Other Parts Discussed in Thread: LMX2820
Good afternoon
I'm using the LMX2820EVM in a feedback mixer configuration. The RF output frequency is 2.2GHz and I have a mixer supplied by a 1.8GHz LO to mix down to 400MHz IF. There is a low pass filter on the output of the mixer to reject any products other than the wanted.
The 400MHz IF signal is fed into the PFD input of the eval board, and the LMX2820 is configured to bypass the N divider, with the PFDinput divider set to 2 so that my 400MHz IF frequency after division matches the 200MHz PFD.
1. Do I need to 'calibrate VCO' when the feedback path is set? I have tried this and my synth loses lock. All the RF levels at the inputs to the EVM are within specification
If I run the eval board normall, 2.2GHz output, using the internal N divider then the eval board locks to 2.2GHz. its when I switch to PFD in and re run the cal that I lose lock.
My feedback signal is taken from the RFOUTAN output (usually terminated with 50R). The LMX2820EVM is configured as shown below:
I can't see why this shouldn't lock, I haven't spotted anything in the datasheet to say otherwise. I haven't changed the loop filter component values from the default setting (for 6GHz) so could this be the issue?
Many thanks
Jonathan