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I am using the Quectel M95 GPRS Modem on my PCB. The hardware reference guide says "pin 39 is the RF antenna pad. The RF interface has an impedance of 50Ω."

I am using a u.efl connector and connecting a QuadBand PCB antenna to the connector. The antenna spec says impedance: 50 ohm.

Now I am confused. My layout designer is saying the manufacturer has to put a 50 ohm trace for the u.efl connector while manufacturing.

There is a Pi circuit on the PCB for RF tuning. How do I use that?

For now only a 0R is put there, but if tuning is required the values were to be put in accordingly.

If the antenna already has 50 Ohm impedance, why does the PCB trace also have be to 50 ohm?

enter image description here

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2 Answers 2

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When you send a signal along a trace, if the length of the trace is of the same order as the wavelength of the signal, then you have to match impedances or, you will get a "reflection". What this could mean in extremes is that the power actually emitted by the antenna is only a fraction of the power being pumped out by the chip - in other words, you are not efficiently coupling the chip to the antenna and you get "reflections" - this can cause your chip to overheat and maybe it might damage it.

So, if you are transmitting 3 GHz (say), it has a wavelength of 100mm and, a rule of thumb is that if your PCB track is longer than 10mm (one tenth the wavelength), you need to ensure its characteristic impedance matches the antenna's impedance.

Further reading here and here

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  • \$\begingroup\$ Thanks Andy, In other words I need that 50-Ohm impedance matched trace from the Pin's output to Connector P1 irrespective to the Antenna having the 50 Ohm Impedance. Correct? \$\endgroup\$ Dec 23, 2015 at 11:39
  • \$\begingroup\$ Dimensionally (looking at your picture and noting that you might operate at close to 2GHz), it's a close call but it barely costs a penny in copper to thicken the track to make its impedance right - there are online calculators that can be used making life easy! If this design was a 433 MHz circuit I wouldn't bother but you have to consider your max frequency. \$\endgroup\$
    – Andy aka
    Dec 23, 2015 at 11:42
  • \$\begingroup\$ I am connecting a PCB Antenna to the u.efl connector of the pcb \$\endgroup\$ Dec 23, 2015 at 11:55
  • \$\begingroup\$ The cable that feeds the actual antenna will have a characteristic impedance of 50 ohms just like the actual antenna so it makes sense to continue this philosophy onto the PCB but having a short length that isn't 50 ohms isn't a big deal if it's short. For instance the track will have to nip-down in width to feed a chip at some point and this could never be 50 ohms but it doesn't matter. \$\endgroup\$
    – Andy aka
    Dec 23, 2015 at 15:10
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    \$\begingroup\$ @user2967920 It's not "irrespective" of the fact that the antenna has 50 ohm input impedance. The output amplifier impedance, trace / feedline characteristic impedance and antenna input impedance all need to be the same as each other, or if they are not then they need to be matched with a suitable matching network or balun. Andy is completely right that over very short distances this can usually be disregarded. \$\endgroup\$ Sep 30, 2016 at 19:25
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I know this is a very old question, but whomever is interested, I actually think there is one important thing wrong in the previous answer. It was assumed that the speed of light within the PCB track is the same as in vacuum, which is not the case. If we take FR4 (7628), its relative permittivity is ~4.5, thus refractive index will be 2.12 (how many times slower the speed is in another medium, which is sqrt of relative permittivity). This changes the wavelength of e.g. 2.45GHz signal to 57.7mm (wavelength = speed / freq). 1/10 of that is around 5.8mm.

Meaning, if the transmission line is longer than 5.8 mm (or better, longer than 5 mm, since different thickness boards will give slightly different relative permittivities), then a matched network needs to be added, otherwise a matching network will only add loses.

UnexpectedMaker has fun series to watch about how he tried matching his TinyPicos on Youtube, which might give a bit more insight into how to do the matching and what happens when you make a poor design...

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