Baseband I/Q Generation with Full Amplitude, Phase, and DC Offset Control

• NI LabVIEW
• NI High-Speed Sources

To test I/Q modulator performance, two baseband analog signals are needed, the in-phase (I) and quadature (Q) signals. These test signals must have good amplitude accuracy as well as low distortion and jitter. Furthermore, these two signals must be tightly synchronized with control for their relative phase, amplitude, and DC offset. Because there is a growing increasing trend for modulator designs to be differential, the I/Q signals may also need to be differential. Thus each I and Q input will need differential test signals in order to examine and study the performance of the circuit. In this case, a total of four inputs are needed for full flexibility: I-, I+, Q-, and Q+.

Typical demands on the test signals can consist of the following criteria:

1. Phase tuning range from -10 to 10 deg, with 0.1 deg resolution.
- Relative phase of each differential line can be set independently.

2. Differential I/Q signals up to 2 MHz in frequency. The base bandwidths of standards such as GSM and CDMA go up to 1.3 MHz

3. DC offset on each differential line from -1.5 to 1.5 V with 1 mV resolution.
- DC Offset of each differential line can be set independently.

4. Maximum output of 500 mV peak to peak output with 10 mV resolution.
- Amplitude of each differential line can be set independently.

Many 2-channel differential signal sources can address the generation of I/Q signals but are not able to address the issue of full independent control of parameters (such as phase, amplitude, and DC offset) of each differential line. The solution to this application is use four single-channel arbitrary waveform generators (arbs) that are synchronized so that the generator outputs correspond to the I-,I+, Q-,Q+ signals. If the requirements are relaxed whereby the relative phase, amplitude control, and DC offset parameter changes affect each differential pair (I-,I+ and Q-,Q+) rather then each individual line (no independent control of each line) then a two-arb solution is viable, with the use of common external signal conditioning. Platforms such as PXI and VXI are designed for applications requiring this level of precise timing and synchronization, because they are equipped with a shared trigger bus whereby individual modules can transmit and receive multiple triggers and clocks with software commands. In addition, with the ability of LabVIEW to generate waveforms with built-in phase offsets, you can easily adjust the the relative phase alignment of each line in the differential pair and the phase between sets of differential I and Q pairs.


The ability to generate baseband I and Q signals with the flexibility of independent control over the phase, amplitude, and DC offset of each differential line can be achieved by using four synchronized signal sources, such as four PXI-5411 Arbitrary Waveform Generators.

Reader Comments | Submit a comment »