The key concepts to setting up multiple source control are:

For MS464xB Series VNAs, up to four (4) external sources can be configured.

The frequency plan is separated into “bands”. There may be as many as 50 or as few as one. One needs multiple bands ONLY if the relationship between sources and the receiver change in an unusual way at some point in the sweep. Examples:

All source frequencies (internal or external) and the receiver frequency must be linearly related. All are expressed as linear equations as a function of a runner variable “f”. This variable f is always the one displayed on the X-Axis although it need not represent an actual frequency (although for convenience it usually does).

These linear relationships can change in different “bands” that the user defines. The band edges are always in terms of the runner variable f.

The band start and stop frequencies are in terms of the runner variable f. Thus they may or may not be physical. To reiterate, the choice is usually based on what one wants the X-Axis of the plots to be labeled in terms of, for example, the RF frequency or the IF frequency for a mixer measurement.

The main multiple source setup menu is shown below in Figure: Main MULTIPLE SOURCE Setup Menu in its ‘Complete’ form. The ‘Simple’ form does not have the band elements, or some of the other features.. The top half of the menu, pertaining to equation editing, will be dealt with first. The special functions of the lower half of the menu will be discussed later.

The top button on the menu toggles Multiple Source mode on or off, similar to the mode selection buttons on the APPLICATION menu (turning Multiple Source mode off here will change the Receiver Config mode on the APPLICATION menu to Standard). When the MULTIPLE SOURCE menu is entered, the MULTIPLE SOURCE tableau will appear in the lower part of the screen (Figure: MULTIPLE SOURCE Tableau below).

When the MULTIPLE SOURCE tableau first appears, the first band is in the table. The Add Band, Delete Band, and Clear Band buttons will have the obvious effects.

A red exclamation point (!) in the first column of the table indicates that an error is detected.

For each band, the following must be defined:

Each equation is of the following form:

Using the multiplier (M) and divisor (D), a rational relationship can be created between the desired frequencies. The offset (OS) completes the remainder of the linear relationship and is the CW frequency when the source is set to CW ON. Any of these parameters may be negative as long as the result of the equation is a valid frequency for that source (or receiver). This can be used for a reverse sweep (in certain mixer measurements for example).

When a cell is highlighted in the table (with the mouse or touch screen), the text entry box becomes active. Text can be directly entered into the table by double-clicking on the cell. The entry must be typed with a space between the number and the frequency units.

When a second band is added, the table adds another block as shown in Figure: MULTIPLE SOURCE Tableau—Two Band Multiple Source Tableau. Note that the bands must be contiguous although the sweep range need not be (by using segmented sweep for example).

As soon as values are changed in the table, the Done Editing button shown in Figure: Main MULTIPLE SOURCE Setup Menu in its ‘Complete’ form. The ‘Simple’ form does not have the band elements, or some of the other features. becomes active. Selecting the Done Editing button will start an error check of all entered parameters. An error dialog may appear if an error is found. During the error checking, the system applies the band limit frequencies to each equation and checks that the results are valid for a given source or the receiver. If an external source is inactive, the error checking will not be performed for that line.

The EXTERNAL MODULE dialog box is accessed off the MULTIPLE SOURCE menu.

This dialog box allows the use of a 3738x or 3739x Test Set and external broadband or millimeter-wave modules. The activation is by band and when turned on, the test set will be informed to activate its modules and use current knowledge of the driving port. This would commonly be used for broadband or mmWave measurements when the broadband/mmWave mode is too restrictive for the desired measurements. This may happen if different IFs are desired, additional frequency converters are involved in the measurement, or other unusual scenarios. The external IF ports are automatically activated in any of the non-Off modes, as are leveling and power control schemes associated with the given test set (see Broadband/mmWave Measurements (Option 7, Option 8x) for more test set information). Only one global mode can be selected per channel but that external module selection can be activated or de-activated on a band-by-band basis.

External mmWave or broadband test sets are not being used. The dialog in this state is shown in Figure: External Module Control Dialog Box with OFF Selected. The 3739x test set does not require external synthesizers, so these non-OFF choices will always be available assuming Option 8x is present. The 3738 test set does not require additional options, but does require the presence of external synthesizers 1 and 2 on the GPIB bus.

