Microwave Engineering Europe - March 2009 - (Page 11) FOCUS ON RADIO 11 conversion stages. The Þnal IF is subsampled in the 2nd Nyquist region of a 16bit ADC and the digital section of the receiver then takes the signal down to complex base-band. A direct conversion receiver was not considered practical because of the difÞculty in keeping the phase and amplitude balance and ßatness over the wide input and IF bandwidths. The high IF is used to keep the image, 1st IF, and LO out of the operating band. This means the complexity of switched/tracking input Þlters is avoided and LO isolation simpliÞed. Also the transmit section required the IF to be more than twice the highest operating frequency to place a 1x2 product out of band as the available mixers were not linear enough to keep this spur within speciÞcation. It was desirable to keep similarity between the receive and transmit frequency plans so parts of the design such as Þlters, synthesiser, and frequency doubler could be reused in both transmit and receive with minimal modiÞcation. This high IF also meant other low order spurious products were out of band. For example the ½IF = 4.5 GHz and the lowest 2x2 will be (2 x 9.4 - 9)/2 = 4.9 GHz. The system was to designed to be capable of operating with a range of sample rates (Fs) from 100 to 125 MHz. Performance of the ADC degrades as the signal frequency is increased. With an IF below 30 MHz the 2,2 response of the 3rd mixer would fall in the IF bandwidth. To avoid this the next Nyquist region of the ADC was used, placing the Þnal IF between Fs/2 and Fs. This means using a 3rd IF of ≈80 MHz places the 2,2 response above 685 MHz so the previous Þlter can provide some rejection. This improves the blocking performance or error vector magnitude (EVM) degradation that could occur with high level signals and the product in band. The 2nd IF at 650 MHz means the 2nd image is moved further from the 9 GHz IF and a high level of rejection from the microwave Þlter is practical. Filters A variety of Þlter technologies were used in system. The Þlters were designed with 3 dB bandwidths much larger than the 20 MHz operating bandwidth to maximise the ßatness in this operating band. Digital equalisation is then used to achieve improved ßatness as required. Microwave The 1st IF microwave Þlter is a combline design from machined silver plated copper and is connectorised. Its design, manufacture, and tuning were subcontracted. The transceiver chain had been split into two boards and the Þlter naturally Þtted between the boards. Due to the step size of LO1 the pass band of this Þlter is ≈70 MHz. At the band edges the Þlters show better than ±0.06 dB and ±0.4 ns variation across 20 MHz. Coaxial resonator The 2nd IF is a 5th order coupled resonator Þlter using ceramic coaxial resonators [1] and high tolerance thin Þlm capacitors. A Chebyshev Þlter was synthesised with a computer design tool and a capacitively coupled parallel resonator implementation selected. The resonator was then replaced with a suitable shorted λ/4 coaxial resonator and parallel capacitor. By selecting a resonator higher in frequency than that actually required by the design it appears inductive. It can then be tuned with a parallel capacitor giving the beneÞts that the design only required resonators cut to one length, it allows tuning of the design after the board is Figure 3: Measured responses of coaxial resonator filters. Surrounding amplifiers and attenuators are included in the measurement hence positive gain. built, and the length of the resonators is slightly reduced. The high order resonances will lead to high frequency re-entrant bands but these are not a problem because the image frequency of the following mixer falls between the main pass band and the Þrst re-entrant band where there is good rejection. Beyond this there is rejection from the preceding microwave Þlter. Measured responses of several Þlters are shown in Þgure 3. The thin Þlm capacitors from AVX Corporation have higher Q values than standard multilayer capacitors but the main reason for selecting them was for the very high tolerance so as to remove need for tuning in production. For example a 3.0 pF part is available with ±0.03 pF tolerance. The selection of Þlters built showed amplitude and phase ßatness achieved was better than ±0.31 dB and ±1.8 ns over 20 MHz. Designing the Þlter for this level of ßatness over the band led to a 3 dB bandwidth of approximately 60 MHz. Rejection was required at the 3rd mixerÕs 2,2 product and Figure 4: Measured passband responses of lumped element filters. Surrounding amplifiers are included in the measurement hence positive gain. Markers show 20 MHz operating band. Microwave Engineering Europe ● March 2009 ● www.mwee.com http://www.mwee.com
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