Hi folks.  I am jumping in because this looks interesting and may be something I want to look at more closely.. I want to comment that I am not clear on the understanding of F1/F2 as expressed in this thread.  I do not see anything in the datasheet that says F1 is the starting frequency and F2 is the end.  From what I see, either F1 or F2 can be that starting frequency. (example in figs 34, 35).

Thanks in advance for educating me.

Ken

KC6PUQ

On Tue, Mar 16, 2021 at 4:28 AM Bill Somerville <g4wjs@...> wrote:

On 16/03/2021 11:16, Bill Somerville wrote:

On 16/03/2021 11:04, Bill Somerville wrote:

On 16/03/2021 10:58, Andy TALBOT wrote:

OK, I now understand the process and can see a way forward in a PIC, sampling at a rate perhaps 32 or 64 times that of the symbol period
From the look of the curves for BT=1 and BT=2, at any one time only the contribution from two pulses need be taken into account.  The contribution from pulses further away are too small to contribute when using a table-based approach to generating the GFSK waveform.

The next stage, when time permits,  is to write some code for a PIC that will sample at 32 or 64 times symbol rate, which is less than 1kHz even for the fastest FST15 mode, then send the results to a 48 bit DDS.
 It needs to be a 48 bit device - I use the AD9852 - as a normal 32 bit one is too coarse to give the correct increments at the slowest modes.

If I'm using contributions from two adjacent symbols, the curve stored in the table need only cover half the entire shape as the second half is a mirror image.  That will possibly,simplify the table lookup process.    I'll be using an 8 bit table, so frequency during the shift  will be quantised and I don't have the mathematical nous to calculate the effect that may have on spurii.

Andy   G4JNT

Hi Andy,

the AD9852 appears to directly support smoothed FSK transitions using what it calls ramped-FSK, I would have though that is a better approach that will avoid possibly noisy small frequency steps between symbols. The data sheet says that even though the ramp function is linear, it may be adjusted on the fly for more complex transitions.

73
Bill
G4WJS.

Hi Andy,

having suggested that, I am not sure how feasible it would be to generate ramped-MFSK using the FSK capabilities of that DDS since you cannot ensure that F1 is always lower than F2 without sometimes having to change either F1 or F2 while it is being used by the DDS. I say this because ramped-FSK has a requirement that F1 always be lower than F2. Perhaps it is a non-starter :(

73
Bill
G4WJS.

Hi Andy,

OTOH you may be able to set F1 equal to F2 (or vice versa depending on the last frequency transition direction) mid-symbol then flip the FSK input when; you need to exchange F1 and F2 to shift down or up (depending on the last frequency transition direction) to a new frequency for the next transition. That way you can do arbitrary frequency transitions while always keeping F1 lower than F2.

73
Bill
G4WJS.