The MAX2644 Meets 10µs Switching Time for 802.11b WLAN LNA
Abstract: This application note measures the turn-on/turn-off time of the MAX2644 low-noise amplifier (LNA). This is needed to support IEEE® 802.11b TDD WLAN systems. The system operates at 2.4GHz. The turn-on time is 8.6µs, and the turn-off time is 5µs. The test set-up is shown along with oscilloscope and vector signal analyzer screenplots.
There are many competitive IEEE 802.11b wireless-local-area-network (WLAN) cards available in the market today. These WLAN cards operating at 2.4GHz unlicensed band are becoming increasing popular in office or home environment, providing variable high data rate transfer up to 11Mbps. Perhaps, one very important parameter that distinguishes a product from its competition is the range performance, or simply put, the receiver sensitivity.
Superheterodyne receiver is predominantly the chosen architecture for WLAN cards available in the market today. Next generation of WLAN cards demands lower bill-of-material (BOM) cost. Direct conversion receivers are favored since expensive IF filters are eliminated and replaced by on-chip integrated lowpass filters for channel selectivity. Depending on the receiver architecture and fabrication processor chosen for the implementation, a low-noise amplifier (LNA) may be required to effectively lower the overall cascaded noise figure (NF) of the system, thus enhancing the range performance of the receiver.
The MAX2644, designed on a low-noise, advanced silicon-germanium (SiGe) technology, is an excellent choice for WLAN application at 2.4GHz. This low-cost, high third-order intercept point (IP3) LNA features a programmable bias, allowing the input IP3 and supply current to be optimized for specific applications. The LNA provides up to +1dBm input IP3 while maintaining a low-noise figure of 2.0dB and a typical gain of 16dB.
Since IEEE 802.11b is a time-division duplexing (TDD) system, the LNA must have fast turn-on and turn-off times in order to fulfill system requirement. Typically, 10µs is an acceptable limit. Lab measurement indicates that the Turn-on time for the MAX2644 is approximately 8.6µs (power ramping up from less than -30dBm to -13dBm ±1dB). The Turn-off time is 5µs for 30dB drop from nominal output power. Both timings meet the system requirement of 10µs.
Table 1. The MAX2644 Switching Time Performance
Settling Time
Test Condition
Turn-on time
8.6µs
Output power settles to within ±1dB.
Turn-off time
5µs
Power level drops to -30dBc from nominal output power level.
Test Procedure
A standard MAX2644 evaluation kit was used with a low-cost NPN digital transistor from ROHM® (DTC144) added to the bias network for shutdown control, as shown in Figure 1. The transistor logic-high level is 2.7VDC to turn on. An equivalent FET switch could also be used for this application.
Figure 1. Standard EV kit modification.
See Figure 2 for the test setup that is used for all measurements.
Figure 2. The MAX2644 switching time measurement test setup.
The arbitrary waveform generator was set to deliver a TTL-compatible logic pulse at 2.8VPEAK in burst mode. The pulse-width is 62.5µs, as shown in Figure 3.
Figure 3. Trigger pulse applied to DTC144EUA digital transistor.
The trigger was routed to both the active-loe shutdown transistor on the EV kit, and the external trigger input on the vector signal analyzer. This arrangement allowed for direct correlation between the on/off trigger pulse and measurement data start point.
The vector signal analyzer was configured to display power (dBm in 1MHz resolution bandwidth) vs. time, and the time scale was referenced from the trigger pulse. This method produced the results shown in Figures 4 and 5. Note that the x-axis (time) is referenced directly to the same trigger event that enables the MAX2644 without any time delay, and the 'off' trigger occurs 62.5µs later on the same plot.
Figure 4. POUT vs. time (10dB/div).
Figure 5. POUT vs. time (2dB/div).
IEEE is a registered service mark of the Institute of Electrical and Electronics Engineers.
ROHM is a registered trademark of ROHM Co., Ltd.
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