LO drive variation also may distort suppression performance. This parameter does indeed impact performance, especially sideband suppression. The nominal LO drive is 4 dBm. Figure 2-7 and Figure 2-8 show the variation of the CF and SBS performance with respect to drive level varying from 3 to 5 dBm. This is actually a sensitive variable showing that suppression performance degrades 15 to 30 dB from the optimum point with just a ±1 dB drive level variation.

Figure 2-7 CF vs LO Drive

Figure 2-8 SBS vs LO Drive Level

LO drive level sensitivity is particularly burdensome because variations in the LO drive are difficult to predict and measure. The synthesizer device providing the LO signal is subject to its own variation over frequency, power supply, and temperature. It is difficult to accurately predict or measure the LO signal in real time to adjust for any variation over operating conditions. The better approach is to select an LO synthesizer that inherently has little variation in output power for environmental conditions. For the actual product LO, the LMX2572LP synthesizer plus harmonic filter is recommended. The LMX2572 has minimal variation in LO drive strength over temperature and power supply and consumes low power. It is an ideal candidate.