Archive for category: Standing Wave Ratio

From around the year 1929 to the late 1960s, large alternating current power systems were modeled and studied on AC network analyzers. These were an outgrowth of the DC calculating boards used in the very earliest power system analysis. These systems were essentially models of the power system, with generators, transmission lines, and loads represented by miniature electrical components with scale values in proportion to the modeled system. Model components were interconnected with flexible cords to represent the schematic of the modelled system. To reduce the size of the model components, the network analzyer was energized at a higher frequency than the 50 Hz or 60 Hz utilityfrequency, and model circuits were energized at relatively low voltages to allow for safe measurement with adequate precision. AC network analyzers were much used for power flow studies, short circuit calculations and studying system stability but were ultimately replaced by numerical solutions running on digital computers. Since the multiple elements of the AC network analyzer formed a powerful analog computer, occasionally problems in physics and chemistry were modelled (by such researchers as Gabriel Kron of General Electric), during the period up to the late 1940s prior to the ready availability of general-purpose digital computers.

One of the most essential pieces of TE in the lab is the network analyzer. It can be used to measure impedance, VSWR, loss, gain, isolation, and group delay of any two ports of a multi-port network (don’t make us draw a potato with arrows here). The two big guys in network analyzers are Agilent, the 800 pound gorilla once known as Hewlett Packard, and Anritsu once known as Wiltron before they turned Japanese.

Network analyzers fall into two categories. Vector analyzers are capable of measuring complex (magnitude and phase) reflection and transmission; scalar analyzers can only measure magnitude.

Scalar network analyzers measure the amplitude portion of scattering or S-parameters, reflection and transmission coefficients between the incident and reflection waves that describe a device’s behavior under linear conditions at the microwave frequency range. Most scalar network analyzers are used to measure transmission gain, transmission loss, return loss, and standing wave ratio (SWR). Traditional devices use diode detectors to convert a radio frequency (RF) input signal to a proportional DC level. This method is less expensive than the tuned-receiver approach, but inherently scalar in nature. Some scalar network analyzers include a 5 ¼” floppy drive or a 3 ½” disc drive. Others include a compact disc (CD) drive for loading programs or storing data.  Tape drivers and display options are also available. For example, analog meters display S-parameter values with a simple visual indicator such as a needle. Digital readouts use numeric or application-specific display. With video displays, data is presented via a cathode ray tube (CRT), liquid crystal display (LCD) or multi-line form.

There are several form factors or instrument styles for scalar network analyzers. Portable or benchtop devices can be moved with relative ease and used in a variety of applications. They may include a case or handle, but are not necessarily designed for hand held use. Fixed scalar network analyzers are kept in one location and meant to be used in one place. They are usually stand-alone devices. PC-based or “black box” instruments and modules do not include an integral display, but instead interface to a computer. They typically plug into the backplane or motherboard, or otherwise interface directly with the computer bus. For each form factor or instrument style, operating temperature and operating humidity are important considerations.

Performance specifications for scalar network analyzers include frequency range, frequency accuracy, frequency resolution, output power range, and nominal input impedance. Typically, applications such a wireless communications require higher frequency capabilities. For example, 900 MHz applications require devices with a high frequency of 10 * 900 MHz for a total of 9 GHz. Other applications must be able to measure lower frequency baseband or intermediate frequency (IF) signals. Frequency accuracyis specified as the sum of several sources of errors, including frequency-reference inaccuracy, span error, and resolution bandwidth (RBW) center-frequency error. Frequency resolution is an important specification for applications that measure close signals that need to be distinguished from one another. Output power is the 1-dB compression point that results in a 1 dB decrease in amplifier gain. Nominal input impedance is the amount of load that an input places on the signal source that drives the load. High input impedance is generally desirable and implies that little change in the signal is expected when the circuit is connected. The most common input impedances for scalar network analyzers are 50  and 75 .

There are several interfaces for scalar network analyzers. RS232, RS422, and RS485 are common serial interfaces. Universal serial bus (USB) is a 4-wire, 12-Mbps serial bus for low-to-medium speed connections. IEEE 1394 or FireWire is an interface standard adopted by the Institute of Electrical and Electronics Engineers (IEEE) for very fast digital data transfers. FireWire is a registered trademark of Apple Computer, Inc. The general-purpose interface bus (GPIB) is designed to connect computers, peripherals and laboratory instruments. Small computer systems interface (SCSI) is an intelligent I/O parallel peripheral bus. Transistor-transistor logic (TTL) is a common type of digital circuit in which the output is derived from two transistors. Some scalar network analyzers use parallel channels or Ethernet networks. Others use modems or communicate via radio transmissions or telemetry.

Special types of network analyzers can also cover lower frequency ranges down to 1 Hz. These network analyzers can be used for example for the stability analysis of open loops or for the measurement of audio and ultra sonic components.

