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Q&A

Filter RF Harmonics With Additional PI Network

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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?

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7 comments

Something doesn't make sense. Why do you care about harmonics at the antenna level? Surely your tuner will filter those out anyway. As long as the antenna resonates well and has the right impedance at the desired frequency, why do you care how much it picks up harmonics too? The first tank circuit in the tuner should squash those pretty well. Olin Lathrop‭ 26 days ago

I guess question #1 here is: is this a simplex or duplex radio? Any antenna switches etc present? Lundin‭ 25 days ago

For me, a schematic (or even a basic shema) would help. coquelicot‭ 25 days ago

We care about the harmonics coming out of the product because they are over the FCC limits! There's no antenna switch, it's a reference design LoRa radio (there's a lump of passive components to tune to 50 ohms out of the chip, and then I've got an empty pi network now to fix these harmonics). I'm just trying to see at a high level how you tackle filtering the harmonics while maintaining the existing matched network. I don't think the solution is super binary. leroy105‭ 25 days ago

You originally said this was a radio. Now you say it's a transmitter. Which is it? Olin Lathrop‭ 25 days ago

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2 answers

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Since we are trying to keep the impedance matched to 50 ohms, what is the best strategy to approach this circuit?

A low pass $\pi$ network seems a good route to go. Basically your $\pi$ network is designed to do two important things: -

  • Impedance match a 50 Ω source to a 50 Ω load
  • Provide significant attenuation above 900 MHz for the reduction of harmonics.

The example below matches to 300 Ω but, the calculator allows you to enter both ends as 50 Ω: -

Low pass pi matching

Picture and calculator can be found here.

So, plugging the numbers into a simulator we get this circuit: -

simple low pass pi filter

And we get this AC response: -

simple low pass pi filter response

So, at 915 MHz we get the usual -6.021 dB input to output attenuation when you have matching input and output resistors (50 Ω) and, at the 3rd harmonic of 915 MHz we see an attenuation of 27.7 dB or, a relative attenuation of about 21.5 dB. Clearly that is very significant and, it's going to be even better at the sixth harmonic (around 40 dB I estimate).

But, remember, that at these frequencies, it's easy to get harmonic leakage to the output with badly chosen inductors and capacitors that are unsuitable for this application so, choose with care. Layout with care also.

If you need steeper roll-off above 915 MHz then the beauty about the $\pi$ network is that they can be cascaded (because they are impedance matching networks and interactions are minimal). So, here's a five stage example of a $\pi$ network schematic (operating at 10 MHz): -

PI cascaded

And here's the frequency response: -

PI cascaded simulation

It's got perfect 50 Ω input and output impedance at 10 MHz and has rolled-off the 2nd harmonic (20 MHz) by around 70 dB.

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14 comments

What's the reason you'd pick a PI low pass over a Chebyshev one? I thought Chebyshev were ideal since they give a much steeper dip after carrier? Lundin‭ 21 days ago

@Lundin if the simpler pi filter provides enough attenuation then no need to go to a more sophisticated filter. However, you can absolutely stack end-to-end pi filters to get both impedance matching and steeper roll-off..... Andy aka‭ 20 days ago

... If you look at the link to the calculator in my answer and scroll down I've made a 5 stage filter and that has a monstrous and slope here. Andy aka‭ 20 days ago

Just curious since my experience (I'm definitely not a RF designer) is that PI filters are mostly used for general EMC, like when you need to block emissions over a very broad frequency range. But harmonics are specific and you know where they will be at, so why not just get rid of them. Lundin‭ 20 days ago

In this case it is LoRa, which is supposed to use spread spectrum to compensate for high output power, so I don't get why the harmonics are that prominent. It's max 14dBm carrier if I remember correctly. Lundin‭ 20 days ago

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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.

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3 comments

Good result Leroy Andy aka‭ 14 days 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 14 days ago

High Q passives -- no real improvement. Within the margin of error in our reading in a chamber. leroy105‭ 6 days ago

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