Teledyne LeCroy QPHY-UWB - Manual (Page 8)

8
QPHY-UWB-OM-E Rev C
CAPTURING A WIRELESS USB DATA PACKET
The block diagram below shows a setup that is useful for capturing a UWB packet within the context of a Certified
Wireless USB (WUSB) system.
Antenna, Amplifier, and Filter
The amplifier is needed in order to be able to move the capture antenna out of the near field of the transmit
antenna. Our experience indicates that the capture antenna will still need to be relatively close to the transmitter
antenna, < 15 centimeters in order to obtain adequate signal to noise ratio. An appropriate band pass filter should
be used in environments where there are larger amounts of ambient cell phone or other wireless transmissions.
These out-of-band transmissions will not at low levels significantly affect the UWB analysis, but if they get too
high, they can use up too much of the oscilloscope dynamic range and decrease the SNR of the UWB
measurement. They also can make it more difficult to trigger on the UWB packet.
Adding a Special Trigger Path
In the above setup, the signal is split into 2 paths so that we can improve the triggering. In a Wireless USB
system, most of the UWB packets are very short “beacon” packets which we do not want to trigger on. We only
want to capture the much longer packets carrying the data payload. In order to accomplish this, we need to use
the Width Trigger mode in the oscilloscope to trigger on only the long packets. The Width Trigger mode has a
much lower bandwidth than the edge trigger, approximately 750 MHz. So we need to create a triggering path
where we use a shottky diode detector (e.g., Anritsu 70KA50) driving a high impedance input to create a rectified
and filtered version of the signal. The signal “envelope” seen by Channel 3 will be pulled slowly low as the packet
starts and will stay low for the duration of the packet and then slowly return to 0V. With this circuit, we can now
use the Width Trigger to trigger only on negative going pulses with width greater than the width of the undesired
short packets. See the oscilloscope Operator’s Manual for help using the Width Trigger. This detector based
triggering circuit can also be used to help trigger on packets in higher frequency band groups where the edge
trigger bandwidth of the oscilloscope is being exceeded or pushed to limited sensitivity.