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Extend probe bandwidth to improve signal fidelity

Posted:01:58 PM August 20, 2018 writer: G

Memory system designs have now shown a trend toward smaller packages, increased capacity, and reduced power consumption. The design is getting more compact, and the data transfer rate is increased from DDR3 to DDR4. This requires a smaller embedded circuit board design, meaning that contacts or connectors will be placed on a smaller board space to detect critical DDR signals (clocks, gates, and data) for verification and testing. For designers, probing has become a challenging task. Probing at unsuitable locations can cause reflections and distortion of measurement results. Improper placement of the probe in the system increases system load and affects signal fidelity and will result in measurement errors such as slew rate, rise time, setup and hold times. To help overcome this detection challenge, engineers are currently using DDR BGA detection. The DDR BGA probes are designed with a small footprint (KOV) that is very close in size to the DRAM and allows the oscilloscope to access signals in the DRAM of the memory device for signal integrity measurements (Figure 1). This mode of use requires soldering the BGA probe to the system and soldering the DRAM to the BGA probe. Welding needs to be done through the BGA rework station. The oscilloscope pads on both ends of the BGA probe provide the connection of the oscilloscope to the solder-in probe.

The DDR BGA probe supports a nominal bandwidth approaching 2 GHz. BGA probes require close to 8 GHz bandwidth to support DDR3 and DDR4 data rates up to 1600 MT/s. After a slight compensation using the probe calibration method, the BGA probe corrects the amplitude and bandwidth losses caused by the probe loss characteristics. Losses caused by BGA probes are more pronounced at high data rates (above 1600MT/s). Previous probe calibration methods allowed the user to apply a transfer function to the oscilloscope through waveform transformation software to compensate for the BGA probe. The transfer function file can be constructed using the S-parameter file of the BGA probe and the solder-in probe. The S-parameter file of the BGA probe can be directly measured by a vector network analyzer (VNA), a time domain reflectometer (TDR) or simulation software. This method is very efficient, but requires the user to be familiar with measuring equipment or simulation tools and conversion software.

Using the oscilloscope's latest probe calibration method allows users to perform accurate AC calibration of the DDR BGA probe from probe to oscilloscope without the need for other instruments such as VNA or TDR. The oscilloscope performs AC calibration by outputting a fast edge. The Agilent 90000X Series oscilloscopes output a 15ps edge to fully characterize VSource, VIn, and VOut (including probe load characteristics) and combine the measured information into a custom correction filter for DDR BGA detection setup. This approach allows the user to make probing corrections for each signal on the BGA probe and eliminate probe characteristics due to manufacturing variations.

DDR BGA probe calibration procedure

To determine the calibration applied to a single probe, a qualitative analysis of the probe is required. To perform DDR BGA probe calibration, you need to set up a pass-through fixture and then use an oscilloscope for AC calibration. The procedure for setting up a BGA probe using a straight-through fixture is (Figure 2): applying the bead to all ground (VSS) signals outside the BGA probe; applying the bead to the BGA probe for a signal that requires probe calibration; by using a large ceramic Gently rub the BGA probe to flatten the bead; cut the two z-axis connecting materials (elastic contacts) and tape them to the ends of the straight-through fixture to provide contact between the bead and the straight-through fixture With a microscope, place the BGA probe flat on top of the straight-through fixture. In this way, only the signal of interest contacts the transmission line, and all grounding beads are connected to the grounding end of the straight-through fixture through the resilient contacts; connect the straight-through fixture to the channel input of the oscilloscope and feed the CAL from the oscilloscope to the through-clamp via the SMA cable; The probe is soldered to the oscilloscope pad on the BGA probe and connected to a channel input. Vin is defined as the signal loaded by the BGA probe on the BGA probe point; VOut is the signal output by the BGA probe; Vout/Vin correction, the probe output signal is the exact performance of the current signal, which is exactly the same as the detection.

AC calibration can be performed using the Agilent oscilloscope with PrecisionProbe software. PrecisionProbe software characterizes and compensates for custom probes with Infiniinium Series oscilloscopes. The PrecisionProbe characterizes the frequency response of the BGA probe (VOut/VIn or VOut/VSrc) and then generates a custom filter that is loaded into the oscilloscope hardware to ensure a flat frequency response of the BGA probe. DDR BGA probe losses will be compensated and the oscilloscope will be able to achieve higher bandwidth. In addition to frequency response (amplitude and phase) measurements, PrecisionProbe provides impedance maps generated during AC calibration, and bandwidth control allows the user to use filters to remove unwanted high frequency noise. This probe calibration method allows the user to simulate an ideal probe without the need for extensive engineering time and capital for design.

The low cost and low power consumption of DDR technology make it popular in mobile applications. Due to the design and technological development of the small size, high memory data transfer rate of the mobile industry, memory verification will be difficult to achieve without a device that can directly access the DDR signal on the DRAM bead. DDR BGA probes help memory designers access DDR signals and use oscilloscopes for signal integrity measurements to ensure products comply with JEDEC standards. While most probes are designed to meet the bandwidth requirements of the signal under test, other factors such as size and design and manufacturing costs need to be measured. With probe calibration tools such as Agilent PrecisionProbe, memory designers can easily transition to new DDR technologies by using existing DDR BGA probe designs. Measuring DDR BGA probes for high-speed memory (DDR3 and DDR4 memory technologies above 1600MT/s) requires the use of PrecisionProbe software to extend bandwidth and increase the margin of signal integrity testing. When generating the transfer function model, you need to pay attention to other system parameters such as source and receiver impedance, transmission line length, loss and characteristic impedance that can easily affect performance. Using these devices to evaluate waveforms (especially for high bit rate systems) requires a combination of de-embedding software (such as Agilent PrecisionProbe) and InfiniiSim (waveform conversion tool suite).

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