Data Sheet

ADL5961

THEORY OF OPERATION

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Rev. 0 | 20 of 34

MULTIPORT VNA

The ADL5961 can also be used to create VNAs consisting of

multiple ports, as shown in Figure 54. Each port connects to one

ADL5961, and RF switches route the RF signal to one ADL5961 at

a time. For optimum phase (and magnitude) accuracy, ensure that

the connections from the RF source to all ports are equal in length.

Furthermore, always terminate the RFIN ports of the ADL5961

devices with 50 Ω, that is, when the RF source is connected

but also when the RF source is not connected to the port. To

achieve this, use an SPxT nonreflective switch with the RF source

connected to the pole. For VNAs with many ports, a cascade of

nonreflective (terminated) RF switches can be used at the expense

of larger insertion loss and potentially increased frequency tilt. Note

that the connections to the RF source as drawn in Figure 54 are

simplified and do not by any means follow the recommendations for

an optimal layout of the system.

Phase synchronization between the ADL5961 devices is achieved

through the LO and offset frequency signals. For optimal accuracy,

match the propagation delays from the LO source to each of the

ADL5961 devices. Residual delay differences can be addressed

through calibration procedures for the VNA system. Routing of the

signal lines to the offset interface is less critical because these lines

operate at much lower frequencies.

The SYNC interface is used to force the LO and offset dividers

in all ADL5961 devices to the same known initial state, such that

no phase ambiguities exist between the devices. A single pulse

applied to this input after power-on is sufficient to synchronize

all devices. Perform this synchronization before an LO signal is

applied. In this case, the timing of the SYNC pulse is not critical.

For optimal accuracy, simultaneous sampling of all ADL5961 IF

channels is recommended. For high port counts, a multichannel

simultaneous sampling ADC can significantly reduce the solution

footprint. Some of these multichannel devices include built-in digital

downconversion (DDC) and decimation filters, which reduce the

amount of processing required in a digital signal processor (DSP).

The performance of the system can further be enhanced by insert-

ing a programmable RF filter in the RF path to filter any harmonic

content of the RF source and a programmable attenuator to com-

pensate for the tilt vs. frequency introduced by any filters, switches,

and transmission lines.

Figure 54. Multiport VNA-Based on the ADL5961 (IFR Is IF Reverse, and IFF Is IF Forward)