ACWWong
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hi aaron,
just tried to read this topic... and am quite confused as its gets really cryptic when a load of the posts going missing....
anyway the most simple way to do this MSK transmitter (and what looks like you are talking about in your original post) is by an IQ modulator (SSB upconverter). The VCO in this case only need to be a CW LO signal (locked by standard PLL at the carrier freq), although you need a quadrature LO for the IQ modulator. The quadrature (hilbert transform) can be divide by 2 if the VCO is at 2*freqtx, or by polyphase filter or by quadrature VCO. The baseband IQ representing your 2Mchip/s MSK can be generated by a digital modulator followed by a DAC to get the ±500kHz analog IQ signals. The "image" rejection depends on your IQ phase and amplitude mismatches in both LO IQ path and baseband IQ paths... 20dB is pretty easy to meet though.. The quality of the transmission will be degraded by the finite "image rejection" and for that matter the carrier suppression (caused by DC offsets) and other spurious tones (due to crosstalk and non-linearity etc.) in the IQ modulator, but the limiting factor i think for the quality of the MSK transmission will be be noise of the final signal caused by PLL jitter (with additonal noise/error due to IQ modulator, PA, DAC etc.)
The MSK transmitter can be also be implemented using a loop modulator... i guess these are the architectures you are referring to when you say "direct VCO modulation". They use a fractional-N PLL in which the data modulation is applied to the feedback divider via a delta-sigma modulator, with the channel number. For PLL BW must be wide enough to follow the modulation, which causes a problem due to the PLL integrated noise will be high due to the wide BW. The get-out here is to use two-point modulation.... Anyway the advantage of the loop modulator techniques is that it can be lower power (no need for quadrature LO or IQ modulator), but is as you say its a bit more "complicated" as an architecture compared to standard IQ modulator approach.
cheers
aw
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