PCB RF trace questions

vector86 · 02-27-2007

I'm designing a telemetry radio, the data sheet for the tranciever I'm using says to make the board traces for the antenna connector like this:

my question: What are the capacitors for? I'm assuming the traces are arranged like this to try to block noise from the rf signal.

P.S. I've been lurking for a while and this is my first post!

Thorn · 02-27-2007

Close. They block DC.

If you don't know that and you're designing a whole telemetry radio, you're probably in way over your head. I'd suggest getting some good books on RF design and reading for a while.

vector86 · 02-27-2007

I was talking about the traces blocking the rf signal. I've already designed a board using Aerocomm's tranciever module, but I wanted to see if it could be made even smaller by using the 1x1 system in chip. I'm a 3rd year computer engineering student, I know capacitors block DC. I just didn't know that there would be any offset in an RF signal. To be honest, I don't take that class until next semester.

Thorn · 02-27-2007

OK, as long as you're taking that class.

Yes, you can have DC in there. By placing the caps in the circuit you allow the AC/RF signal to be passed out to the antenna, yet block and DC from going out. It also works on the inverse; any DC at the antenna is blocked from getting back into the radio.

vector86 · 02-27-2007

Looking at it more closely, it looks like a bandpass filter. Any idea how to calculate what value caps I need? The data sheet makes no mention of what kind of impedance the rf port has.

wrzwaldo · 02-27-2007

Quote:

Originally Posted by vector86

Looking at it more closely, it looks like a bandpass filter. Any idea how to calculate what value caps I need?

http://www.arrl.org/aarrl.html

Quote:

The data sheet makes no mention of what kind of impedance the rf port has.

Are you sure it's actually a data sheet and not a sales pamphlet? Contact the manufacturer?

vector86 · 02-28-2007

I emailed someone at Aerocomm today and they recommened I use a 22pF series capacitor without the other parallel capacitors. They didn't tell me how this value was calculated so I can only guess until they answer my second email.

02-27-2007	#1 (permalink)
vector86 Registered Member Join Date: Mar 2005 Posts: 4	PCB RF trace questions I'm designing a telemetry radio, the data sheet for the tranciever I'm using says to make the board traces for the antenna connector like this: my question: What are the capacitors for? I'm assuming the traces are arranged like this to try to block noise from the rf signal. P.S. I've been lurking for a while and this is my first post!

02-27-2007	#2 (permalink)
Thorn Did you do the math? Join Date: Apr 2002 Location: Villa Straylight Posts: 10,361	Close. They block DC. If you don't know that and you're designing a whole telemetry radio, you're probably in way over your head. I'd suggest getting some good books on RF design and reading for a while. __________________ Thorn "Read Altas Shrugged. Compare it to today. Repeat as necessary"

02-27-2007	#4 (permalink)
Thorn Did you do the math? Join Date: Apr 2002 Location: Villa Straylight Posts: 10,361	OK, as long as you're taking that class. Yes, you can have DC in there. By placing the caps in the circuit you allow the AC/RF signal to be passed out to the antenna, yet block and DC from going out. It also works on the inverse; any DC at the antenna is blocked from getting back into the radio. __________________ Thorn "Read Altas Shrugged. Compare it to today. Repeat as necessary"

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02-27-2007	#3 (permalink)
vector86 Registered Member Join Date: Mar 2005 Posts: 4	I was talking about the traces blocking the rf signal. I've already designed a board using Aerocomm's tranciever module, but I wanted to see if it could be made even smaller by using the 1x1 system in chip. I'm a 3rd year computer engineering student, I know capacitors block DC. I just didn't know that there would be any offset in an RF signal. To be honest, I don't take that class until next semester.

02-27-2007	#5 (permalink)
vector86 Registered Member Join Date: Mar 2005 Posts: 4	Looking at it more closely, it looks like a bandpass filter. Any idea how to calculate what value caps I need? The data sheet makes no mention of what kind of impedance the rf port has.

02-28-2007	#7 (permalink)
vector86 Registered Member Join Date: Mar 2005 Posts: 4	I emailed someone at Aerocomm today and they recommened I use a 22pF series capacitor without the other parallel capacitors. They didn't tell me how this value was calculated so I can only guess until they answer my second email.