Comments on Filter RF Harmonics With Additional PI Network
Parent
Filter RF Harmonics With Additional PI Network
I have a 900MHz radio with a reference design matching network. The radio feeds to a PCB antenna (PCB antenna hasn't been verified with a VNA, and may not be a great match; this is a project to now fix this system).
There are over the limit harmonics from the 6th harmonic (~5,400MHZ) to (~8,100MHz).
The trace width of the antenna feed are (ostensibly) 50 ohms based on the board stackup.
After the reference design matching network, we did put down an empty PI filter so I can add some additional filtering.
Since we are trying to keep the impedance matched to 50 ohms, what is the best strategy to approach this circuit?
Do you start with a low pass filter and then tweak the low pass filter to re-match as best as possible to 50 ohms?
Post
We got a copy of the hardware and took a first crack using a lowpass Chebyshev filter at 950MHz (radio transmits up to ~930MHz).
On the first pass, we got a .2dBm drop on the fundamental [margin of error in our test chamber] and a 12db drop on the most severe harmonic.
I don't know if the Chebyshev cutoff at 930MHz vs. 950MHz, is going to have a huge impact on harmonics coming off the system 3GHz+. I'm not going to investigate further since we got such a great and easy result on the first pass.
Because we don't have any issues with other harmonics of the system, until like the 7th harmonic or so, a low pass filter totally worked here.
I didn't layout the board, but I'm super glad the layout guy decided to add in an additional PI network for future filtering.
I've impedance matched a few RF systems using a VNA, and if you have the space, those extra three components are life savers.
I've always gotten rock star results (way above FCC limits) after tuning up our systems.
1 comment thread