The mixer delay measurement is an extension of the scalar mixer measurement: The network analyzer generates a two-tone RF signal as a mixer input signal and measures the converted IF signal at the mixer output. The mixer delay is derived from the relative phases of the two-tone signals at the mixer input and the mixer output.

The group delay τg of a circuit is defined as the negative derivative of its phase response (see Delay), hence, for two tones with phases Φ1 and Φ2 and a frequency difference ("aperture") Δf:   

ΔΦin and ΔΦout are the phase differences of the two tones at the input and output of the DUT, respectively.

The phase difference of the source signal ΔΦin and the aperture Δf are known quantities. ΔΦout depends on the DUT and can be measured. As a phase difference, ΔΦout is stable against variations of the LO frequency, because those will affect both signals in the same way. This means that the mixer delay measurement does not require any synchronization between the analyzer and the LO signal, even if the LO shows a noticeable frequency drift.   

Compared to conventional measurement methods, the mixer delay measurement offers several additional advantages.  

• No external mixers are needed

• A network analyzer with standard functionality is sufficient, see description of test setups below

• Easy calibration using a calibration mixer; see Mixer Delay Meas Calibration.

The mixer delay measurement requires two independent RF signal sources. The two IF signals can be measured simultaneously at a single analyzer port. For an R&S ZVA analyzer that is equipped with option R&S ZVA-B16, Direct Generator/Receiver Access, a test setup of the following type is recommended:

Test setup 1: External combiner

The lower tone signal is generated at port 1, the upper tone is provided by port 3. Both signals are combined externally and fed to the SOURCE IN connector at port 1.Thus the superimposed signals are available at test port 1 and can be fed to the DUT input. The IF output signal is measured at port 2. It is possible to use an external generator for the upper tone signal, so that two analyzer ports are sufficient.

On R&S ZVA-Z67 network analyzers, all ports have independent internal sources. You can use true differential mode with an arbitrary combination of two source ports.
On an R&S ZVT20 network analyzer that is equipped with option R&S ZVT-B11, you can combine the two source signals internally without using the SOURCE IN/OUT connectors and an external combiner. You can also replace the external combiner by an Extension Unit R&S ZVAX with installed option R&S ZVAX-B211.

As an alternative to the test setup 1 shown above, you can also combine the two tones using the coupler in the test set of port 3:

Test setup 2: Coupler used as an internal combiner

When using test setup 2, open the Channel – Mode – Port Configuration dialog and adjust the source powers: To compensate for the coupling loss of the internal coupler, the source power at port 1 should be 10 dB above the source power at port 1. See alsoLevel Handling during Power Calibration.  

For network analyzers with upper frequency limits of 20 GHz and above, test setup 2 causes some restrictions:
—The lower frequency limit is 500 MHz. Use external combiners at lower frequencies, especially if you want to measure at high RF source powers.
—If an high-frequency RF signal is down-converted to in IF signal with a frequency below 500 MHz, the IF signal should be measured directly at MEAS IN (rather than at test port 2).   

With option R&S ZVA-Z10, the mixer delay measurement can be performed with two different R&S ZVA or R&S ZVT network analyzers, one providing the source ports, the other the receive port. The two instruments can communicate with each other via LAN using LXI event messages. This can be advantageous e.g. for measurements on DUTs with large dimensions where the connection to a single instrument would require very long RF cables. The test setup for a source instrument with internal coupler is shown in the Mixer Delay Measurement Setup dialog.  

Test setup with external receiver  

It is also possible to use an external coupler; in analogy to the Test Setup with Internal Receiver, or establish a direct (dedicated) LAN connection between the two analyzers.

The measurement with external receiver is performed as follows:

1. The sender network analyzer (VNA1) transfers its channel settings to the receiver analyzer (VNA2), i.e. VNA2 is fully controlled by VNA1 and shows its remote screen.

2. VNA1 generates the dual-tone source signal for the first sweep point, VNA2 measures the b-waves and returns the results to VNA1.

3. VNA1 calculates and displays the mixer delay results.

4. Steps 2 and 3 are repeated for all sweep points.

Conditions and Restrictions

The mixer delay with external receiver requires a single-channel test setup. During the measurement, use the 10 MHz reference frequency to keep both analyzers synchronized and do not switch off any of the instruments.

If the NWA application is closed and started again, the measurement must be re-started manually (Channel – Sweep – Restart).    

The mixer delay measurement depends on the frequency difference between the upper and lower tone termed the Aperture. Conversion of the two-tone RF signal in the mixer preserves the aperture. To ensure that the analyzer can detect the two IF signals and calculate reasonable group delay results, observe the following rules:

• Select a measurement bandwidth (IF bandwidth) which is significantly smaller than the aperture. High selectivity IF filters have steeper edges and improve the detection of the two adjacent signals.

• Detection fails if the frequency drift of the LO is larger than the IF bandwidth. An IF bandwidth larger than approx. 3 times the maximum LO drift is generally sufficient.

• The aperture must be adjusted to the mixer properties. A small aperture increases the noise in the group delay; a large aperture tends to minimize noise, but at the expense of the frequency resolution. Large aperture settings are preferable if the mixer shows little dispersion (i.e. almost constant delay).
  For most broadband mixers, large aperture values (10 MHz to 15 MHz) are preferable. For satellite applications, smaller apertures (2 MHz to 4 MHz) are recommended. See also Trace – Format –Aperture.  

• The calculation of the mixer delay fails if the phase shift difference between the upper and lower tones caused by the mixer exceeds 180 deg.  

When a mixer delay measurement is active, the wave quantities a and b in the More Wave Quantities and More Ratios dialogs correspond to the lower tone. The primed wave quantities a' and b' correspond to the upper tone.   

Connecting cables and other components in the test setup have an impact on the phase difference between the upper and the lower tone in the RF and IF signals and thus affect the measured mixer delay. Hence, an uncalibrated mixer delay measurement cannot provide the accurate "absolute" delay of a mixer.

A mixer delay calibration uses a mixer with known absolute delay as a calibration standard. Absolute mixer data can be acquired in a vector mixer measurement or in a simulation. The analyzer measures the calibration mixer and compares the measured group delay with the actual group delay in order to determine a set of correction data. The correction data is applied to subsequent measurements, if Correction On is selected in the Define Mixer Delay Measurement without LO Access dialog.

The following guidelines will help you find the appropriate calibration method:

• For many applications, the "relative" mixer delay of different mixers is a sufficient information. Relative measurements do not require any calibration; no calibration mixer with known absolute delay is needed.

• In cases where the delay of the calibration mixer is much smaller than the delay of the measured mixer (and small compared to an additional delay caused by the test setup), its absolute delay may be assumed to be zero. With this approximation, a calibration is still possible, but no previous vector mixer measurement for the calibration mixer is required.

• To determine accurate absolute delays, in particular for mixers with small delay values, a calibration using a calibration mixer with known (constant or frequency/power-dependent) delay is needed.  

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