The use of frequency offset through digital frequency shift is at the expense of restricted instantaneous bandwidth. For example, for an automatic offset, the driver places the LO outside the signal bandwidth which restricts the signal bandwidth to 50% of the device instantaneous bandwidth after subtracting an instrument-dependent LO guard band.

The following figures illustrate a 1,000 MHz direct conversion acquisition versus a 1,000 MHz frequency offset acquisition. Notice the different constraints and distortions in each figure.

The direct conversion acquisition illustrated in the following figure involves a single acquisition to cover 1,000 MHz using the PXIe-5841. There are distortions present that effectively constrain the dynamic range of the spectrum, however. A primary example of this is the residual LO power, also known as carrier or LO leakage. Residual LO power is an artifact of the quadrature architecture used in direct conversion devices. Use the equation |x|>(BW/2) to calculate the minimum frequency shift needed to avoid LO leakage, where x is the minimum digital frequency shift and BW is the required bandwidth.

The following figure illustrates that you can acquire the same 1,000 MHz using frequency offset with minimal distortion present from direct conversion architectures. However, when you use frequency offset, the effective bandwidth of the device is restricted. The 1,000 MHz acquisition that took a single acquisition using direct conversion takes three acquisitions using frequency offset configuration.

Images can occur when using the PXIe-5841 with frequency offset because of imperfections of the gain imbalance and skew between the I and Q channels. Residual sideband images are suppressed by calibration, but images can still be a complication when using frequency offset.