Microwave Engineering Europe - March 2009 - (Page 17) FOCUS ON RADIO 17 little coding gain make them very susceptible to IP2 distortion in a zero IF receiver. More modern wideband modulations schemes such as WCDMA, LTE and WiMAX are not as susceptible to this form of distortion and make zero IF receivers a simpler proposition. By retuning the LO in the zero IF receiver to provide an offset at some low IF and by using the I and Q branches to form an image reject receiver it is possible to turn the zero IF WCDMA receive chain into a low IF chain suitable for GSM. The selection of IF and the choice of IF filter BW is important as blockers are specified on the high and low side of the wanted signal. Hence making the IF higher in frequency keeps it away from the DC IP2 product, which also allows the RF-offset blocker to get through the IF stage low pass filter. The IF frequency can be set somewhere between 400 kHz to 600 kHz IF. A low pass filter (LPF) bandwidth of around 600 to 800 kHz is optimum to ensure that analog-to-digital converters following the LPF are not compressed. The example in figure 2 uses the largest blocker from the GSM900 Mobile Station requirement using the following parameters: • Blocker Level = -23 dBm at 1.6 MHz • Wanted signal = -99 dBm • Rx Input Referred IP2 = 35 dBm • Low IF = 500 kHz • Rx IF BW = 750 kHz It can be seen that the wanted signal is recoverable after BB down conversion and filtering with an SNR > 10 dB which is sufficient to reliably demodulate in baseband. Hence with a small overhead in the baseband processing to carry out the single sideband low IF down conversion, it is possible to share the same receiver hardware resource for both WCDMA and GSM receive modes. In order to enable this reuse of resource, the receiver must have sufficient linearity, and compression to avoid distortion in the presence of the blocking signal, together with RF frequency agility and IF filter BW selectability as indicated above. Implementation A transceiver IC developed at Lime Microsystems with an operational range between 375 MHz to 4 GHz covers all of the frequency bands specified in 3GPP. It also provides 2 transmitter outputs, 2 main receiver inputs and a broadband receiver input optimized for the listen mode discussed in the previous section. The Lime transceiver is shown in figures 3 and 4. As the transceiver has not gained flexibility by adding additional circuitry but by providing reuse of existing design blocks then it offers a cost effective solution for both a single frequency system targeted at a single market (e.g. WCDMA band I in Europe) or a multi-band, multi-standard system capable of dynamically reconfiguring to any cellular band. A single low cost receive path with a sufficient frequency range, BW flexibility and linearity can be shared for both WCDMA and GSM receive paths. Having enabled the possibility of sharing the same hardware for multiple bands and standards, an optimum solution for transceiver connectivity has been identified. Using a single low cost receive path for multiple receive modes reduces the need for multiple receiver devices (whether separate devices or on chip), the LO, filters and ADC are reused offering significant saving on part count and/or silicon cost. Testing microwave systems, sub-systems or components? With the 6820A Series Microwave Scalar Analyzers with integrated source or the 6840A Series Microwave System analyzers including integrated spectrum analyzer Aeroflex has your test needs covered. Whether you are testing individual microwave components, complete radar systems, ground stations or satellite links, the series provides a comprehensive suite of easy to use configured measurements. Available in various frequency ranges up to 46 GHz the 6800 Series has the ideal tools for measuring antennas, cables, filters, LNAs, etc. All the basic standard measurements you would expect are available VSWR, Insertion Loss, Fault Location, Group Delay, LNA gain, 1 dB compression, 3 dB bandwidth - and all in automatically configured form. If you are looking for a simple and effective means of testing frequency translation devices, mixer/convertor gain and group delay are all covered. For radar installation stimulus there are pulse generator and modulator facilities available for use with the integrated source. Unique applications are also available to measure end-to-end group delay and amplitude variation across a Satellite or Microwave network. Channel Equalizers can be evaluated by taking synchronised intercontinental measurements across the complete satellite system. To order a FREE Microwave info pack including the 6820A/ 6840A data sheets, a microwave datamate and brochure visit: www.aeroflex.com/me0309 Microwave Engineering Europe ● March 2009 ● www.mwee.com http://www.aeroflex.com/me0309 http://www.aeroflex.com/me0309 http://www.mwee.com
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