Advanced Signal Routing with the NI PXI-2593 and NI SCXI-1193 RF Switches

This document describes how to use the xx93 in custom configurations:
Unterminated 3x1, 17x1, and 35x1
Terminated 4x1, 8x1, and 16x1
Dimensionally Flexible Sparse Matrix

Overview

The NI PXI-2593 and NI SCXI-1193 (collectively xx93) 50 Ohm switch modules are designed to carry high frequency signals in excess of 500 MHz. They can be configured into various sizes of multiplexer or sparse matrix while maintaining RF signal integrity and without the need for additional external cabling. Below is a list of configurations for the xx93.
The xx93 switches can be configured either in the Measurement and Automation Explorer (MAX) or programmatically using niSwitch Initialize with Topology in NI-SWITCH 2.0. There are predefined topologies for the xx93 as unterminated multiplexers: 8x1, 16x1, and 32x1. The RF performance for these topologies is given in the specifications for each module. Specification sheets for all National Instruments switch products are installed with the NI-SWITCH 2.0 driver, and are available online from ni.com.

The xx93 switches have many more configuration possibilities than the standard unterminated multiplexers. The innovative topology of the xx93 has internal signal paths that allow any pair of terminals (Channel or Common) to be connected while maintaining RF performance. Custom configurations are obtained by using the Independent topology. This topology, just like the standard topologies, is selectable in MAX or available programmatically using niSwitch Initialize with Topology. In the Independent topology, switch modules may be controlled using Connect/Disconnect function/VIs or using niSwitch Relay Control with NI-SWITCH 2.0. NI Switch Executive can implement these configurations with Connect/Disconnect functions/VIs. The following sections describe how to select channels and control the xx93 in various custom configurations.
Connect/Disconnect function calls take two “channels” as input parameters. These channels can be external terminals (COM0, CH15, etc.) or internal routes between relays (A0B0, B0B1C0, etc.). Internal channel names for the xx93 are derived from the names of the relays they connect. Combine the names of all the relays connected by the channel, in alphabetical order and remove the “K”s. For example, the channel connecting KA0 and KB0 is A0B0.

niSwitch Relay Control uses “Open” and “Close” commands to actuate each relay in the module. Figure 2 shows the initial state of the relays in the Independent topology. These are considered the “Open” positions for each relay.

3x1 Unterminated Multiplexers

Each group of four channels (0:3, 4:7, 8:11, etc.) can be configured as independent, unterminated 3x1 multiplexers. Choose one channel as the “common”, and route it to the other three channels.

For example, choosing CH0 as a 3x1 common, route CH1, CH2, and CH3 to it with the following command options:


The PXI-2593 can be configured as quad 3x1 multiplexers using its 16 channels. The COM terminals are unused. The SCXI-1193 has 32 channels that can be divided up into eight 3x1 multiplexers. Additionally, the four COM terminals can route together creating a ninth 3x1 multiplexer as shown below. Routing function options are given in Table 3.

In the 16x1 unterminated multiplexer topology, channels 0:15 route to COM0. Similarly, channels 16:31 route to COM2. The other two COM terminals (COM1 and COM3) are not used. In the 32x1 topology, all channels route to COM0. The other three COM terminals are not used. With the Independent topology, these COM terminals can be used as extra channels, producing 17x1 and 35x1 unterminated multiplexers.

Terminated Multiplexers

It is sometimes impossible to disable a signal source when it is not connected to a load through the switch. If the switch is unterminated, the signal reflects off of the open relay, back into the source. This doubles the voltage on the transmission line, increasing crosstalk, and potentially damaging some sources. While the xx93 switches do not have integrated terminators, they can be configured such that half of the channels are terminated with external loads, such as NI P/N: 761930-01. The following diagram shows an 8x1 unterminated bank configured as a 4x1 terminated multiplexer.

NOTE: Even as a terminated multiplexer, the xx93 switches use “Break-Before-Make” relays. During relay actuation, a channel will be unterminated. Do not switch active sources if they will be damaged by the transient reflections of these actuating channels.

In the 4x1 terminated multiplexer shown above, many of the relays can be pre-switched, minimizing the number of commands necessary to connect or terminate channels.

