Agilent Spectrum Analysis Basics - Agilent Technologies

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There are three common methods used to perform time gating: • Gated FFT • Gated video • Gated sweep Gated FFT Some spectrum analyzers, such as the Agilent PSA Series, have built-in FFT capabilities. In this mode, the data is acquired for an FFT starting at a chosen delay following a trigger. The IF signal is digitized and captured for a time period of 1.83 divided by resolution bandwidth. An FFT is computed based on this data acquisition and the results are displayed as the spectrum. Thus, the spectrum is that which existed at a particular time of known duration. This is the fastest gating technique whenever the span is not wider than the FFT maximum width, which for PSA is 10 MHz. To get the maximum possible frequency resolution, choose the narrowest available RBW whose capture time fits within the time period of interest. That may not always be needed, however, and you could choose a wider RBW with a corresponding narrower gate length. The minimum usable RBW in gated FFT applications is always lower than the minimum usable RBW in other gating techniques, because the IF must fully settle during the burst in other techniques, which takes longer than 1.83 divided by RBW. Gated video Gated video is the analysis technique used in a number of spectrum analyzers, including the Agilent 8560, 8590 and ESA Series. In this case, the video voltage is switched off, or to “negative infinity decibels” during the time the gate is supposed to be in its “blocked” mode. The detector is set to peak detection. The sweep time must be set so that the gates occur at least once per display point, or bucket, so that the peak detector is able to see real data during that time interval. Otherwise, there will be trace points with no data, resulting in an incomplete spectrum. Therefore, the minimum sweep time is N display buckets times burst cycle time. For example, in GSM measurements, the full frame lasts 4.615 ms. For an ESA spectrum analyzer set to its default value of 401 display points, the minimum sweep time for GSM gated video measurements would be 401 times 4.615 ms or 1.85 s. Some TDMA formats have cycle times as large as 90 ms, resulting in long sweep times using the gated video technique. RF step attenuator Mixer IF resolution bandwidth filter IF log amplifier Envelope detector (IF to video) Video bandwidth filter Peak/sample detector Analog-digital converter RF input -$ dB Gate control Reset Display logic Local oscillator Scan generator Display Figure 2-40. Block diagram of a spectrum analyzer with gated video 42
Gated sweep Gated sweep, sometimes referred to as gated LO, is the final technique. In gated sweep mode, we control the voltage ramp produced by the scan generator to sweep the LO. This is shown in figure 2-41. When the gate is active, the LO ramps up in frequency like any spectrum analyzer. When the gate is blocked, the voltage out of the scan generator is frozen, and the LO stops rising in frequency. This technique can be much faster than gated video because multiple buckets can be measured during each burst. As an example, let’s use the same GSM signal described in the gated video discussion earlier in this chapter. Using a PSA Series spectrum analyzer, a standard, non-gated, spectrum sweep over a 1 MHz span takes 14.6 ms, as shown in Figure 2-42. With a gate length of 0.3 ms, the spectrum analyzer sweep must be built up in 49 gate intervals (14.6 divided by 0.3), or. If the full frame of the GSM signal is 4.615 ms, then the total measurement time is 49 intervals times 4.615 ms = 226 ms. This represents a significant improvement in speed compared to the gated video technique which required 1.85 s for 401 data points. Gated sweep is available on the PSA Series spectrum analyzers. RF step attenuator Mixer IF resolution bandwidth filter IF log amplifier Envelope detector (IF to video) Video bandwidth filter Peak/sample detector Analog-digital converter RF input Display logic Local oscillator Gate control Scan generator Display Figure 2-41. In gated sweep mode, the LO sweeps only during gate interval Figure 2-42. Spectrum of the GSM signal 43
Page 1 and 2: Agilent Spectrum Analysis Basics Ap
Page 3 and 4: Table of Contents — continued Cha
Page 5 and 6: Some measurements require that we p
Page 7 and 8: Figure 1-3. Harmonic distortion tes
Page 9 and 10: While we have defined spectrum anal
Page 11 and 12: Since the output of a spectrum anal
Page 13 and 14: We need to pick an LO frequency and
Page 15 and 16: To separate closely spaced signals
Page 17 and 18: Agilent data sheets describe the ab
Page 19 and 20: This allows us to calculate the fil
Page 21 and 22: Some modern spectrum analyzers allo
Page 23 and 24: On the other hand, the rise time of
Page 25 and 26: The width of the resolution (IF) fi
Page 27 and 28: The “bucket” concept is importa
Page 29 and 30: Peak (positive) detection One way t
Page 31 and 32: The normal detection algorithm: If
Page 33 and 34: EMI detectors: average and quasi-pe
Page 35 and 36: The effect is most noticeable in me
Page 37 and 38: Thus, the display gradually converg
Page 39 and 40: In some cases, time-gating capabili
Page 41: Timeslots 0 1 2 3 4 5 6 7 Timeslots
Page 45 and 46: In Chapter 2, we did a filter skirt
Page 47 and 48: Custom signal processing IC Turning
Page 49 and 50: Chapter 4 Amplitude and Frequency A
Page 51 and 52: Following the input filter are the
Page 53 and 54: Improving overall uncertainty When
Page 55 and 56: Examples Let’s look at some ampli
Page 57 and 58: In a factory setting, there is ofte
Page 59 and 60: Because the input attenuator has no
Page 61 and 62: Noise figure Many receiver manufact
Page 63 and 64: Using these expressions, we’ll se
Page 65 and 66: Let’s first test the two previous
Page 67 and 68: This is the 2.5 dB factor that we a
Page 69 and 70: When we add a preamplifier to our a
Page 71 and 72: 2D dB 3D dB 3D dB 3D dB With a cons
Page 73 and 74: We can construct a similar line for
Page 75 and 76: Figure 6-2 shows the dynamic range
Page 77 and 78: Dynamic range versus measurement un
Page 79 and 80: Let’s see what happened to our dy
Page 81 and 82: The range of the log amplifier can
Page 83 and 84: Chapter 7 Extending the Frequency R
Page 85 and 86: Next let’s see to what extent har
Page 87 and 88: In examining Figure 7-5, we find so
Page 89 and 90: Can we conclude from this discussio
Page 91 and 92: Amplitude calibration So far, we ha
Page 93 and 94:
From the graph, we see that a -10 d
Page 95 and 96:
Pluses and minuses of preselection
Page 97 and 98:
Table 7-1 shows the harmonic mixing
Page 99 and 100:
Figure 7-15. Which ones are the rea
Page 101 and 102:
Figure 7-18. The image suppress fun
Page 103 and 104:
Other examples of built-in measurem
Page 105 and 106:
Digital modulation analysis The com
Page 107 and 108:
Data transfer and remote instrument
Page 109 and 110:
Summary The objective of this appli
Page 111 and 112:
Delta marker: A mode in which a fix
Page 113 and 114:
Frequency stability: A general phra
Page 115 and 116:
LO feedthrough: The response on the
Page 117 and 118:
Residual responses: Discrete respon
Page 119 and 120:
Video: In a spectrum analyzer, a te
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