- Confirmed the Phase DAC to be initialized at mid-scale.
- Confirmed the Phase DAC step resolution for fine clock shifting.
The clock synchronization algorithm relies on the Phase DAC to fine
shift the sampling clocks on each daughterboard.
Only a certain number of DAC codes are required for the actual clock
adjustment, thus a different range of codes may be chosen by
initializing the Phase DAC with a given value. With the selected range,
one may measure the Phase DAC's linearity and step resolution, which
defines how many steps are required when performing the fine shifting
of the clocks.
After initializing the 16-bit Phase DAC at 25%, 50% (mid-scale), and
75%; it was found that the clock distribution PLL locks relatively
faster when using mid-scale (2^15). By testing the Phase DAC's
linearity, it was confirmed that the circuit resolution is 1.11 ps per
code.
- Optimized JESD204B RX/TX links' latency.
- Made JESD latency constant across supported frequencies.
- Checking RX SYSREF capture in the FPGA deframer block.
The JESD204B standard can be linked in such a way to produce a
repeatable, deterministic delay from the framer to deframer. This is
accomplished by setting up a LMFC (local multiframe clock) in both
devices.
The LMFCs are reset whenever a SYSREF edge is captured by the framer
and deframer. Therefore, it is simple to control the LMFC rising edges
in each device by implementing variable delay elements on the SYSREF
pulses to the framer and deframer.
Latency across the JESD204B TX/RX links should remain constant and
deterministic across the supported sampling_clock_rate values. By
testing the roundtrip latency (i.e. FPGA -> TX -> RX -> FPGA) with
different delay values in the FPGA, one may decrease the latency and
provide enough setup and hold margin for the data to be transfered
through each JESD link.
It was found that a different set of SYSREF delay values are required
for sampling_clock_rate = 400 MSPS to match the latency of the other
supported rates.
