 loss, whereas most antenna designs should pable of operating over the desired range of frequencies. actually provide a modest signal gain. Therefore, tuning In addition to the typical low NF and high IP3, it may be the electrical length of the antenna is desired over simply desirable to have the ability to adjust the gain and po- changing the matching of the antenna. tentially scale the power down (often NF and IP3 track Next in the signal chain is the band-select filter elec- bias current) when possible, which will allow for a variety tronics. This element is provided to limit the range of of signal conditions that exist across the bands of input frequencies presented to the igh-gain stage to operation. minimize the effects of intermodulation distortion. Even Mixers are used to translate the RF spectrum to in the case where intermodulation is not a problem, it is a suitable IF frequency. Although only one mixer is possible that strong out-of-band signals could limit the shown in Figure 2.3-7, many receivers may use two or amount of potential gain in the following stages, resulting three mixer stages, each successively generating a lower in limited sensitivity. This is especially true for receivers frequency. (Note: Receiver IFs are not always lower than Analog Control Loops Channel Channel Filter 1 Filter 1 Filter 2 Antenna Output Tuning Control Loops Figure 2.3-7 Single-carrier Rx example. CHAPTER 2.3 Software defined radio the RF signal. A common example is found in HF re- Sometimes the AGC is controlled from an analog control ceivers where the desired RF signal may only be a few loop. However, a digital control loop can also be used to MHz. In these cases, they are frequently mixed up to IF implement difficult control loops not possible using frequencies of 10.7 MHz, 21.4 MHz, 45 MHz, or higher analog feedback. In multi-carrier applications, use of an IF frequencies because of the availability or performance AGC may at best be difficult. If insufficient dynamic of the required component.) Each successive sta