Q: What is DigRF?
A: DigRF is a digital interface standard defined and supported by the Mobile Industry Processor Interface (MIPI) Alliance (www.mipi.org). MIPI also has interface standards for both LCDs and cameras used in cell phones.
Q: Where is this interface used?
A: It is used primarily in between the RF transceiver IC and the baseband (BB) IC in a mobile handset.
Q: What does the interface do?
A: It provides a standard way to transmit I/Q data between the RF transceiver and the BB chip. The receiver circuits generate the I and Q signals from the radio signal. These are digitized and otherwise processed and sent to the BB circuits for further processing. In the transmit mode, the BB chip creates the digital versions of the I and Q signals and sends them to the RF chip over the interface.
The RF chip performs digital-to-analog conversion to create the analog signals that become the radio signal to be transmitted. In the past, a variety of RF and BB chips generated both digital and analog I/Q signals. There was no standard approach, meaning designers had to add extra circuits to make conversions as required to allow the two chips to talk to one another. The DigRF interface standardizes digital transfers.
Q: What is the actual physical interface?
A: The digital interface is basically a serial data bus with six paths—one for receive (Rx) I/Q data, one for transmit (Tx) data, one for a clock signal, and another for a clock enable. The Rx and Tx paths are differential low-voltage differential signaling (LVDS).
Q: What is the data rate?
A: In the new version 4, the data rate is 1.5 Gbits/s. That speed is needed to support the newer Long Term Evolution and WiMAX standards that also use multiple-input multiple-output (MIMO). The older version 3 transmitted at a 312-Mbit/s rate.
Q: Is there a protocol involved?
A: Yes. MIPI defines a packet-based protocol that transmits the data as well as control information. Control packets send control signals between the RF and BB chips. Data packets send data in alternate I and Q words using 8B/10B FEC encoding. When MIMO is used, multiple I/Q words must be sent interleaved. The protocol overhead is typically 20% of the serial data stream.
Q: What makes the testing so difficult?
A: The new higher-speed v4 standard bumps the data speeds up significantly, meaning that some serious signalintegrity measurements are needed to ensure that the chips are functioning properly. In addition, the more complex protocol makes it necessary to perform protocol analysis again to make sure the chips are correctly exchanging data.