The 3738x test set is being used with conventional OEM mmWave modules (the ME7828 family of systems). In this case, a check box will be available per-defined band to allow activation of the test set in that band. This option is only available if External Sources 1 and 2 are connected and active. There are no other parameters to select other than to activate or de-activate on a band-by-band basis.

As an example to show how this selection might be used, consider a DUT that is a downconverter taking 250 to 300 GHz down to a fixed IF of 1 GHz. The LO is sub-harmonically driven with an effective multiplier of 18 and will be driven from External Source 3 (EXT. SRC 3). The RF is supplied by an external mmWave module (WR-03 waveguide in this case), which also has an effective source multiplier of 18, and this will be driven from External Source 1 (EXT. SRC. 1). This external module also has an LO input (x20 effective multiplier) that will be driven from External Source 2 (EXT SRC 2). It is desired to measure both return loss (at the RF) of the DUT as well as its conversion loss. This will require two different multiple source setups since the conversion paths in the VNA are different.

Connect all external synthesizers and make sure the GPIB addresses match, and that 10 MHz clocks are synchronized (or higher frequency references if those are being used). It is desired that the DUT LO port be driven since that can affect return loss. Since the mmWave module IF outputs will be used (and routed to the VNA rear panel), Ext Module Ctrl for BB/mmWave must be enabled.

In this case, Ext Module Ctrl must be OFF since we will not be using the rear panel IFs. Note that this also disables the test set so some care is required with RF signal cabling.

The mmWave (3739) selection also assumes the use of OEM mmWave modules, but with the 3739x test set. Details on some applicable modules, from a hardware perspective, are discussed in Broadband/mmWave Measurements (Option 7, Option 8x). Any multiplied transceivers can be used here as long as the frequency and power plans are consistent with the VectorStar system with the 3739x test set. The test set offers additional power control options and assumes the use of the internal VNA sources so there are some additional selection options as suggested by Figure: External Module Control Dialog as Configured for a Single Active Band of mmWave (3739). Note that as additional bands are added to the multiple source definition, the band check boxes must be manually checked for each in this EXTERNAL MODULE CTRL dialog.

The selection choices per band are described below:

For use with 374xx and MA25300A/MA25400A modular components. This right side of the radio button array is for the broadband operation and for banded operation. The selection for BB/mmWave is straightforward. When checked in a given band, the test set will be activated, the VNA’s internal transfer switch will be shut down, and the rear panel IF ports on the VNA will be activated. There are choices for broadband and banded operation. The distinction between these selections is based on the imposed frequency range limits when activated. Broadband allows from the lower instrument limit up to the upper limit of the system (110 GHz, 125 GHz, 145 GHz (150 GHz operational), or 220 GHz (226 GHz operational) for ME7838E/EX, A/AX, D, and G systems respectively), E band allows 56-95 GHz (under-range to 54.000000001 GHz allowed) using 3743x modules, and W band allows 65 GHz to 110 GHz using 3743x modules. For systems loaded with Option 80/081/084/085, three broadband buttons will be available for 125 GHz, 145 GHz (operational to 150 GHz), or 220 GHz (operational to 226 GHz) situations. Not all of the selections make sense depending on which modules are in use (e.g., if 3743A/AX modules are being used, only the first selection is useful). For ME7838E/EX systems (using Options 86/87) only operation to 110 GHz is allowed. Otherwise, the selections and instrument operation are the same. The dialog in the case of broadband set for two bands is shown in Figure: External Module Ctrl Broadband Mode (3739x-based) with Two Bands Configured. Note that as additional bands are added to the multiple source definition, the band checkboxes in this External Module Ctrl dialog must be manually checked.

These MA25x00A, 3743x, and 3744x modules have independent source and receiver paths that can be selected from the above dialog. The selections can be interpreted as follows:

Figure: EXTERNAL MODULE CTRL Dialog Box Variants—OFF, mmWave 3738, and mmWave 3739 shows the EXTERNAL MODULE CTRL dialog box variants for OFF, mmWave 3738, and mmWave 3739. In Figure: EXTERNAL MODULE CTRL Dialog Box—Broadband (1 of 2), examples are shown for Broadband, and Figure: EXTERNAL MODULE CTRL Dialog Box—E-Band and W-Band shows the variants for E-Band 56 GHz to 95 GHz and W-Band 65 GHz to 110 GHz.

