biff44
Advanced Member level 6
- Joined
- Dec 24, 2004
- Messages
- 5,047
- Helped
- 1,376
- Reputation
- 2,748
- Reaction score
- 1,056
- Trophy points
- 1,393
- Location
- New England, USA
- Activity points
- 37,908
In my understanding, GFSK exactly means filtering of the originally binary FM signal with a Gauss filter. The datasheet doesn't tell about the involved filter, but to deserve the name Gauss filter, it should be at least 2nd order in my opinion. I don't know if it's performing better, but ADI ADF7023 is showing a 300 kbps GFSK spectrum in the datasheet, you should compare to your results.Unfortunately, all GFSK appears to mean to TI is to lowpass filter the analog control voltage going to the VCO tune line.
The reason, why GFSK uses a gaussian filter with a soft frequency characteristic is in it's time domain behaviour. A steep ("brick wall") filter would cause considerable time domain distortions respectively bit errors. As long as you don't implement a modulation method involving multi-bit symbol coding, you won't achieve a smaller transmission bandwidth than the said chips have. As I said, I'm not completely sure with the CC1101, but the all digital modulation of ADI chips is very likely at the theoretical limit.I am talking about filtering out the TX to a "~ brick wall" bandwidth
I was also going to revisit the GFSK vs 2FSK modulation. If 2FSK is more constant-envelope, then I would have less 3rd order spectral regrowth (maybe 2FSK is actually smaller bandwidth for 2nd adjacent channel suppression purposes).
Biff
as FvM said the spectral regrowth has nothing to do with modulation sidelobs.
Both FSK and GFSK are constant envelope (infact nonlinear PAs are widely used to have high efficency with this modulations), so don't expect to have lower regrowth with FSK respect to GFSK.
You idea is interesting: be aware of choosing Transceiver freq and external PLL freq to avoid image.
Mazz
Two dominate items effect broadband noise floor. The GMSK DAC modulation filter typically only shapes the primary modulation. The ultimate selectivity of the baseband filtering will determine DAC higher frequency attenuation. Second item is sideband noise performance of sythesizer. The best filterless design produces -165 dbC/Hz at 400 kHz offset. This meets GSM spec.
You may be able to find a 800/900 MHz GSM transceiver that will give you what you need.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?