RE: Dual Conversion crystal PPM Rx
PCM mode is only operational with like original equipment. This means only a Futaba PCM RX will work with a Futaba TX in the PCM mode. The code is proprietary and does not work with any other PCM equipment...
For the most part you'll have to use a crystal that meets the requirements of the specific RX in use, single/dual conversion is a secondary consideration but an absolute must. Crystals are not generally interchangeable from one manufacturer to the next. The crystal does not differentiate between PPM/PCM modulation modes, this true at of the TX and RX.