Currently, many wireless systems use a wideband LNA (Low Noise Amplifier) in the receiver chain. Increasingly, wireless systems are operating over a wide frequency range: for 802.11a, the frequency of operation is from 4.9 GHz to 6.0 GHz and for UWB, the band of operation covers several GHz. The LNA, which is the first stage of a receiver chain (after the antenna(s) and the front-end filter(s) if any), needs to provide a high enough gain and a low noise figure over the entire band of operation.
A conventional half duplex wireless transceiver, such as for 802.11a/g (or any other half duplex wireless system), has an architecture as shown in
Defining the beginning and the end of the band of reception by Fbegin and Fend, the reception bandwidth is: BWRX=Fend−Fbegin. A wireless system has to be able to receive any signal in the frequency range from Fbegin to Fend. That is why the LNA in today's receiver path is designed to have a high gain that is as flat as possible over the entire receiver band, as shown in
Note that the number of operating channels in the reception band is defined by
where BWCH is the bandwidth of one channel, as shown in
The following reference is exemplary of the state of the art:
[1] “A 2.5 dB NF direct-conversion receiver front-end for HiperLAN2/IEEE802.11a” Paola Rossi, Antonio Liscidini, Massimo Brandolini, Francesco Svelto, ISSCC 2004 conference.
The present wireless transceiver includes a tuneable narrow band LNA which rejects dramatically any out of band interferers.
The present may be more fully understood from the following description in conjunction with the appended drawing figures. In the drawing:
Referring now to
The wireless system includes a narrow band tuneable LNA 429, a Tx to Rx coupling element 427, and a LNA tuner control signal 428 coming from the baseband processor 411 which controls the LNA tuner. (The elements 410, 420, 421, 423 and 425 correspond to like elements described previously in relation to
The tuneable LNA response should be designed such that:
it covers the entire receiver band. However for a particular tuned frequency Ftuned, the tuned LNA response should show a bandwidth such that:
BWCH<BWtuned_LNA<BWRX.
The LNA tuning range should cover more than the reception bandwidth BWRX to compensate for any chip process and temperature variations
The Tx to Rx coupling element may be composed of two directional couplers, for example, as shown in
A Tx to Rx coupling element can also be designed with microstrip lines on a PCB to which the wireless transceiver is soldered.
Due to process variations, the tuneable LNA has to be calibrated. To perform this calibration, the transceiver should be capable of RF loopback (this means that the transceiver should be able to transmit and receive at the same time even if the system itself is TDD only, as is the case for the 802.11a/b/g system). After calibration, the transceiver should be able to down-convert its own transmitted signal to get nominal signal swing at the receiver baseband output and maximum out of band rejection at the LNA as illustrated in
The calibration can run when the system is powered up but also (if necessary) in response to any large temperature variation the transceiver may experience (this implies that the transceiver should have a temperature sensor which most of today's wireless systems have).
The following calibration procedure may be used to adjust properly the LNA tuner:
Step #1:
Turn on transmit chain, receive chain, the TX to RX coupling element and switch off any antenna switching element in the front-end.
Step#2:
Set the transmit frequency=receive frequency=CH#1 (channel of the receive operating band; see
Step #3:
Set the transmit power to a low power which can be sufficiently amplified by the receiver chain (after going through the TX to RX coupling element) to get a nominal baseband demodulated signal of amplitude Abb_RX.
Step #4:
Sweep the LNA tuner frequency Fi from lowest frequency Ftuner_min to the highest frequency Ftuner_max (see
Step #5:
Find the index i of the tuner frequency Fi for which the baseband receive amplitude is maximum. Call the found index iCH—#1
Step #6:
Repeat step #2 to step #5 for all other channels of receive operating band (see
Step #7:
Save in the baseband memory a lookup table (LUT), an example of which is shown in
Step #8:
The calibrated receiver will use the LUT defined in step #7 during normal operation.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/IB2006/052596 | 7/28/2006 | WO | 00 | 1/29/2008 |
Number | Date | Country | |
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60703794 | Jul 2005 | US |