Microwave Engineering Europe - April 2008 - (Page 20)

20 MILITARY/AEROSPACE FOCUS success is in the receive path, because most of the signal processing functions are performed in the receive path. Yet, some functions in the receive path are strictly physical and therefore are better implemented using passive (analog) devices. This approach is especially effective when considering power consumption in battery operated equipment. Our argument is based on the generic block diagram of an ISWR receiver, shown in Figure 3. The receiver includes an ADC located at the receive path input. This ADC circuit is required to possess rather extraordinary performance, to meet typical communication system speciﬁcations. For example, a VHF tactical radio must simultaneously possess high sensitivity (a few tenths of a microvolt), very high dynamic range (at least 120 dB) and, most important, tolerate high power interference (up to 1 W) from nearby equipment operating on channels close to the receive frequency. To ﬁlter out the interference, the ADC must have a sampling ampliﬁer capable of linear ampliﬁcation for signals up to 8 Vrms (20 Vp-p). Just consider how much power must this circuit consume, and what its noise ﬁgure can be! What makes matters even worse, the interfering signals can be anywhere within 2 MHz to 2 GHz range, and even higher. So undersampling is not possible — some interference will always be folded back into the frequency band of interest. Therefore, front end analog ﬁltering is necessary. Considering the passive nature of analog ﬁltering, in this case the undesired signal is simply rejected with no power consumed. This kind of ﬁltering will always be preferable over any digital ﬁltering of high power signals, especially in battery operated equipment. Another inherent drawback of an ADC component placed close to the antenna is its noise performance. ADCs have a lot of noise sources: sampling ampliﬁer NF, aperture jitter, quantization noise, poor isolation from backend switching circuits, etc. Commercially available high speed ADCs have effective noise ﬁgures of 30 to 40 dB; even higher values are often found. To achieve reasonable sensitivity, a low noise ampliﬁer (LNA) must be used in the front end. Unfortunately, this brings back the ADC dynamic range problem. Even with automatic gain control (AGC) the problem will not go away because of selectivity degradation issues. These considerations lead to the conclusion that the ideal receiver ADC must be replaced by the more practical equivalent shown in Figure 4. The ﬁrst function is a front end ﬁlter. Usually, this ﬁlter is has a relatively wide Figure 3: Software receiver. Figure 4: Practical implementation of ideal ADC functionality. passband, because its function is to protect against strong interference from signals at frequencies relatively far away from the desired carrier frequency. However, if the interfering signal falls within the front end ﬁlter passband, the LNA gain (50 dB in this example) will boost its level at the ADC input, relative to the level at the antenna connector. For 80 dB adjacent channel selectivity (100 dB for tactical VHF radios), the interference level at the ADC input will be 130 dB higher than the smallest desired signal at the antenna input. Assuming a sensitivity of 110 dBm, the interference voltage at the ADC input can reach more than 6 Vp-p! Moreover, to achieve 80 dB selectivity, the ADC dynamic range must exceed 90 dB. Let’s compare this requirement with the capabilities of the TI ADS8401, claimed to be the industry’s fastest 16 bit ADC: it has 90 dB SNR (full scale), but the sampling rate is only 1.25 Msps — three orders less than required! It is quite clear that any medium or high performance system must use RF signal downconversion and hardware IF ﬁltering. This is probably the only practical way to implement a medium or high performance radio (although this solution requires a frequency synthesizer). • Digital ﬁltering limitations It is often assumed that digital ﬁltering, implemented by software in DSP or in FPGA, can provide high performance, low cost and save space. While this is normally correct for baseband signal processing, for RF, IF or RFI ﬁltering this approach is simply not feasible. Digital ﬁltering is active ﬁltering: it requires linear processing of the input signal within the passband as well within the stopband. Digital ﬁltering becomes impractical when: • The input is a high level signal (for example, PA output signal); • The undesired signal has a high amplitude; • Wideband interference (up to several GHz) at any frequency in the range of interest, makes undersampling impossible; • Power consumption is a crucial issue (battery operated equipment). EMC and RFI Just consider the typical battery of requirements a radio must comply with: • Many different standards and regulatory requirements that determine the interference levels it may produce; • Black and red segregation, a common requirement in military equipment, imposes severe requirements to prevent information leakage to outside world and needs special physical means to meet such requirements; • Withstand hostile environments, with conducted and radiated interference levels outlined in various standards, as well as collocation with similar or different equipment. The mechanical construction of any radio set is affected to a high degree by such requirements. Beyond appropriate mechanical Microwave Engineering Europe ● April 2008 ● www.mwee.com 018-019-020-021-022_MWEE.indd 20 26/03/08 18:07:22 http://www.mwee.com

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Microwave Engineering Europe - April 2008 News Contents Comment Test and Measurement: Comprehensive WiMAX and Wi-Fi Product Design Demands Effective Channel Emulation Military/Aerospace Focus: Hardware Needs Limit Software Radio Interview — Mitsubishi Electric Europe: GaAs Technologies Spanning High-End Space and Radar Through to Cost-Sensitive Handset and LNB Applications How Do You Test ZigBee Transmitters? Advanced Receiver Design Boosts Performance CMOS PAs Pave the Way for One-Chip Phones Products Calendar

Microwave Engineering Europe - April 2008

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