RFX400 daughterboard modification

The RFX400 can work from 400Mhz to 500 Mhz with a total power up to 200 mW. To use it for the band III on the channels 10,11,12 we need to make some modification.

The conversion is done with a quadrature modulator (AD8345) taking the two DAC's USRP TX port signal and a carrier from a built in programmable local oscillator (PLL using one ADF4360). The card include also for the TX path 2 stages of amplification before a low pass filter. The bandwidth of the last stage will not go under 300 Mhz (According to the spec. of the RF3315 rated for 0.3 to 3 Ghz).

If you try to use this card under 400 Mhz you need at least to change the inductances value of the PLL VCO. (It won't lock outside the original band otherwise). According to the chip data-sheet the correct value to operate on the VHF band III is around 22 nH. Here's a table showing the range of operation depending on the inductance value (derived from ADF4360 formula):

L	VCO frequency	Operation bandwidth	Min ouput frequency	Max output frequency
18nH	464'936'224.70	61'991'497	201'472'364	263'463'861
22nH	422'383'112.10	56'317'748	183'032'682	239'350'430
27nH	382'668'034.10	51'022'405	165'822'815	216'845'219

Locate the TX PLL:

Then replace L105 and L115 inductances (in // with R105 and R106)

(Note: Since the RFX400 use the same PLL design beetween RX and TX side, this could be done at the RX path, if you want to receive signal on the same band, just replace L31 & L32 as done at the TX side)

We had no problem after this modification to lock beetween 200 to 235 Mhz. (Note: When having RFX400 and BasicTx installed simultaneously inside the USRP the software will adress by default the BasicTX !)

Whithout changing the amplifier (remember, we are bellow 300 Mhz the lower limit of the last stage) we still have an average of 5 mW at the output.