Originally posted by hilleja
In a dual conversion RX there are basically 2 separate circuit combinations, the crystal reacts to 2 separate frequencies interpreted by the circuits. These two circuits are then compared with each other to arrive at a concensus of the center frequency the RX is sensitive to.
There are currently 3 different modulations being used in most RC radios: AM FM and PCM(pulse code modulation -- information is interpretted by measuring the width of signal pulses)
In dual conversion they don't work like that, there is no concensus comparison. An Rx generates its own internal frequency, controlled by the crystal. It's a little bit different to the incoming frequency from the Tx. The two waves are mixed together, producing a third wave which is much lower frequency, being the difference between the Tx and Rx's own frequency. It's the same effect that makes the wah wah noise of two engines on a twin that are not quite at the same revs. That wah wah, called the Intermediate Frequency or IF is typically 455khz in radios, compared to the many Mhz of the transmitted wave, but it still contains all the data. That low frequency is much easier to amplify then filter off than the high Tx frequency. That mixing of the two frequencies, one from the Tx and one from the Rx, is the single conversion.
The snag is, an incoming frequency of 455kHz above, and 455kHz below the Rx's own internal frequency will both get through. One of them is your Tx, and one is an image frequency from some other transmitter that is 0.9Mhz away from your Tx. Also there can be images from half and whole multiples of your frequency. your Rx sees them as being perfectly valid IFs. Dual conversion stops this by doing the mixing process twice. The Rx generates two internal frequencies. The first is mixed with the incoming frequency to make an IF usually of 10.7Mhz, which is then mixed with the second internal frequency to make an IF of 455kHz. So it does nothing to block interference on your frequency, what it does is block images caused inside the Rx by frequencies quite well removed from your own.
Your radio has only 2 modulations. AM or FM. PCM is not a radio modulation, it is a method of data encoding and is nothing to do with radio per se. We use 2 methods of data encoding, PPM or PCM. We could send these down a wire, or switch a light beam on and off etc. We choose to send by radio, so the PPM or PCM data is impressed onto a radio signal by means of AM or FM. Your radio is one of: PPM AM; PPM FM; PCM FM. You could make a PCM AM if you really wanted to but no-one bothers. In American modelling it is very common to talk about FM when you really mean PPM which is why you often run into confusion. Your PCM radios are FM. You can not choose between PCM and FM because they are different things. You can choose between PPM or PCM, and between AM or FM.
PCM is not interpreted by measuring the width of the pulses. PPM uses the analogue time length, often referred to as the width, of the pulses. PCM is binary encoding and is interpreted simply as "is it on or off?" as the data is sent as a binary number in a series of on-offs.
Harry