Hello

I have designed two PCB's which utilize modem I2Cs to transmit signals at 700Mhz and 900Mhz respectively. I have UFL connectors which connect 50ohm impedance aerials to the PCB. The UFL connectors are connected to the output of the I2Cs and ground.

I have positioned the UFL connectors <10mm away from the I2C outputs and have a PCB trace connecting the output to the connector. Using this calculator (https://www.eeweb.com/tools/microstrip-impedance) and specifying to my PCB manufacturer the following paramaters:
Copper weight = 1oz
PCB thickness = 0.8mm
PCB material = FR4 TG130
and assuming Er = 3.5

I have calculated a trace width of 1.76mm to maintain 50 ohm impedance. This is somewhat hard to do given the output pins of the chip are ~1mm in width. Many aerial manufacturers call for matching circuits to be implemented on the aerial input, but the LC values will obviously differ across different PCB designs. I do not own a VNA so verifying that the impedance will be correct after manufacturing will be difficult.

I have read on various websites that provided the PCB trace length is <1/20th of the wavelength (700Mhz = 429mm & 900Mhz = 333mm), then the PCB trace impedance is not that critical? Is this true?

If this is the case, I will try to make the PCB trace 1.76mm wide but keep the length to <10mm and not implement a matching circuit on the aerial input.

 

I2C is limited to 3.2MHz, how are you getting 900MHz? Are you using a transmitter circuit and modulating it with the I2C signal?
Please show the schematic.

 

I2C is limited to 3.2MHz, how are you getting 900MHz? Are you using a transmitter circuit and modulating it with the I2C signal?
Please show the schematic.

Schematic is located here https://imgur.com/a/ccrckZp

This is one of the pcbs. Using a ublox chip to communicate to a narrowband network. A similar one has been designed using a different chip.

 

Hey @rfwifi123,
Just an FYI -- those microstrip trace calculators only provide estimates. You should use a 3D simulator for better results. Better yet, contact the board house you are going to order it from and have them do the math for you.
Mark

 

Thanks for the replies. I take it it’s still critical to get the characteristic impedance of the trace right then, no matter how short it is?

 

@rfwifi123,
The reflections happen when the impedance changes. As you're moving from the PA to the antenna, every change in impedance reflects energy back, decreasing your emitted power. Now, if you're dealing with high-power applications -- that reflected energy can cause heating that destroys the amplifier. With your application, you'll just not transmit as far.
You also might want to check the datasheet for your radio IC -- plenty of sources have complex impedances -- it probably is not the 50 Ω you are designing to, but something along the lines of (46-4j)Ω or other such nonsense.