Thank you for your response.
OK, I understand the converter bit, and why I would need it. With that understanding I feel I would be better off with the FrSky X9R Pro radio. The reason I did not go for that unit in the first place is because I could not find a receiver for it that was PPM or PPM-SUM capable. But, if I am still not able to find a PPM SUM receiver for the X9R Pro I could just get a straight forward PWM receiver and buy a converter for it correct?
On the topic of transmitter to receiver compatibility, I am sure I don't need to purchase a FrSky receiver to match the X9R Pro. It must be compatible with at least a few other brands. But what spec do I look for that would tip me off that any particular receiver was a good match for the X9R?
Thanks again for your quite valuable help