Why GSM IM3 test freq so?

Hi, All,
I was instructed to test GSM IM3 as following:
Say for link freq RX is 935MHz,
then set the 1st interference freq to 935+17.6MHz,
and set the 2ndt interference freq to 935+2*17.6MHz.
Then does the IM3 test.

I got the perfect result so that I doubt the interference freqs are not right?
Who can explain why the interference freqs are so setting?
many thanks
best
tony liu

GSM spec specifies the spacing of the two tones to be 800kHz.
For example if the RX signal fo is 935MHz (3dB above sensitivity level) the 1st tone f1 (CW) should be at 935.8MHz (-43dBm), and the 2nd tone f2 (GMSK modulated) should be at 936.6MHz (-43dBm).
If you use tones with different spacing than those mentioned in the GSM spec, should be no results differences between setups IF all 3 signals (fo,f1,f2) are inside of the GSM900 RX band (925MHz-960MHz).

Hi, vfone,
Your freqs sound sense.
Thanks.
Tony Liu