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controlling an VCO with SPI bus with compact Fieldpoint 2120

Hi I have a problem with our compact field point 2120, we are using it to control a VCO chip, ADF4360.  This particular chip has an SPI serial interface on it.
My initial thought was that I could use a MAX3100 convertor chip, this would convert the RS232 to SPI.  But looking at the data sheet on the VCO it requires a 24-bit string to be passed into it to set it up.  I realsie that the RS232 sends data out in 8 bit packets.
and this would convert the rs232 but it would add spikes on the data every 8 bits, which would cause it not to communicate properly.  this was  a cheap 2 chip solution that I was looking for.
So the only other way I can think of it to use a microcontroller or similar cheap device that u can send rs232 commands to and it has in its program to send out appriopate 24bit SPI packets according to the data i send it. this may actually prove more elegent because all we are doing is varying the frequency of the VCO, this could mean we send just a value e.g.2.7gig to the chip and it works out all the rest.
Is there any cheap off the shelf unit that exists, or can the RS232 port be controlled controlled driectly to make a signal?  I know the development board uses the PC's parrellel port to controll the VCO, but thats not very elegent.
IF someone knows please could they reply, many thanks, CHris
ps. our 4 slot backplane is full and we dont have any spare connections there.
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