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Vector network analyzers

 

A word about acronyms concerning network analyzers… vector network analyzers (VNAs) are often called “ANAs” by old engineers. ANA stands for “automated network analyzer”. A long time ago during the Carter administration, the original network analyzer (H-P 8409) was not automated, in the sense that TE error correction was done by hand. Return loss measurements could not exceed the VSWR of the equipment, so you couldn’t resolve beyond 20 dB return loss in most cases. Gain and insertion loss and phase were calculated from the subtraction of two measurements (first the through connection, then the DUT connection). It was a bad time to be alive.

Then the first automated network analyzers were introduced. A minicomputer (about equal to a 1000 watt, five dollar calculator) grabbed the vector data from the 8409, and did some fancy manipulations that resulted in automatic error correction and accurate magnitude and phase of the four S-parameters. It was considered magic. The next step was to build the error correction into the test equipment (no external computer) and display the error-corrected measurements in nearly real time (the original HP 8510, circa 1982). Today vector network analyzers are all automated (error correction is built in). And the acronym ANA has stuck.

This type of network analyzer consists of a sweep oscillator (almost always a synthesizer so that measurements will be repeatable), a test set which includes two ports, a control panel, an information display, and an RF cable or two to hook up your DUT. Each port of the test set includes dual directional couplers and a complex ratio measuring device. Other options include a means for bias voltage/current injection, and a computer controller to manipulate and store data. The “classic” vector network analyzer is the Agilent (HP) 8510, shown below. Depending on how much you spend, this analyzer can make measurements from 45 MHz to 110 GHz.

Before you jump into vector network analyzer measurements, you will have to calibrate the network analyzer. There are many types of calibration techniques, and even more types of calibration standards. A typical calibration will move the measurement reference planes to the very ends of the test cables. You will have the choice of calibrating for reflection or transmission only, using either of the two ports or both of them together. For most tasks you will probably calibrate both test ports for reflection and transmission, which will allow you to measure full two-port scattering matrices (S-parameters for your device under test (DUT). This is referred to as “twelve-term error correction”.

Before you perform a calibration, you should do a little “preflight” check-out of the TE and DUT.  The following are the guidelines to follow before you proceed with calibiration:

What frequency range do you need to measure?
Does the cal kit, cables and any adapters you need operate over the desired band?
Are the cables in good condition? (Connect them together and see what the effects of gently bending them have on uncalibrated transmission and reflection parameters).
Will the cables reach the DUT? (This seems obvious, but I have seen people waste time calibrating only to discover that the test cables are too short to reach both ports of the DUT).

 

Guidelines in calibrating a vector network analyzer

The reflection calibration for each port requires three standards, typically: an open circuit, a short circuit, and a matched 50-ohm load (for waveguide calibration, a pair of offset shorts and a load are used. An open in waveguide usually acs closer to a load due to radiation). The matched load can be a “broadband load”, meaning that it has very low reflection coefficient over a lot of bandwidth, or a sliding load. Sliding loads are expensive and fragile standard which should only be used if your measurement requires great accuracy (perhaps you want to be able to tell the difference between a 1.01:1 VSWR and a 1.02:1 VSWR). The sliding load recognizes that a “perfectly matched” 50 ohm calibration standard can never exist, but a series of loads with equal mismatch but varying phase can be used to draw a circle around the center of the Smith chart, thereby solving for the perfect load. My advice to you: unless someone takes the time to show you how to use the sliding load properly and remember to:

The particular set of cal standards (and test cables) that you use will depend on what frequency band you need to cover. Coaxial calibration kits come in type N, 7 mm, 3.5 mm, 2.92 mm, 2.4 mm, and 1.0 mm. There are waveguide calibration kits for every waveguide band. Be sure not to exceed the frequency capability of the test set, cables, adapters and calibration

that Cal kits are expensive, and pieces of the cal kit should NEVER be used as adapters loads in any test set. And always put the little plastic covers onto the calibration pieces, you want to prevent dirt, skin, grease, etc. from degrading the accuracy of future calibrations. To check the validity of your calibration, as well as the general health of the test equipment, you need to look at a few things after you calibrate. If you are doing transmission measurements, check the residual error in a “through” connection (connect the test cables to each other). You should see 0 dB plus or minus 0.05 dB or better. The phase should be very close to 0.0 degrees as well. The return loss of both ports should be at least 40 dB but can be better than 60 dB if you are using good equipment. The transmission and reflection parameters should not vary significantly when you gently bend the test cables, or you have a bad connection. If you see an issue with the calibration you just did, figure out the problem before you perform another calibration, or you will be wasting your time and adding needles wear and tear to the cal kit and test cables.

