Wednesday, March 26, 2008

Why is my PPM Transmitter not on frequency? -- Piet le Roux

We have had a lot of reports of interference on 35MHz lately. One suggestion to avoid planes being "shot down" was to get a radio scanner or frequency monitoring equipment to detect the interference before it becomes a problem. With the introduction of relatively cheap frequency measuring devices and scanners, it has become more and more common to find these devices at flying fields. It is a good idea to check your radio's frequency regularly but with basically only two service centers in South Africa it is not practical to send it away every time so it is great that more clubs can have access to suitable equipment. But users must note that they will display the frequency of a positive shift PPM (pulse position modulation) transmitter about 1,25 KHz low in frequency although its is perfectly on frequency. All JR equipment and most other makes in South Africa uses positive shift FM for PPM transmissions.

Both PPM and PCM information are transmitted via FM so in order to understand the reasons for this incorrect reading we must first understand the basics of FM (frequency modulation). When information is transmitted via FM the frequency (that is the frequency on your crystal) is varied in accordance with the data signal. If the data signal were a perfect sine wave the transmitter would be on frequency when the data signal crosses the zero point and at maximum deviation (1.8 KHz) at the peaks of the sine wave. The negative side of the sine wave would cause deviation in the opposite direction as the positive halve of the signal. We would the end up with a radio signal that varies +- 1.8 KHz around our channel frequency and have a bandwidth of 3.6 KHz. When this signal is measured the average frequency measured would be the center frequency (crystal frequency) because the two halves (positive and negative halves) of the data signal are equal and thus also the amount of positive and negative frequency deviation of the radio signal. This is how a PCM (pulse coded modulation) is transmitted and the frequency of a PCM transmitter would be displayed correctly.

Most transmitters these days can transmit both PCM and PPM, in order to achieve this flexibility the modulator has to do a balancing act in order to keep the transmitter inside its allotted channel. It does this by Making the zero point of the PPM data signal - 1.8 KHZ instead of the center frequency like with PCM. The PPM data signal consists of short positive pulses. The peaks of these pulses would cause the radio signal to deviate to + 1.8 KHz from the center frequency. Because the data signal consists of short positive pulses and a long reset pause(where the transmitter stays at -1.8 KHz ) the radio signal would spend most of its time in the negative halve of it allotted channel and thus the average frequency would be about -1.25 KHz from the center frequency and this is displayed by most measuring equipment.

To cap. If you want to check the frequency of your PPM transmitter, set one of your unused model programs to PCM mode and select this model when you want to check your frequency. If your transmitter does not support PCM just remember that your frequency would be displayed as about 1.25 KHz low. NB. Remember that only technicians that have been authorized by ICASA can make adjustments to transmitters. So if you think your transmitter is faulty send it to the nearest service center and do not attempt to DIY!

1 comment:

Piet Le Roux said...

After doing more testing with both digital and analogue measuring equipment I have concluded that the reading of a positive shift PPM signal appears to be normally about 1.18 KHz low in frequency.

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