Cell phones and other mobile Internet devices (MIDs) typically
use two primary chips—the RF section or radio transceiver
and the baseband (BB) section, which handles the digital
processing associated with modulation and demodulation
and other physical-layer functions. Since the radio generates
and receives analog RF signals and the baseband chip performs
digital operations, some data conversion between the two is
involved. The big question has been where to put the analog-todigital
and digital-to-analog converters (ADCs and DACs).
Are they in the RF chip, the baseband chip, or maybe both?
How can chip vendors make their ICs so any RF chip is compatible
with any other baseband chip? The answer lies in creating
an interface standard that all parties buy into. Known as DigRF,
that standard is now in its fourth (v4) iteration (see “DigRF
FAQs,” p. 26). So while the standard now solves the interface
problem, the issue turns to how best to test and troubleshoot
that interface given its added speed and complexity.
The RDX Test Platform from Agilent Technologies is a complete
solution to testing the integration of the RF and baseband
ICs in a wireless product using the DigRF v4 (or v3) interface.
It’s designed to speed and simplify testing with non-invasive
measurements. With this system, digital and wireless engineers
alike can perform radio digital cross-domain testing, debugging,
and characterization.
This system is the result of the major changes in the DigRF
standard that, in turn, are in response to the rapid adoption of
the Long Term Evolution (LTE) and WiMAX 4G standards,
which enable downlink speeds of more than 300 Mbits/s in
multiple-input multiple-output (MIMO) configurations.
The interface between the RF and BB chips is more critical
than ever. Version 4 of DigRF is not only faster, it also adds
multiple data lanes, power-saving modes, faster power recovery,
8B/10B encoding, and a link layer protocol. Suddenly, the v3
test setups just cannot handle these changes.
The RDX test system supplies tools for digital and RF designers
to provide gigahertz cross-domain system characterization.
It consists of two key modulesthe N5343A Exerciser and the
N5344A analyzer. These modules are housed in a small Agilent
N2X mainframe that’s built to accommodate future MIMO
designs (see the figure).
Both modules support DigRF v3 as well as v4. The Exerciser
has combined stimulus and capture capability up to 1.5 Gbits/s.
It also has both speed and mode change tests, bit to packet level
generation, and an application programming interface (API)
for automation. The Analyzer features clock recovery capability,
speed and mode change tracking, simultaneous transmit and
receive monitoring, a protocol decoder, and a packet viewer.
SPECIAL PROBES AND SOFTWARE
Additionally, the RDX modules offer active probing with
ultralow capacitive loading (less than 0.15 pF) and high sensitivity
that have a minimum disturbance effect at gigabit speeds.
Two probe choices also are available. The N5345A Midbus
Probe includes Soft Touch technology for fast probing on prototype
boards. With the B5346A flying leads probe solution,
designers can monitor the v4 links in space-constrained designs.
The test software, which includes protocol generation and
analysis, interoperates with Agilent’s popular Signal Studio
software and 89600 vector signal analysis (VSA) software. RF
engineers can save some time by using familiar vector signal
generation analysis software that supports the RDX modules as
well as other signal analyzer and signal sources.
HOW IT WORKS
The Agilent RDX system solves the problem that engineers
have in testing the interface between the RF IC and the BB IC. It
lets mobile handset designers quickly evaluate product behavior
by monitoring the digital serial bit stream between these chips.
RDX control packets are decoded and displayed on the system
controller PC to provide visibility into the configuration,
status, and control flow. IQ data is extracted from the data
packets and analyzed using Agilent’s 89600 VSA software.
With these capabilities, handset integrators can characterize the
interactions between the RF IC and the BB IC to isolate defects
and optimize performance.
This is crucial because in some cases, the RF IC and BB IC
are developed by different vendors and integrated by an ODM
into a final handset design. Also, the RDX lets RF IC development
teams characterize their components independently
of the BB IC. It can enable the RF-IC manufacturer to prove
interoperability with a number of various BB ICs from different
vendors as well.
The RF IC transmitter is characterized by developing a digital
IQ representation of the RF signal using the signal studio and
loading it into the System Controller PC. The exercising module
packetizes the IQ data, inserts the specified control packets,
and drives the bit stream into the RF IC digital baseband input
over the v4 interface.
Continue on Page 2