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Table of Contents Introduction Product Description Why Measure Group Delay? Group Delay Techniques Direct Phase Modulation Delay Key Features and Benefits Measurement Overview 3 3 3 3 3 4 4 Direct Demodulation Mode (Zoom FFT Off) Zoom FFT Mode Marker Options (Zoom FFT on) 5 5 5 5 6 6 7 8 Normalization 9 Measurement Block Diagram Modulation Source Synthesizer Support Measuring in the Frequency Domain How to do normalization What frequency span to use? Unambiguous delay range Negative delay Measurem
Introduction Measuring absolute group delay through frequency translating devices such as downconverters or receivers is difficult at best, particularly at microwave frequencies. Vector network analyzers are excellent tools for measuring group delay relative to a “golden device”, but they are difficult to use for absolute measurements. For microwave devices without access to internal local oscillators (LOs), the problem is especially difficult as vector network analyzers cannot be used at all.
The direct phase technique requires that both the test and reference signals be at the same frequency in order for phase to be measured. For measuring frequency translating devices, a reference DUT or external mixer is needed to ensure this condition. Modulation Delay The second technique measures modulation delay. A modulated carrier is passed through the DUT, and then demodulated.
Simplified user interface. Custom menukeys tailor the MTA’s user interface for making group delay measurements without the need for an external computer. Limitations The key limitations of this application are that it is slower than most other solutions, and the relative accuracy, while adequate for most measurements, may not be as good compared to what customers have come to expect from instruments such as the HP 8510 vector network analyzer.
Measuring in the Frequency Domain This program makes extensive use of the Fast Fourier Transform (FFT) to perform the group delay measurement. FFTs are used to demodulate the carrier and compute the phase of the baseband signal. Zoom FFTs are also used as a noise reduction technique. A zoom FFT takes a larger time record to get better frequency resolution, and displays only a portion of the frequency domain data. The program operates in two basic measurement modes.
Zoom FFT Mode With this mode, the user specifies the amount of zoom used, from 1 to 128. Higher zoom values result in better accuracy, but slower measurement times. Larger zoom values are analogous to smaller resolution bandwidths in a spectrum analyzer. A typical value which represents a good trade-off of accuracy versus measurement time is 16. Each factor of two increase in the zoom FFT value will slow the measurement time down by two as well.
When the program is running, the top trace of the display shows the first order sideband pair, and the bottom trace shows the baseband signal, but this time the signals are centered on the screen (figure 7). With the zoom FFT mode, both amplitude and phase versus frequency can be measured, with no additional measurement time. Marker Options (zoom FFT on) When the zoom FFT mode is selected, the group delay program defaults to placing the marker on signals using the highest peak function.
Normalization Trace normalization is desirable for relative measurements, and required for absolute delay measurements. For obtaining the best relative accuracy, performing a normalization removes the group delay contributions of the interconnect cables and the test equipment. An absolute group delay measurement can only be done by first establishing a reference, which is the group delay of the test setup without the DUT in place.
When the group delay flatness of the cables is not a significant contribution to measurement error, the normalization process should be done with both the input and output cables present. If cable contribution is likely to be a significant factor over one or both of the frequency ranges used in the measurement, then the normalization can be done with only one of the cables.
Measurement Time vs. Trace Length min 27 Obtaining the Program 24 21 18 15 Zoom=128 Zoom=64 12 9 6 3 0 Using the Personality Zoom=32 Zoom=16 Zoom=Off 1 4 8 16 32 64 Zoom=8 128 The HP 71500A group delay personality is distributed free of charge on an “as is” basis, without a formal HP product number. It is neither a fully supported nor specified product. The user is free to modify the IBASIC code to customize it for their particular measurement application.
involves using an external computer, and the other uses the mass storage capability of the HP 70004A color display/mainframe. The latter method can load the program either from a floppy disk or a memory card. Since the program is likely to be supplied on a floppy disk, it is desirable to copy it to a memory card as soon as possible. Subsequent downloadings can then be done from the memory card, which alleviates the need to have an external computer or floppy disk drive. address and volume number.
Either an HP-HIL keyboard can be used for entering the command (plugged into the front of the HP 70004A display/mainframe), or the “typing aids” key under the Utility menu of the USER keys. As the program is stored on the card, the LED to the left of the card will flash intermittently. If there is a problem with the mass storage definition, refer to chapter 3 of the HP 71500A Instrument BASIC User’s Guide (70820-90055).
Setting Up and Doing a Measurement The order of keystrokes for setting up a group delay measurement is not crucial. The HELP key under the Sweep menu describes one possible sequence: 1. Configure the sources (Sources menu): the personality allows for the HP-IB control for up to three sources (RF, LO, and modulation). At a minimum, you must have control of the RF source to perform a measurement. When entering an HP-IB address, use the last two digits only (for example, enter 19 rather than 819).
The circuit ON/OFF softkey is used to display or disable the block diagram drawing. Although the drawing shows a connection of the 10 MHz reference between the sources and the HP 71500A, it is usually not necessary to do so. to select between the two marker modes, LOCAL or HIGHEST (see discussion of Marker options on page 8). The phase ON/OFF softkey determines whether a phase trace is displayed below the group delay trace.
6. Start the measurement (Sweep menu): to invoke a measurement, press the MEASURE key. At any time during the acquisition of data, the ABORT key can be pressed to stop the measurement. It may take a few seconds before anything happens because the abort is processed after the current trace acquisition is completed. If the instrument appears to be frozen after pressing ABORT, press INSTR PRESET followed by the USER key to return to the personality.
Accuracy Considerations Typical Accuracy Tables 1, 2, and 3 show typical measurement repeatability/accuracy versus zoom size at 1 and 20 GHz. These numbers do not take into account mismatch errors, or frequency normalization error, so they represent best case conditions (mismatch is discussed in more detail on page 18). Each data point in the table was derived from two nonfrequency translating measurements of a cable, each with a 100 MHz span centered at the carrier frequency.
Signal-to-Noise Ratio Mismatch Error The accuracy of the measurement is directly proportional to the signal-to-noise ratio (SNR) of both signals. For best measurement results, the signals at both inputs to the HP 71500A should be as close to (but not above) 0 dBm as possible. Tables 4, 5, and 6 show the accuracy degradation due to sub-optimum RF input signal levels.
When using the FFT zoom mode, the highest peak function of the markers can be used to track an LO that is drifting slightly during the measurement. Larger zoom factors result in less tolerance to LO drift during the measurement, since the frequency span becomes smaller. If the sidebands drift out of the displayed frequency span, then obviously the measurement will be invalid. The direct demodulation mode has the most tolerance to LO drift and residual-FM.
Appendix A Comparison to Other Instruments This discussion is meant to clarify where the HP 71500A fits in compared to other group delay measurement solutions. Vector Network Analyzers The most common tool for measuring group delay of linear devices is the vector network analyzer (VNA). VNAs utilize the direct phase measurement technique. They are very fast and accurate. A VNA by itself can only measure non-frequency translating devices.
increasing the system cost and complexity. The HP 71500A solution is much simpler, because it directly provides RF-IF or IF-RF measurements without additional components. HP 11770A Link Measurement Personality This personality provides the HP 8590 E-series spectrum analyzers with group delay measurement capability very similar to an MLA. This solution can measure devices from 300 KHz to 2.9 GHz; it is not limited to 70/140 MHz IFs as with a conventional MLA.
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