Mixers are one of the fundamental components of every superheterodyne receiver (Rx), but evaluating them has never been easy, even with powerful vector-network analyzers (VNAs). The techniques employed to characterize the phase and group-delay performance of mixers are especially cumbersome, lengthy, and prone to error. To solve this problem, Agilent Technologies (Santa Rosa, CA) has created new calibration techniques for its PNA Series of VNAs that reduces or eliminates traditional problems inherent in characterization of mixers and converters. It is available for the E8362B, E8363B, and E8364B VNAs, with coverage from 10 MHz to 20, 40, and 50 GHz, respectively, as well as for the new E8361A network analyzer with coverage from 10 MHz to 67 GHz.

The 67-GHz E8361A (Fig. 1) should find a following in manufacturers of passive components and subsystems designed for satellite communications, point-to-point digital radio, broadband wireless access, and OC-768 (40 Gb/s) optical-communications systems. The instrument has all of the features and capabilities of previous PNA Series analyzers, including trace noise of less than 0.03 dB at a 1-kHz bandwidth, dynamic range greater than 90 dB at 67 GHz, and measurement speed of less than 26 µs per point. Similar to all of the PNA Series instruments, the E8361A is based on the Windows 2000 operating system, which provides the operator with a familiar operating environment, provides a multitude of connectivity choices, and allows programs to be run inside or outside the analyzer.

The frequency-offset measurement capability is implemented as a hardware and firmware solution in the analyzers. The hardware provides the ability to make basic offset-frequency measurements, including mixer-conversion loss, intermodulation distortion (IMD), and harmonic and spurious responses. The firmware automates the mixer-measurement process, making it possible for users without extensive knowledge of mixer measurement to set up, calibrate, and characterize their devices accurately and quickly. The firmware's advanced calibration choices include vector correction of conversion loss, phase, and group delay, and match-corrected absolute-power measurements, both of which increase the overall accuracy of the process compared to other methods in use today.

Any mixer-based superheterodyne receiving system requires that the mixers within it have well-controlled amplitude phase, and group-delay responses. Characterizing the amplitude response (conversion gain or loss) is the easiest measurement. Conversion phase and group delay, however, continue to be difficult to measure with high accuracy and repeatability, and the test set-up employed in the process usually requires multiple external components, with many connections and reconnections. This process creates mismatch and connector repeatability errors, and increases the chance that operator error will occur, creating a high level of uncertainty in the measurement results.

Agilent's new vector mixer calibration accommodates conversion loss as well as phase and group delay, resulting in a far more accurate and comparatively simple technique that requires fewer external components and connections. This is best understood by comparing it with two other methods that are commonly employed to characterize mixer phase or group-delay responses.

The first method requires the designer to make three measurements on three pairs of mixers. The amplitude and phase responses of each mixer are calculated by solving the three linear equations created by the three measurements. The technique uses upconversion and downconversion and employs an intermediate-frequency (IF) filter between the mixer pairs to keep the unwanted mixing product from being reconverted. The method also assumes that at least one of the mixers is reciprocal (it has the same conversion loss and group delay in upconversion as downconversion). Its most obvious drawback is the fact that three sets of measurements must be made and the mixer pairs must be reconnected with the filter. Errors can creep into the process due to connector repeatability and the mismatch effects between the filter and mixer pairs, as well as between the mixers and test equipment.