A switch matrix allows row terminals to connect to column terminals. A full matrix topology has relays at every row-column crosspoint. While this topology is flexible and allows as many simultaneous routes as the smaller row or column dimension, it is expensive to provide relays for every crosspoint. Full matrices are not ideal for carrying high frequency signals because the unused portion of the connected traces adds capacitive load and RF stubs to the transmission lines. This results in reflections that can distort and attenuate the signal.

An alternative to a full matrix is a sparse matrix. This topology allows only a limited number of simultaneous row-to-column connections – often only one connection at a time. Sparse matrices are generally made from two multiplexers with their COM terminals tied together. They use fewer relays and are less expensive than full matrices. As shown in the following diagram, a typical sparse matrix can make a single, stub-free connection:

The flexible topology of the PXI-2593 and SCXI-1193 allows any pair of terminals to be internally connected, while maintaining RF performance. Therefore, a single xx93 module can be conceptually treated as a sparse matrix of any dimension. Examples of this Dimensionally Flexible Sparse Matrix capability are shown below.



Determining Simultaneous Connections with the Routing Resources Diagram
An advantage of the Dimensionally Flexible Sparse Matrix configuration over traditional sparse matrices is the ability to make multiple simultaneous connections. The routing resources diagram abstracts the relays and paths in the switch to a form that is useful for determining possible simultaneous routes. In the diagram, traces are represented as rooms, and relays are walls. A signal can pass from room to room via the relay walls. Only one signal can exist in a room at any one time, so once a room resource is taken, it may not be used by another route. Additionally, there are impassable walls indicated. Front-panel terminals are located around the outside diagram.


It is possible to make multiple simultaneous connections within a Dimensionally Flexible Sparse Matrix. Carefully selecting terminal assignments in a particular application can efficiently make use of this capability.

The following example shows the routing resources used in a PXI-2593 conceptually treated as a 4x14 sparse matrix. Four possible simultaneous connections are shown.

Example


It is sometimes necessary to distribute a source to several points simultaneously, like power busses, or excitation signals. The xx93 modules allow any or all terminals to be connected together. Since every additional channel added to the path splits the transmission line, this is not appropriate for high frequency signals that require consistent impedance.

As an example, a power source can be connected to COM0 and simultaneously routed to CH0, CH2, CH3, CH12, and CH13. The following diagram shows the connection path.
There are often two possible paths between any pair of terminals; however there is only one optimal path for RF performance. When routing high frequency signals, choose the path that minimizes stubs. The following diagrams show the two possible routes from CH0->CH1, along with the performance impact of an RF stub in the sub-optimal route.


When routing signals in the Independent topology, it is important to switch all tributary relays away from the signal path to reduce capacitive loading and stub-effects. For example, when connecting CH0->CH2, KB1 should be opened. While it is not directly involved in the connection, it can load down the transmission line, if closed, by hanging A1B1 off the path (shown in the following figure). This produces reflections that increase VSWR and insertion loss.

When routing between "Channel" terminals and not connecting to a COM, be sure to switch the COM relays (KD0, KD5, KD6, and KD11) away from the signal path, or they will add a significant stub.

Configuration Channels and RF Stubs
To simplify programming, define all internal channels (A0B0, B0B1C0, etc.) as Reserved for Routing Channels in MAX. NI-SWITCH 2.0 will use these channels to find routes inside the module. For example, using the single connection call: CH0->CH1 will find one of the two possible routes described above. To prevent NI-SWITCH from finding the sub-optimal path, disable B0B1C0 as a configuration channel before making the connection. NI-SWITCH will return the optimal, and only remaining, route through B0B1. Be sure to re-enable B0B1C0 before trying to make a connection requiring it, such as CH0->CH2. This technique, while tedious, may be more efficient than explicitly connecting each internal channel.

Differential Signal Routing

The xx93 switches may be used for differential signal routing (using two banks in tandem), or in situations where minimized phase skew between channels is important. Each bank of 8x1 channels is phase matched with typical propagation times within 100 ps of each other. Cables add delay to signal propagation through the system. Be sure all of the cables are the same length. Mismatched cables may introduce significant skew between channels defeating the switch’s inherent phase matching.

Reader Comments | Submit a comment »