Version 3 (Matt Ettus, 11/16/2006 06:27 am)
h3. Changing the RFX400 Frequency Range
The Flex400 uses 2 of the ADF4360-7 VCO/PLL chips from Analog Devices, one on TX and one on RX. The way the board is designed, the output frequency of the VCO/PLL is divided by 2 to give the actual channel frequency. For example, since the RFX400 covers 400-500 MHz, the VCO/PLL actually tunes from 800-1000 MHz. If you want to cover 136-174MHz, you would have to design for 272-348 MHz.
The frequency range of the ADF4360-7 is controlled by 2 inductors, L31 and L32 on RX, and L105 and L116 on TX. They are 0402-sized, and ship with 4.3nH installed. The ADF4360-7 datasheet has instructions on how to select the inductors for your desired frequency range. If you are ordering the inductors, I would suggest not only ordering the value you think you need, but also ordering at least one bigger and one smaller value since the calculations in the datasheet are not exact.
Other things to note:
* The fractional bandwidth covered is approximately constant, so if we cover about 120 MHz when at 450 MHz, then you can probably only cover 40 MHz or so around 150 MHz. At 800 MHz, you should be able to cover about 200 MHz.
* There is a low pass filter on the TX/RX connector with a cutoff around 550MHz. If you are going higher in frequency, you should make these changes:
*** L224 and L225 should be zero ohm resistors (0603 size)
*** C221, C222, and C223 should all be removed.