Technical Insights

DDR5/LPDDR5 Signals Can Be a Real Challenge to Measure

2020-01-14  |  8 min read 

Overview 

Today’s mobile and computer bus technologies are driving the need for higher speed. Memory buses such as LPDDR5 / DDR5 use on-die termination (ODT) modes, which eliminates the need for external termination resistors and, as a result, improves signal integrity. It is a real challenge for probing technology that supports a continuously variable termination mechanism. The ODT mode switches termination impedance back and forth between low and unterminated high impedance. A probe’s input impedance becomes a significant factor when connecting to this technology. As the data rates of modern memory systems get faster, the bandwidth of the probing system is something you cannot ignore.

 

Figure 1. Dynamic ODT enables the DRAM to switch between high and low termination impedance. When the termination impedance goes high, the probe impedance needs to be high enough to reduce probe loading. 

 

Figure 2. RCRC probe (N7003A) for probing LPDDR4 signal

 

Figure 3. RC probe (1169B) for probing LPDDR4 signal

An RC probe (Figure 3) is a better choice when probing buses that transition to a high-Z state or when dealing with a signal with high impedance.

As you probe these new types of buses, consider the impact of using a probe on the target signal. The probe becomes an additional load (resistive, capacitive, and inductive) that can affect the measurement results or even change the operation of the device under test (DUT). Making sure that these loading effects are within acceptable limits is a key step to successful oscilloscope measurements.

 

The Measurement System

Oscilloscope 

Bus speeds are significantly faster, which adds complexity when accessing the DUT. With LPDDR5 / DDR5 data rates reaching 6.4 Gbps, measurement system bandwidth will have a direct impact on making a proper measurement. Oscilloscope bandwidth, probe bandwidth, and probe impedance (RLC) may have a detrimental effect on the signal if not selected properly. For LPDDR / DDR5, consider using a high-performance oscilloscope with a high effective number of bits (ENOB). Keysight’s Infiniium UXR-Series oscilloscopes (figure 4) offer the most extensive selection for your current and future measurement needs. Today you need 25 GHz. It is a safe bet that tomorrow you will need more. So pick a measurement platform that will allow you to move to higher bandwidths (up to 110 GHz today).

Figure 4.  UXR-Series oscilloscopes

 

The probing system

As for the required probing system, this is a real technological challenge. DDR5/LPDDR5 application, imposes high probe bandwidth and high impedance probe input maintained over a wide frequency range to avoid the probe loading effect.

The existing probing solution based on InfiniiMax 1169B 12 GHz with RC input impedance is not fast enough. The InfiniiMax III/III+ with up to 30 GHz bandwidth meets the bandwidth requirements. However, it often causes a considerable loading effect as the midband impedance of the probe is not quite high enough to make low loading measurements.

Keysight’s new MX0023A InfiniiMax RC probe (Figure 5) provides up to 25 GHz differential bandwidth. It delivers an RC input impedance profile for the extremely low midband loading necessary to address modern high-speed probing requirements. Keysight’s new MX0106A InfiniiMax differential solder-in head and MX0105A InfiniiMax differential SMA probe head offer flexible connectivity options.

 

Figure 5.  MX0023A InfiniiMax RC probe 

 

Keysight Has You Covered

The MX0023A provides a variety of flexible connectivity solutions, covering today’s emerging signaling standards. Standards covered include LPDDR/DDR4, LPDDR/DDR5 with up to 6.4 Gbps data rates, MIPI D-PHY 3.0, C-PHY 2.0, and M-PHY 4.0 bus signaling with > 8 Gbps of data rates. 

  • 25 GHz high bandwidth and RC input impedance profile for low midband loading 
  • probe amp-specific S-parameter correction filter for flattening the magnitude and phase response of the probe for high-accuracy measurements
  • auto-switchable 1:1 and 4:1 attenuation ratio for superior noise performance, while maintaining 25 GHz bandwidth
  • support for a variety of probe heads and accessories, including three new probe heads and most of InfiniiMax I/II accessories, which saves investments on probe heads
  • AutoProbe II interface for easy direct connection to Infiniium oscilloscopes

 

MX0106A InfiniiMax differential solder-in head, 23 GHz

The MX0106A is a solder-in head. It allows a soldered connection into the target for a reliable hands-free connection. This probe head provides 23 GHz bandwidth and low capacitive loading.

 

Figure 6. MX0106A InfiniiMax solder-in probe head 

 

Figure 7. Zoomed-in view of MX0106A probe head tip

 

MX0105A InfiniiMax differential SMA probe head, 20 GHz

The MX0105A is a differential SMA probe head. It provides 20 GHz of bandwidth. The MX0105A allows you to connect two SMA cables to make differential measurements on an oscilloscope channel.

 

Figure 8. MX0105A InfiniiMax SMA probe head front view

 

Figure 9. MX0023A InfiniiMax RC probe amp with MX0105A SMA probe head

 

MX0100A InfiniiMax micro probe head, 25 GHz

The MX0100A is a solder-in head for use with InfiniiMax I/II/RC probe amps designed to access small geometry target devices. The probe head is made out of flex printed circuit, making it light, flexible, small yet highly usable. It provides up to 25 GHz of full probe amp bandwidth when used with the MX0023A and excellent probe loading characteristic (170 fF). Gold plated nickel tip lead is replaceable and user trimmable. 

The probe head offers wide operating temp range of -55 to +150 °C (per JEDEC JESD22-A104 revision E spec), making it ideal for environmental chamber testing with the probe head soldered to the DUT inside the chamber.

Figure 10. MX0100A micro probe head

 

 

 

Conclusion

The measurement system used for technologies such as LPDDR5 / DDR5 has a direct impact on your ability to see what is happening on your DUT from a signal integrity and signal compliance perspective. An oscilloscope with high ENOB and probing technology that does not impair the target signal is key. It helps you to deliver your design on time without being in the critical path of the project.

 

Click here to learn more about DDR5 / LPDDR5.