Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

Comments on Filter RF Harmonics With Additional PI Network

Parent

Filter RF Harmonics With Additional PI Network

+2
−0

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?

History
Why does this post require moderator attention?
You might want to add some details to your flag.
Why should this post be closed?

1 comment thread

General comments (7 comments)
Post
+2
−0

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.

History
Why does this post require moderator attention?
You might want to add some details to your flag.

1 comment thread

General comments (3 comments)
General comments
Andy aka‭ wrote almost 3 years ago

Good result Leroy

leroy105‭ wrote almost 3 years ago

@ Andy aka - I'm ordering some high Q components to see if we can get that 12dB drop to increase a little more. I have some folders of some rando. low grade passives for tuning work. I did a few passes tweaking some values and the filter response is still not exactly what a calculator would say, but we are getting a better result being a bit off calculation. (I'm a very practical RF engineer... I'll take the empirical over paper calculations any day of the week!).

leroy105‭ wrote almost 3 years ago

High Q passives -- no real improvement. Within the margin of error in our reading in a chamber.