned to receive the signal from the Tx whilst the Crystal Oscillator (local oscillator) produces a similar signal but at a lower frequency (455Khz lower), to be mixed with the incoming Tx signal. The difference in frequency between the two signals is then passed to the IF Strip. The IF Strip is a filter which will only allow signals of the IF frequency to pass through it. With single conversion PPM receivers this frequency is 455 kHz which is the difference between transmitted signal frequency and the local oscillator frequency assuming of course that both are operating on the same channel number! After the IF Strip the signal is then rectified i.e. converted to a DC signal similar to that of the Encoder and passed to the Decoder. The Decoders job is that of a postman's i.e. to post each piece of channel information to the correct output channel socket for onward dispatch to the servos. It must do this 100% accurately every time and irrespective of the number of channels the Tx or Rx has such that an 8 channel Rx will work with a 4 channel Tx and vice versa etc. It is aided in this by the synchronisation pulse inserted by the Encoder which tells the Rx when one chain of information is complete and another is about to start. The effect of this pulse is to tell the Decoder counter to stop counting and go back to zero again.

System Variations

Transmitter

There are two types of transmitter, one involves pulsing the transmitter (AM) i.e. switching the output on and off a bit like Morse Code whilst the other involves 'swinging' the Tx frequency (FM). AM refers to Amplitude Modulation whilst FM stands for Frequency Modulation. Most r/c equipment on the 27Mhz band is AM whilst all current 35Mhz sets are FM. An FM receiver is different from an AM receiver and neither will work with the others transmitter. AM transmitters are all PPM (Pulse Proportional Modulation). PCM (Pulse Coded Modulation) transmitters are generally switchable between PPM and PCM modes.

Receivers

There are four main types of receiver, AM - PPM for 27Mhz equipment, FM PPM Single Conversion, FM PPM Dual Conversion and PCM which can be either single or dual conversion. Needless to say with so many options and so many different makes there is the inevitable confusion, most of which seems to centre around the dual and single conversion PPM system. Most modellers accept that an AM Rx will only work with an AM Tx, likewise it is accepted that to drive a PCM Rx you need a PCM transmitter of the SAME make as the receiver. The reason for this is that with PCM there is not as yet a common encoding standard as there is with PPM. A bit like the British and American television standards which are incompatible.

The main difference between a single conversion receiver and dual conversion receiver is the dual conversion receiver has two crystals whilst the single conversion receiver has only one. Theoretically a problem can arise with a single conversion Rx if there is another transmitter operating on a frequency which is a harmonic away from the IF frequency of that receiver. A harmonic of the IF frequency would be 910Khz (2 x 455) or 227.5Khz (455 / 2). Now 227.5Khz is almost the difference in frequency between Channel 60 and Channel 83 which we know to be 230Khz (23 x 10Khz). The same would apply to channels 61 and 84 (either way), 62/85 and 63/86.

In a dual conversion receiver there are two crystals, the one we change to change channels and a fixed crystal. The idea is that by reducing the frequency of the incoming signal down to the IF frequency in two stages we can increase receiver selectivity and overcome the theoretical problem associated with single conversion Rxs mentioned above. The first crystal (the one we plug in) produces a signal between 24 and 25Mhz whilst second produces a 10.7Mhz signal to be mixed with the difference of the incoming signal and the first oscillator signal i.e. 35Mhz - (24-35Mhz). The difference in these two signals (455 or 470Khz) is then fed to the IF strip as before. Because Dual Conversion receivers are more complicated than single conversion receiver they are more expensive. Also as a rule Dual Conversion Rxs need to be fitted with cyrstals of the same make as the Rx which is not always the case with single conversion Rxs.

The Myths

The main myth is that you can only operate Dual Conversion receivers from a 'Dual Conversion' Tx and a Rx similar to the one that came with the set. This is simply not true. There is no difference between a PPM Tx that came with a Dual Conversion Rx and one that came with a Single Conversion Rx. Often a retailer will swop a dual conversion Rx for a single conversion Rx to reduce the total cost of the set. A point worth checking when you see a set advertised at less than you would expect it to be!

Another misapprehension a lot of modellers are under is that different brands of PPM Rxs will not work with other brands of Tx. In fact there are a number of receivers on the market that do not have accompanying transmitters Webra and Jeti are two that come to mind. Likewise all the leading brands of servos will work with all the leading brands of R/C equipment providing they have the appropriate leads. I personally use a JR388 transmitter with mainly Hitec airborne equipment. With some single conversion Rxs there is even some compatibility with different makes of crystals i.e. Futaba Hitec and JR. If mixing and matching I would strongly advise a full range / compatibility check before flying. Range checks should be carried out in a controlled manner i.e. a comparison made between a receiver known to be operating as expected and the new one.

Synthesised Transmitters and Receivers

In synthesised radio control equipment the crystal is replaced with a synthesiser which allows the operator to change to any frequency in the 35Mhz band without recourse to crystals simply by selecting a new frequency on the transmitter and then retuning / changing the crystal in the receiver to the new frequency. If the receiver is synthesised the normal procedure is to switch on the transmitter close to the receiver. Switch on the receiver and activate channel scanning. The receiver will then lock on to the strongest signal which should be the parent transmitter. After tuning the Rx will stay on the new frequency until retuned to a new one. A synthesised Tx can used with a crystal Rx and vice versa. A synthesised module in the Tx or Rx is completely independant of the other. Using a synthesised transmitter does place a greater responsibilty on the operator to ensure that they know the channel number they are operating on. There are some Txs that can be fitted with a channel checker that prevents the Tx being switched on if there is another transmitter transmitting on that frequency. A few transmitters have a double switch on procedure i.e. on first switching on the Tx displays the transmitting frequency but does not start transmitting until a second switch is activated.

Summary

I hope this article has been of value and more importantly easy to understand. I have kept the explanations as simple as possible as we do not need to know how it works just the basic concepts so we can make informed decisions when buying new equipment.

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