Other Parts Discussed in Thread: LMX2571, LMX2571EVM
I'm using an LMX2571 as a synthesizer driven by I2S audio. We designed and produced a board with a TI CODEC that uses a digital mic as input and outputs I2S to the LMX2571. We can program the LMX2571 registers and get good NBFM audio on our intended frequency (Fc), but also have good audio on frequency multiples of 130 Khz above and below. For example, if my Fo is 100.000 Mhz, I will have a good signal there and at 100.130, 100.260, 100.390... and 99.870, 99.740, 99.610 as well. These sidebands appear to cover about 3% of the intended carrier frequency. For example, for 100 Mhz intended operation, I have these 130 Khz modulated spurs to about 1.5 Mhz below and 1.5 Mhz above the 100 Mhz center and then they drop off sharply in amplitude. Note: these undesired sidebands are equal in strength to our desired carrier.
We're using a 20 Mhz LVCO as our OSCin and have probed for 130Khz on the board, but found nothing. Any ideas would be appreciated.
Using the same register settings on the LMX2571EVM, we have no problem obtaining a clean carrier with no undesirable sidebands.
Thanks,
Steve
