I am trying to do a project that involves proximity detection. I need a few devices omnidirectionally detecting each other at short distance (under 20 feet). It seemed to me that sub-1Ghz RF is the way to go. I got a few MRF49XA transceivers. The datasheet for the chip provides a reference balun circuit schematic, but considering my requirements, I wonder if I really need one and if I do, can I get a pre-made balun IC rather than building my own. All I need is for the devices to be able to detect each other at around 15-20 feet range, hopefully with the lowest possible analog RSSI value and get to highest RSSI when right next to each other.

I do not have EE education and I am very new to the field, so if the question doesn't make sense or requires more information, please be patient with me.


1 Answer 1


If you read quickly the datasheet (page 73), you'll see that you have to use a balun circuit in order to match the IC's pins high input impedances to the antenna's impedance which is likely to be 50 \$\Omega\$

If you go further and read page 13 :

The LNA has approximately 250 \$\Omega\$ of differential input impedance which functions well with the proposed antenna (PCB/Monopole) during signal transmission. The LNA, when connected to the 50 \$\Omega\$ device, needs an external matching circuit (Balun) for correct matching and to minimize the noise figure of the receiver.

So you definitely need a 50 \$\Omega\$ unbalanced to 250 \$\Omega\$ differential impedance matching circuit, the technology you want to use is up to you.

In RF electronics, impedance matching between interconnected circuits is mandatory, should it be for signal integrity reasons or hardware safety reasons.

  • \$\begingroup\$ Hmm... so, do I have to make the exact circuit in the datasheet or can I use something like this maybe? mouser.com/ds/2/249/TP-103-477720.pdf \$\endgroup\$
    – Mad Wombat
    Mar 29, 2016 at 16:14
  • \$\begingroup\$ See edit. You can for example use something LIKE the TP-103 but not the TP-103 because it outputs 200 ohm diff. instead of 250 diff. \$\endgroup\$
    – MaximGi
    Mar 29, 2016 at 16:25
  • \$\begingroup\$ Thank you, that clarifies things. I am too noob to figure out what numbers to look for :) \$\endgroup\$
    – Mad Wombat
    Mar 29, 2016 at 16:31
  • \$\begingroup\$ A possibly stupid question. What if I don't plan to use an antenna? What is the impedance of "no antenna" or "strip of metal on PCB"? \$\endgroup\$
    – Mad Wombat
    Mar 29, 2016 at 17:48
  • \$\begingroup\$ Approximately infinity at your frequencies (it's a small strip line terminated by an open circuit) which would cause no damage since you are doing reception, but you'd not receive anything though \$\endgroup\$
    – MaximGi
    Mar 29, 2016 at 19:50

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