Performing FEI on an RFM69HCW without preamble

[davethomaspilot]

First time post ...

I'm exploring using the RFM69HCW as a replacement for an obsolete part from a Linx 915 Mhz transceiver. 

I have no flexibility to redefine the message formats, since the design must be compatible with equipment that's been in the field for 20+ years.

The messages have no preamble --as defined by the RM69 spec (010101 ...), but I've got things working using the RFM69 in continuous mode and a bit bang serial interface. 

But, the FSK signal is narrow band and I'm worried that frequency accuracy and/or drift may be an issue.  The LINX design calibrates its frequency by doing an FEI by temporarily setting a wide bandwidth and capturing the FEI measurement when the incoming data is a logic one.  Many samples are taken, and a frequency correction is determined from the captured samples.

I can find no detail on how the RMF69 does FEI, other than "The operation must be done during the reception of preamble".

The calibration needs to work with a predefined message that does NOT have a preamble.

But, I'm wondering why the operation must be done during a preamble.  Maybe it's just necessary for both both a logic one and logic zero (carrier + freq Dev, carrier - freq Dev) to be observed?

Or, maybe there needs to be the same amount of logic 1 time as logic zero time--maybe the average of the two frequencies is used to calculate the frequency error.  But, if that's the case, perhaps by knowing the number of 1's and 0's in the incoming message,  a modified frequency correction can be calculated using the FEI from the RFM.

Any insight on how the RFM actually does the REI would be greatly appreciated, or where to ask this question.

Also, what frequency accuracy can I expect with the RFM69HCW--or, what is the crystal oscillator part that is used.

Thanks!