60 System Level Architecture and Design Considerationsif the oscillator also has energy at the same offset Df from the carrier, then the block-ing signal will be mixed directly to the IF frequency as illustrated in Figure 3.15.Example 3.7: Calculating Maximum Level of Synthesizer SpursFor the specifications given in the previous example, calculate the allowable noise in a synthesizer in the presence of the blocking signals. Assume the IF bandwidth is 200 kHz. Solution: Any tone in the synthesizer at an 800-kHz offset will mix with the blocker, which is at –43 dBm, and mix it to the IF stage where it will interfere with the wanted signal. The blocker can be mixed with noise anywhere in the 200-kHz bandwidth. Since noise is specified as a density, total noise is obtained by multiplying by the bandwidth, or equivalently in dB by adding 10 log 200,000 or 53 dB. We note that to be able to detect the wanted signal reliably, as in the previous example, we need the signal to be about 11 dB or so above the mixed-down blocker. Therefore, the mixed-down blocker must be less than –113 dBm. Therefore, the maximum synthe-sizer noise power at 800 kHz offset is calculated as –113 + 43 - 53 = -123 dB lower than the desired oscillating amplitude measured in a 1-Hz bandwidth. 3.2.7 DC OffsetDC offset is caused primarily by leakage of the LO into the signal path. If the LO leaks into the signal path and is then mixed with itself (called self mixing), this mix-ing product will be at zero frequency as shown in Figure 3.16. This will produce a DC signal or DC offset in the baseband, which has nothing to do with the informa-tion in the modulated signal. This offset will be amplified by any gain stages in the baseband and can saturate the ADC if it is too large. DC offsets can also be caused by PA leakage and jamming signals received from the antenna. This problem is of-ten worse in direct conversion radios where there is usually much more gain in the baseband of the radio and the LO is often at a much higher frequency, thus reducing the LO isolation of the radio. Figure 3.15 Phase noise can cause nearby channels to get mixed down on top of the desired signal.