Andy's R.C. Information Page

Transmitters

Most Transmitter crystals represent the channel they are transmitting on; for example Futaba, Hitec and JR (Macgregor) Channel 69 transmitter crystal frequency is 35.090MHz.

Some manufacturers (Fleet and Multiplex for example) use a transmitter crystal at half the operating frequency and double it in the transmitter. In these cases the Channel 69 Transmitter crystal operates at a frequency of 17.545MHz. Other transmitters are synthesized and these do not require crystals as all channels are programmed in (Mobile phones can cause interference with this type).

The Golden Rule is::
Only use the transmitter manufacturers crystal in the transmitter!



Receivers

The receiver must receive the correct frequency while at the same time remove every other signal that its aerial picks up. To do this the receiver includes 'Filters' which 'filter out, or reject, all other frequencies. Having a fixed IF frequency, much lower than the original transmitted frequency, means that very tight and stable filtering is possible - Regardless of what channel you are on, the filter only has to work at 455kHz. The receiver is able to reject any frequency more than ±2.5kHz from that required. The IF, supplied as an output, is then amplified and fed to a 'Demodulator' - a device used to extract the information from the radio signal - in our case, turn it back into a form that the servos can understand.

There are two types of modulation systems in use. They are PPM (Pulse Position Modulation) also known as FM (Frequency Modulation) and the other type is PCM (Pulse Coded Modulation). PCM has built in fail safe system, that hold the servo settings if interference is detected. PPM is prone to interference and shows up as a glitch, causing the servos to move at random. If the interference continues you loose total control of the model.



Single Conversion Receivers

Intermediate Frequencies of 450kHz, 455kHz and 460kHz are used in single coversion Receivers. If you minus this frequency from the Transmitting frequency, you are left with the required Crystal frequency; for example Futaba receiver crystal frequency is 34.635MHz for Channel 69 proving an IF of 455KHz as 34.635 + 0.455 = 35.090MHz. This signal is demodulated and fed to the PPM Channel Decoder.

Different manufactures use different IF's; The Golden Rule is::
Only use manufactures crystal in a receiver!



Dual Conversion Receivers

The first Intermediate frequency used with the majority of Dual Conversion receivers is 10.7MHz and the Second Intermediate Frequency of 455kHz is used. To work out the crystal frequency we minus the 1st IF's from the frequency being received for example - Using Channel 69; 35.090MHz - 10.700 = 24.39MHz. Ignore the second IF in the equation as this is taken care of by using a fixed internal Xtal Oscillator of 10.245MHz. 10.7 minus 10.245 = 0.455 giving us the 2nd IF. This signal is demodulated and fed to the PPM Channel Decoder. Again, the Golden Rule is:: Only use the manufactures crystal in a receiver!



Glitches and Interference

Glitch is any type of interference that reduces your ability to control your model. This interference has many forms and levels of severity; it might be a slight hesitation to respond to a radio input, or a severe glitch that causes your RC model to crash. Hunting down the cause of a bad glitch can be labourious but most can be eliminated.

Firstly, check with a scanner that nobody else or thing is transmitting on your channel. If your channel is clear, then using the scanner ensure that you transmitter is on frequency, for example Channel 69 is 35.090MHz. You should hear a perring noise coming out of the scanner speaker if you are using PPM. If it is not on channel, replace the Transmitter Crystal.

Next do a range check, 30 paces with the transmitter aerial down. If the range is short, then it can be the transmitter or receiver at fault. Swap the receiver and receiver crystal first, proving which is at fault.
If the range is good, but still glitching follow this procedure until cured.

1. Ensure that all wires are run away from servos, batteries and the engine.

2. Wrap the receiver in protective foam as vibration will cause glitches.

3. Bring the aerial straight out of the fuselage, next to the receiver and pin it via an elastic band to the fin, ensuring that the aerial is not doubled back.

4. Check the battery voltage and all power cables for corrosion - this makes the wire black and is a poor conductor, causing loss of power when current is being drawn, ie when the servos are operated.

5. Ensure that you do not have any metal to metal servo or linkages. If you have to have these, the connect a length of wire to each connector, this will filter any metalic interfertence.

6. If Glitching occurs in a certain area, when flying, it is possibly microwave or 455KHz interferrence. This interfers with the receiver components. Wrapping aluminium foil around the reveier will reduce this interference.