: During calibration, if you are measuring the loss of some test cables and don’t expect to see transmission data under -20 dB, go ahead and omit the isolation cal standard. But if you want to see the steep skirts of a filter or the reverse isolation of a multi-stage amplifier, you should perform the isolation step.

:  This will improve the accuracy of your data, so long as you do it during the calibration as well as the actual measurement. But it will slow down the measurement process noticeably, a consideration if you have a lot of data to collect in limited time.

This is an option on most new network analyzers, reducing IF bandwidth also improves measurement accuracy. Try reducing from 35 kHz down to 500 Hertz.

: Smoothing is cheating. Smoothing reduces the “bumpiness” of a frequency response by averaging data across a couple of frequency points and using the result at one frequency. But if you need to cheat to get some data for the boss who is standing behind you, go for it. The only time that smoothing may actually improve measurement error is in group delay mode (note: this is referred to as the “aperture” setting when you are using Anritsu (Wiltron) equipment. The group delay is actually calculated from the slope of the phase angle versus frequency, and the “aperture” allows the user to define how much frequency band top take the slope over.

: When you use the auto scale, it quickly displays visual information on the parameter you are investigating. But when you actually plot the data on a pen plotter or printer or using an Excel spreadsheet, use a scale that makes sense. Like 2 or 5 or 10 dB per division. NOT 3 or 6 dB per division. If I have to explain why you should do this, you should seriously consider a new career outside of engineering. Also, when you are plotting the same type of data for units of the same type that you are measuring, Try to use the same scale for all of them, or real engineers will consider you a flake when they have to check out your data.

 

Scalar Network Analyzers

 

Scalar network analyzers measure the amplitude portion of scattering or S-parameters, reflection and transmission coefficients between the incident and reflection waves that describe a device’s behavior under linear conditions at the microwave frequency range. Most scalar network analyzers are used to measure transmission gain, transmission loss, return loss, and standing wave ratio (SWR). Traditional devices use diode detectors to convert a radio frequency (RF) input signal to a proportional DC level. This method is less expensive than the tuned-receiver approach, but inherently scalar in nature. Some scalar network analyzers include a 5 ¼” floppy drive or a 3 ½” disc drive. Others include a compact disc (CD) drive for loading programs or storing data. Tape drivers and display options are also available. For example, analog meters display S-parameter values with a simple visual indicator such as a needle. Digital readouts use numeric or application-specific display. With video displays, data is presented via a cathode ray tube (CRT), liquid crystal display (LCD) or multi-line form.

 

There are several form factors or instrument styles for scalar network analyzers. Portable or benchtop devices can be moved with relative ease and used in a variety of applications. They may include a case or handle, but are not necessarily designed for hand held use. Fixed scalar network analyzers are kept in one location and meant to be used in one place. They are usually stand-alone devices. PC-based or “black box” instruments and modules do not include an integral display, but instead interface to a computer. They typically plug into the backplane or motherboard, or otherwise interface directly with the computer bus. For each form factor or instrument style, operating temperature and operating humidity are important considerations.

 

Performance specifications for scalar network analyzers include frequency range, frequency accuracy, frequency resolution, output power range, and nominal input impedance. Typically, applications such a wireless communications require higher frequency capabilities. For example, 900 MHz applications require devices with a high frequency of 10 * 900 MHz for a total of 9 GHz. Other applications must be able to measure lower frequency baseband or intermediate frequency (IF) signals. Frequency accuracyis specified as the sum of several sources of errors, including frequency-reference inaccuracy, span error, and resolution bandwidth (RBW) center-frequency error. Frequency resolution is an important specification for applications that measure close signals that need to be distinguished from one another. Output power is the 1-dB compression point that results in a 1 dB decrease in amplifier gain. Nominal input impedance is the amount of load that an input places on the signal source that drives the load. High input impedance is generally desirable and implies that little change in the signal is expected when the circuit is connected. The most common input impedances for scalar network analyzers are 50  and 75 .

There are several interfaces for scalar network analyzers. RS232, RS422, and RS485 are common serial interfaces. Universal serial bus (USB) is a 4-wire, 12-Mbps serial bus for low-to-medium speed connections. IEEE 1394 or FireWire is an interface standard adopted by the Institute of Electrical and Electronics Engineers (IEEE) for very fast digital data transfers. FireWire is a registered trademark of Apple Computer, Inc. The general-purpose interface bus (GPIB) is designed to connect computers, peripherals and laboratory instruments. Small computer systems interface (SCSI) is an intelligent I/O parallel peripheral bus. Transistor-transistor logic (TTL) is a common type of digital circuit in which the output is derived from two transistors. Some scalar network analyzers use parallel channels or Ethernet networks. Others use modems or communicate via radio transmissions or telemetry.

 

 

This is a good video so many people get the theory wrong.
Video Rating: 5 / 5

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