|Figure 1: Probes are a key element of the total signal|
The first matter to consider is the oscilloscope to be used for the testing. Testing DDR interfaces always means using probes, so it's a good idea to look at the oscilloscope and probes as a complete acquisition system (Figure 1). Critical attributes of that system include a) adequate bandwidth, b) low loading, and c) easy mechanical connection to the DUT. Let's look at each of these attributes in turn.
Conventional wisdom dictates that "oscilloscope bandwidth should be at least five times the bit rate. However, DDR interfaces have very fast slew rates relative to their transition rates (Figure 2). Thus, characterizing the system with acceptable rise-time accuracy demands an oscilloscope and probe pairing with a relatively high bandwidth.
|Figure 2: The high slew rates of DDR|
interfaces boost bandwidth needs
In the vast majority of DDR test applications, solder-in probe tips make the mechanical connection to the DUT. Because of the many signals being probed, these tips must be of sufficiently small size. They should also offer physical flexibility to reduce torque on the relatively delicate solder connections as the probe amplifier is moved. Finally, they should serve several different types of measurement tasks to reduce the complexity of test setup.
|Figure 3: Poor usage of the oscilloscope's dynamic range|
reduces the SNR, while clipping the signal can result
in an overdriven front end
|Figure 4: Best practices include separating signals into|
their own respective grids and setting vertical gain so
that signals occupy about six vertical divisions
Figure 4, on the other hand, illustrates some best practices (not only for DDR measurements but for all oscilloscope applications). Rather than leaving the display as a single large grid, it is better to separate the signals into their own respective grids. Avoid signal clipping by setting vertical gain so that the signals occupy about six vertical divisions. These two practices will give you the maximum usage of your oscilloscope's dynamic range.
|Table: Specified signal levels for DDR variants|
Heeding the above advice should make for smoother sailing in the waters of testing a DDR interface's physical layer.