Other Parts Discussed in Thread: DS160PR410,
A common question when using a signal conditioning device is can the device support a data rate that is lower than the target data rate.
The answer to this is device dependent, but there are several key criteria to look for depending on the device type. For redrivers, CTLE boost is the primary consideration. For retimers, there are a few additional considerations beyond just CTLE boost.
CTLE Boost
Continuous time linear equalizer, or CTLE, is a functional block within both redrivers and retimers. The CTLE boost curve is responsible for compensating for channel insertion loss in order to reduce ISI jitter. Typically when selecting a signal conditioning device, it's important to know how much channel loss it can compensate for. The CTLE boost is a good indicator of this. CTLE boost is a frequency dependent parameter. For example, DS160PR410 provides around 18 dB of boost at 8 GHz, but only around 7 dB of boost at 2.5 GHz. Because of this, it's important to consider the nyquist frequency of your intended application. Selecting a device supporting a much higher data rate than intended your application will generally mean that the device is not able to provide as much boost in your application compared to what is listed on the first page of the device data sheet.
CTLE boost curves or tables are often provided in device documentation. These can be used to better understand the compensation capability of a device across frequency.
Retimer Specific Considerations
A retimer is typically bound to more specific data rates than a redriver. This is because the clock and data recovery (CDR) functional block is limited to locking to a fixed range for rates. A typical CDR circuit will support a range of data rates, and several sub-rates of this range. For example, the DS280DF810 supports retiming the following data rates:
| Test Condition | Min | Max | Unit |
|---|---|---|---|
| Full Rate | 20.2 | 28.4 | Gbps |
| Half Rate | 10.1 | 14.2 | Gbps |
| Quarter Rate | 5.05 | 7.1 | Gbps |
However, it is also possible for the retimer to support additional data rates through a CDR bypass configuration. For example, the DS280DF810 can support 1.25 Gbps signaling through the CDR bypass configuration. Typically a retimer operating in CDR bypass can also provide CTLE boost, but digital filters such as DFE and FFE are bypassed.
An additional consideration when operating in CDR bypass mode is that some retimers have limited output amplitude control while operating in CDR bypass. This may be important to consider if using the retimer in an application where the output amplitude is critical for meeting a specification.