In the BB/mmWave receiver configuration of multiple source when the instrument has two sources, there are a number of additional measurement combinations possible including manual IMD (exclusive of Option 44), mixer measurements, desense measurements, and others. There are, however, some intricacies that warrant further discussion.

Key Point: In dual source systems operating as 2-port instruments, source 2 always drives the mmWave test set (true for pure mmWave 3739 modes as well).

As such, source 1 is the free source to act as drive for a 3rd module, to act as a subharmonic LO, etc. The source 2 equation state value will determine the test set state (assuming the corresponding box has been checked) and >54 GHz power control values will affect the 3739 test set drive for BB/mmWave when in the appropriate frequency range. The port 1 <54 GHz power control field will always affect source 1 drive (available from the VNA port 1 or the access loops if so equipped) and the label for that power field will change (Src1 to Aux Module). Note also that both sources must manually be made active (under Int. Src. Control) if it is desired that they drive simultaneously.

When both sources of Option 31 are active in a mmWave configuration (using this auxiliary module for source 1), power calibrations will be more complex. When in this state (multiple source with both sources active, one of the mmWave Ext. Mod. Ctrl. buttons clicked and in a mmWave frequency range), the power menu will have ‘Src1 driving aux module’ for the first field and the power calibration selector will have three choices (Port 1, Port 2 and Aux Module) in a 2-port system. The Target Power and other concepts are similar in all cases except the Aux Module calibration does not have a factory calibration as a starting point. At this stage the power associated with the Aux module is referencing the Port 1 test port of the baseband VNA. Because the output power of the module has a large offset from the VNA test port power it may take longer to converge and one may have to adjust the main power menu setting to get closer to the desired target. When one of these power calibrations is performed, the unindicated source will be deactivated so that unindicated source will not interfere with the calibration (if it, for example, was connected into the power measurement path via a combiner that was used for an intermodulation distortion measurement). After the calibration is complete, the sources are returned to their desired driving state.

The module block diagram below in Figure: MA25x00A or 3743x Millimeter-Wave Module—Block Diagram of Leveling Options illustrates where different leveling detection points are located.

Some of these choices can be made clearer with the following measurement examples in DUT Measurement Example #1—Mixer and DUT Measurement Example #2—Up Converter.

When in a broadband or mmWave version of modular 3739 modes, power calibration can be an important topic as these modes are frequently used for pseudo-absolute power measurements such a multiplier or mixer conversion, harmonics or IMD. Since the mmWave module can only have limited internal filtering of its source chain, the internal source harmonics can sometimes be relatively high (on the order of –10 dBc at some frequencies). Since the power meter used for the power calibrations can be sensitive to this harmonic energy but the tuned response of the VNA is not, there can be a disconnect between integrated and fundamental power that can impact measurement accuracy.

A function, known as fundamental power correction, is available as part of the user power calibrations in these modes that adjusts the power calibration to take into account these differences related to spectral composition of the source signal. When enabled (by the checkbox as shown in Figure: Fundamental Power Correction Checkbox in Power Calibration Dialog—(Appears Only in Modular Broadband or Banded mmWave Mode); this checkbox is only visible when in modular mmWave/Broadband modes), the user power calibration appears to proceed the same as always but, behind the scenes, the system receiver is used to estimate source harmonic levels (at each calibration frequency and/or power) and adjust the target power to take those contributions into account. When the calibration is finished, if one goes back with the power meter to check readings, they may appear high (e.g., the target value entered was –10 dBm but the power meter now reads –9 dBm at a calibration frequency) but this is because the calibration is now trying to get fundamental power to the target rather than integrated power.

Because of the extensive bookkeeping of receiver frequency plans needed for this process, the fundamental power correction is only available if the multiple source definition includes only one band. The correction only applies when the module is sourcing power (above the sourcing breakpoint frequency which is 54 GHz by default but can be changed with the configuration file as described elsewhere).

