Mixer Measurements

Mixer Measurement Methods

The Analyzer allows you to perform measurements of mixers and other frequency translating devices using scalar and vector methods.

The scalar method allows measurement of the scalar transmission S-parameters of frequency translating devices. Phase and group delay measurements are not accessible in this mode. The advantage of this method is the simplicity of measurement setup (no additional equipment necessary). See Figure 37.

The scalar measurement method is based on frequency offset mode. Frequency offset mode enables a frequency offset between the Analyzer test ports as described in detail in section 4.11.2. Frequency offset mode can be combined with various calibration methods.

When performing scalar measurements of a mixer, the most accurate method of calibration is scalar mixer calibration (See section 5.6).

An easier but less accurate method is using absolute measurements in combination with receiver calibration and power calibration (See sections 4.7.3, 5.5 and section 5.4). This method often results in transmission S-parameter ripples due to mixer input and output mismatch. This can be partially compensated by using matching attenuators of 3-10 dB at the mixer input and output.

The vector mixer calibration method allows for measurement of mixer transmission complex S-parameters including phase and group delay. The method requires additional equipment (See Figure 38): an external mixer with filter, which is called calibration mixer, and a LO common for both the calibration mixer and the mixer under test.

The vector mixer calibration method doesn’t use frequency offset. The vector mixer calibration method ensures same frequency at the both test ports of the Analyzer, in normal operation mode. The vector mixer calibration procedure is described in the section 5.7.

Frequency Offset Mode

The Frequency Offset mode allows for S-parameter measurement of frequency translating devices including vector reflection measurements and scalar transmission measurements. In this context, frequency translating devices include both frequency shifting devices such as mixers and converters, as well as devices dividing or multiplying frequency.

This measurement mode is based on a frequency offset between the ports. The frequency offset is defined for each port using three coefficients: multiplier, divider, and offset. These coefficients allow for calculation of a port frequency relative to the basic frequency range.

M – multiplier,

D – divider,

F_ofs – offset,

F_base – basic frequency.

In most cases it is sufficient to apply an offset to only one of the ports, leaving the other one at the basic frequency (M=1, D=1, Fofs=0).

Below are some examples of offset coefficient calculation for different types of frequency conversion. Here the mixer RF input is connected to Port 1, and the mixer IF output is connected to Port 2. The basic frequency range is set to the mixer RF frequency range and the first port of the Analyzer does not use frequency offset. The second port of the Analyzer is set to the IF frequency range and use frequency offset mode as follows:

In frequency offset mode, the bottom part of the channel window will indicate each port’s frequency span.

User can set Start and Stop frequency for each port directly instead of using Multiplier, Divider and Offset values. Using Start/Stop values will set Multiplier and Offset, which can be determined from the specified frequency and the base frequency, while maintaining the preset Divider.

Source/Receivers Frequency Offset Feature

Conventional frequency offset mode uses frequency offset between the ports, while the source and receivers of each port operate at a common frequency. Frequency offset between the ports allows for S-parameter measurement of frequency translating devices including vector reflection measurements and scalar transmission measurements.

The source/receivers frequency offset feature introduces a frequency offset between the source and receivers within a single port. Frequency offset between the source and receivers allows for absolute measurements only.

Automatic Adjustment of Offset Frequency

When you perform mixer measurements in frequency offset mode, you need to set the offset frequency equal to the LO frequency. The error of the offset frequency setting must be less than IF filter bandwidth, otherwise, the receiver will not receive the output signal from the mixer. In practice, there is always an LO frequency setting error (unknown to the user) when the tested mixer has an independent LO.

The Analyzer offers automatic adjustment of the offset frequency. This function enables you to accurately set the offset frequency equal to the frequency of the independent LO of the DUT.

Automatic adjustment of the offset frequency can be activated only for one port. The value of the offset frequency automatic adjustment will be indicated in the line of the respective port in the channel window (See Figure 40).

Automatic adjustment can be made within a ±500 kHz range from the offset frequency set by the user. The function can be enabled/disabled by the user. Adjustment can be performed upon key pressing, or periodically at a set time interval.

Figure 40 Channel window in frequency offset mode with enabled automatic adjustment function of the offset frequency

The typical error of automatic adjustment of the offset frequency depends on the current IF filter bandwidth (See Table 13).

Table 13: Typical error of offset frequency automatic adjustment

Setting of Offset Frequency Automatic Adjustment

Softkeys related to Automatic Frequency Offset Adjustment allow you to enable/disable the automatic adjustment function of the offset frequency, to select the port, to enter the adjustment value, to enable/disable continuous adjustment, and to initiate a single adjustment.