Other Parts Discussed in Thread: LMX2572
Hello TI Team.
We have developed a VHF and UHF FM transceiver that uses the LMX2572LP for frequency generation. This has worked well, and we have approved the devices with 500 mW transmit power. The operating range of the devices is from 140–175 MHz and 410 MHz to 470 MHz.
Now we had the idea of turning it into a 6-watt device. However, there is a problem with spurious emissions.
We use direct digital FM modulation
with a sampling rate of 153 kHz. These 153 MHz appear in the spectrum around the carrier.
In the VHF range, these spurious emissions are very small and hardly visible, but in the UHF device they are clearly noticeable. In the middle of the band at 400 MHz, they would be within the permissible range, but at the band limits at 440 MHz and 470 MHz, they are too strong for approval because they exceed the required -36 dBm at a transmission power of 6 watts.
We now have the question of why these spurs are so frequency-dependent and what we can do to reduce them. The loop filter is the one from the evaluation board and we are using the lowest charge pump current. With higher charge pump currents, the situation gets worse.
Best regards.
D.W.
