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.