LBXXXA Series USB Powe
™
Product Manual PMA (Power Meter Application)
Revision: 08/02/11
- 25 -
Measurement Example: GSM (LB4XXA models only)
GSM (Global System for Mobile Communication) is a TDMA (Time Division Multiple Access) world standard and the
most widely used for mobile phones. The modulation technique is 0.3 GMSK (Gaussian Minimum Shift Keying) with a
modulation data rate of 270.8 kb/s. Bit rate relates to amplitude modulation bandwidth, however, GSM transmission
uses a constant signal envelope so average power can be measured.
The GSM TDMA structure is shown below with data shown in a single time slot per frame or 1/8 (12.5%) of the time.
The transmitter synchronizes during 3 tail bits and is silent during 8.25 guard bits. That amounts to 11.25 bits of
transmitter inactivity out of 156.25 bits total. The transmitter is then active or transferring data within 0.928 (92.8%) of a
time slot:
(156.25
– 11.25) / 156.25 = 0.928 of a slot
The duty cycle of the transmitter power is then 12.5% x 0.928 = 11.6%
Make the measurement by connecting the sensor and opening the application.
1.
Select:
Measurement > Set Frequency
2.
Enter:
Carrier frequency, (select one)
850/900/1800/1900 MHz
3.
Select:
Measurement > Anti-alias
(enable)
4.
Select: Measurement > Pulse Power
5. Select: Triggering > Continuous > Internal
The sensor can now measure the GSM power in the active time slot. Verify the sensor automatically reports the
correct duty cycle.
Alternately, you can use the more traditional method previously described to compare the pulse power readings:
1.
Select
:
Measurement > Average Power (CW)
2.
Select: Offsets & Response > Duty Cycle > Setup
3.
Enter: 11.6
(%)
4.
Select: Offsets & Response > Duty Cycle > Enabled
(or activate the DC icon
on the toolbar)