As an example, consider a power calibration over the range 81 GHz to 86 GHz and with a target power of –15 dBm using 3739 modular broadband mode. If one performs a standard power calibration, the power meter, after calibration, will generally measure quite close to –15 dBm (subject to drift, connector repeatability, etc.). In this case, however, source harmonics ranged from about –20 dBc at the edges of the range to about –15 dBc in the middle of the range. This leads the fundamental power (using the basic power calibration) being about 0.9 dB to 1.3 dB lower than the target value. By applying the fundamental power correction, the fundamental power is closer to target (which will make the VNA measurements more accurate) but the power meter will read a slightly higher value since it is measuring the fundamental plus the harmonics. The plot in Figure: Fundamental Power Correction Checkbox in Power Calibration Dialog—(Appears Only in Modular Broadband or Banded mmWave Mode) shows the difference in power meter readings after the calibration for the two different calibration choices. The fundamental power correction adjusts the integrated power target higher to get the fundamental power closer to the desired target value. In this case example, the target was –15 dBm and the harmonics were in the –15 dBc to –20 dBc range.

Although the labeling may not be obvious, External module control= mmWave (3739) can also be used for a variety of remote antenna measurement applications even if in the microwave band. In these applications, an external downconverter (often at a measuring antenna mount) is used to allow the measured signal to be sent long distances back to the VNA at a lower frequency. This structure is pictured in Figure: Example: Remote Antenna Measurement. The IF of this remote downconversion process can be at any VNA-accessible frequency and it can be routed to a VNA port (or a receiver direct access port on instruments with option 51, 61 or 62) for final downconversion by the VNA. In this case, External Module Control is NOT used. The LO for the external conversion can be from an external synthesizer or routed from the VNA (if option 31 is installed-the second source) and can be configured using standard multiple source control. Also the receiver equation can be set to look at the external IF frequency generated by the remote downconverter. If the internal source(s) is(are) used, they are available from the standard ports (or direct loop access, if so equipped) and option 08x is not required.

Alternatively, if the remote downconverter can be set to produce an IF at 12.35 MHz (the VNA internal IF in most cases) and the instrument has option 08x, the remote IF can be routed directly to the rear panel IF ports that are normally used for the mmWave applications discussed in previous sections. Such a direct IF injection can provide noise advantages since an internal downconversion is omitted.

The mmWave (3739) External Module Control selection is important in that it enables the rear panel IF ports while imposing minimal other constraints on the frequency plan (unlike the broadband and E/W-band selections on that dialog). The internal sources can be used up to 40 GHz (for a MS4644B or MS4647B, 20 GHz for a MS4642B) while external sources can be controlled over their valid frequency ranges. The VNA source power is routed to the RF spigot on the front panel associated with option 08X (RF1 and RF2 for dual-source systems (those with option 031)). For the dual-source systems, note that the RF spigot being driven will be controlled by which source is active and both can be made active within multiple source control as discussed elsewhere in this chapter. It is also important to note that these RF spigots are coupled off of the main source drive paths so the power levels are 10-15 dB below that indicated on the power menu.

The Receiver equation can be set in this direct IF mode, but because the rear panel IF spigots are being used, the internal receivers are not active. The LO spigots on the front panel are active and the signal present there can be used, but note that the internal LOs are capped at 10 GHz so the receiver equation must be set appropriately. As an example, suppose one wanted to use the internal LO to act as sub-harmonic (1/3) drive for a remote converter and suppose the RF to be converted is at 28 GHz. We wish a 12.35 MHz IF, so the final LO (internal to the remote converter) should be 28.01235 GHz assuming a high-side LO. The sub-harmonic drive should be one-third of that or 9.33745 GHz. The multiple source system adds 12.35 MHz to the equation value so the equation should be reduced by the same amount to compensate. Thus, the receiver equation entry should yield 9.3251 GHz. An example setup is shown in Figure: Multiple Source Harmonic Conversion Example that accomplishes this measurement over a finite range. Note the -24.7 MHz used in the receiver equation. For a simple harmonic conversion using a factor of N, this number will generally be -(N-1)*12.35MHz. If multiple conversions are used or there is a more complex IF plan, the equation will differ. The universal point is that the LO will be programmed to 12.35 MHz above the output value of the equation.

Some of the other selections that are part of the external module control dialog are not particularly relevant to this measurement application. Leveling should be left at RF (leveling within the VNA source chain). Common offset should only be selected if the source(s) and receiver are within about 50 MHz of each other (when multiplication factors are removed)… this is generally not the case. Start delay and VCO overrange can be used (behavior detailed in the previous section) but they are not normally changed for this type of measurement application. The band and receiver checkboxes must, of course, be enabled for the IF switching to happen.