Discussed S_sub_bf = 512 instead of 488.

applications/lofar2/doc/prestudy/station2_sdp_ring.txt

@@ -47,6 +47,19 @@ the data rate for one full band station beam is N_pol * S_sub_bf * f_sub * N_com
 W_beamlet_sum = 2 * 488 * 195312.5 * 2 * 18 = 6.8625 Gbps. Using L_lane = 7.8125 Gbps this leaves
 about 1 - 6.8625 / 7.8125 = 12% margin for packet overhead, which is sufficient.
 
+On the lane:
+Using s_sub_bf = 488 yields 6.8625 Gbps, so 1 - 6.8625 / 7.8125 = 12.1% margin
+Using s_sub_bf = 496 yields 6.975  Gbps, so 1 - 6.975  / 7.8125 = 10.7% margin
+Using s_sub_bf = 512 yields 7.2    Gbps, so 1 - 7.2    / 7.8125 =  7.8% margin
+
+In the dp_clk domain:
+Using 488 yields 512 / 488 = 4.9 % margin
+Using 496 yields 512 / 496 = 3.2 % margin
+Using 512 has no margin, so requires higher dp_clk rate to be able to insert headers.
+
+Design decsision:
+  Use dp_clk = 200 MHz, so do not overclock to support S_sub_bf = 512. It may be feasible to
+  support S_sub_bf = 496, but assume 488 because that is required.
 
 Design descision:
   Use W_beamlet_sum = 18 bit for both critically sampled beamlet and oversampled beamlets.