On turning the RFX900 to an RFX1800 and back again

To convert the RFX900 to an RFX1800 you only have to do 3 things:

  1. Cut the traces which go to FIL1.
  2. Put an 0603 capacitor in C204.
  3. Run the following command:
    ./burn-db-eeprom -A --force -t rfx1800

The filter is a 902-928 MHz band-pass which is placed on the TX/RX path.
It is installed on the RFX900 to reduce receive noise from potentially
strong nearby cellphone signals.

If you cut the filter out you need to complete the signal path by
populating C204 with a capacitor in roughly the 100pF to 10,000pF range.
Very high capacitor values have bad parasitic effects – 0.1uF is fine
but don’t use 10uF.

You can actually leave the filter in and also populate C204. The only
problem is that the two paths will have different phases that vary with
frequency and this will cause weird resonances. Your power will vary
+/-10dB in 2MHz wide ranges.

Once you’ve done this, you can use the RFX900 just as if it were a

To turn it back into a RFX900 again, or to turn your RFX1800 into a
RFX900, you can run the command:

./burn-db-eeprom -A --force -t rfx900

An important (!) note to make is that if you cut the filter and forget
to install a capacitor, if you install your capacitor incorrectly, or if
you run the eeprom burn command and turn the RFX900 into an RFX1800
without making any hardware changes, your signal will not get through
and you will burn out your transmit amplifier!

To summarize:

with filter and with capacitor
	weird resonances

without filter and without capacitor
	nothing gets through, burns out tx amplifier

with filter and programmed to be an RFX900
	902-928 MHz on the TX/RX port
	800-1000 MHz on the RX2 port

without filter, with capacitor and programmed to be an RFX900
	800-1000 MHz on both ports
	decreased rx noise rejection on TX/RX port

without filter, with capacitor and programmed to be an RFX1800
	1.5 to 2.1 GHz on both ports

with filter, without capacitor, programmed to be an RFX1800
	nothing gets through, burns out tx amplifier.

Joshua L., PhD. – [email protected]

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