Mixer Conversion-Loss and Group-Delay Measurement Techniques

Introduction

This application note compares techniques and instruments for measuring Conversion Loss and Group Delay on a single-stage converter with an embedded low pass filter.  

Conversion Loss using a:

• Spectrum analyzer
•  Scalar network analyzer
• Vector network analyzer with frequency-offset mode 
• Vector network analyzer with frequency-offset error correction

Group Delay using a:

• Vector network analyzer: golden-mixer technique
• Vector network analyzer: three-mixer technique 
• Vector network analyzer with frequency-offset error correction

Measurement Setup

Each measurement technique is compared in the following areas:

• Test setup complexity 
• Measurement accuracy
• Suggested implementation and costs

Test setup complexity

This includes physical setup, calibration techniques, time, and effort required. 

Measurement accuracy 

The measurement accuracy section examines the additional considerations that need to be addressed to get the most accurate results. 

An airline experiment is performed to isolate the systematic errors present in the transmission path for measurement. The conversion loss or group delay measurement of the device-under-test (DUT) by itself is compared to the measurement of the DUT and airline. The airline is added to the measurement configuration is shown in Figure 2. It affects the phase relationship between the reflected error signals and the actual measurement signal. They will add together to produce a different result. In an ideal environment, the only change will be the loss of the airline. By comparing the measurement differences between the techniques, one can infer the level of uncertain ripple caused by the systematic errors present in each method.

Suggested implementation and costs  

With a variety of instruments from different suppliers, it is hard to generalize cost by simple categories. This final section aims to identify some relative price points in comparison to other alternatives and will identify other sources of cost including additional instruments required, operator training, test accessories, additional options, etc.

Conversion Loss Spectrum Analyzer

A popular technique for making a conversion loss measurement is to use a spectrum analyzer. A standard spectrum analyzer can be used to simultaneously measure absolute power at multiple frequencies. This is ideal when measuring signals with unknown frequency and power levels. 

Test setup complexity 

The setup and use of a spectrum analyzer is very straightforward. The output of the converter is connected to the input of the analyzer. All signals at the output of the mixer are immediately displayed. These signals include both the sum and difference mixing products, input signal, LO signal, and all of the spurious mixing products. 

The measurement can be configured in about 15 minutes. It requires an external source for the input signal and another for the LO signal. 

Measurements can be made at a single input frequency or swept over a specific bandwidth. 

The measurement can be made in at least two ways: 

• The simplest way is to use the max hold feature, available on most spectrum analyzers, by increasing the dwell time of the input source to be greater than one spectrum analyzer sweep. Figure 4 shows the results of an absolute power measurement using the Keysight Technologies, Inc. PSA Series spectrum analyzer. For this measurement, 50 milliseconds per point dwell time was used requiring approximately 23 seconds for one complete sweep.
• Another alternative is to control the sources and spectrum analyzer remotely (through a program you write). The program sets the CW frequency of the sources and spectrum analyzer, triggers the sources, and makes the power measurement at the correct frequency. The data is saved at that data point, the frequency incremented, and the processes repeated. The saved data can then be printed or plotted. Converters with internal LO’s, with or without a lockable time base, can be tested with this method without any special considerations.

Spectrum analyzers measure power: not conversion loss, therefore, conversion loss must be calculated externally. To use the equation in Figure 3, the input power to the converter must be measured as a reference. The best way is to connect the v indirectly to the spectrum analyzer input, save the response, and remove it from the measured response. 

A common mistake is to connect the test cable in and test cable out together using a thru connection, which would remove the response of both test cables. However, test cable out will see lower frequencies out of the converter during the measurement and therefore less loss than at the input frequencies of the normalization. To accurately remove the cable loss, a vector network analyzer (VNA) can be used to characterize the insertion loss of the test cable out over its intended frequency range. Figure 5 shows the addition of this correction factor in the general conversion loss equation. Note that the insertion loss of the cable is negative and should be added to the conversion loss.