SPECTRUM ANALYZER; Sweep time

Sweep time

Analog resolution filters
If resolution were the only criterion on which we judged a spectrum analyzer, we might design our analyzer with the narrowest possible resolution (IF) filter and let it go at that. But resolution affects sweep time, and we care very much about sweep time. Sweep time directly affects how long it takes to complete a measurement.

Resolution comes into play because the IF filters are band-limited circuits that require finit times to charge and discharge. If the mixing products are swept through them too quickly, there will be a loss of displayed amplitude as shown in Figure 2-14. (See Envelope detector, later in this chapter, for another approach to IF response time.) If we think about how long a mixing product stays in the passband of the IF filter, that time is directly proportional to bandwidth and inversely proportional to the sweep in Hz per unit time, or:

Time in passband = RBW/(Span/ST) = (RBW)(ST)/Span

where RBW = resolution bandwidth and ST = sweep time. 

Figure 2-13. Phase noise can prevent resolution of unequal signals

Figure 2-14. Sweeping an analyzer too fast causes a drop in displayed amplitude and a shift in indicated frequency

Figure 2-12a. Phase noise performance can be optimized for different 

measurement 

Figure 2-12b. Shows more detail of the 50 kHz carrier offset region

On the other hand, the rise time of a filter is inversely proportional to its bandwidth, and if we include a constant of proportionality, k, then:

The value of k is in the 2 to 3 range for the synchronously-tuned, near-Gaussian filters used in many Agilent analyzers. 

The important message here is that a change in resolution has a dramatic effect on sweep time. Most Agilent analyzers provide values in a 1, 3, 10 sequence or in ratios roughly equaling the square root of 10. So sweep time is affected by a factor of about 10 with each step in resolution. Agilent PSA Series spectrum analyzers offer bandwidth steps of just 10% for an even better compromise among span, resolution, and sweep time. 

Spectrum analyzers automatically couple sweep time to the span and resolution bandwidth settings. Sweep time is adjusted to maintain a calibrated display. If a sweep time longer than the maximum available is called for, the analyzer indicates that the display is uncalibrated with a 'Meas Uncal' message in the upper-right part of the graticule. We are allowed to override the automatic setting and set sweep time manually if the need arises. 

Digital resolution filters

The digital resolution filters used in Agilent spectrum analyzers have an effect on sweep time that is different from the effects we've just discussed for analog filters. For swept analysis, the speed of digitally implemented filters can show a 2 to 4 times improvement. FFT-based digital filters show an even greater difference. This difference occurs because the signal being analyzed is processed in frequency blocks, depending upon the particular analyzer. For example, if the frequency block was 1 kHz, then when we select a 10 Hz resolution bandwidth, the analyzer is in effect simultaneously processing the data in each 1 kHz block through 100 contiguous 10 Hz filters. If the digital processing were instantaneous, we would expect sweep time to be reduced by a factor of 100. In practice, the reduction factor is less, but is still significant. For more information on the advantages of digital processing, 

refer to Chapter 

Envelope detector6

Spectrum analyzers typically convert the IF signal to video7 with an envelope detector. In its simplest form, an envelope detector consists of a diode, resistive load and low-pass filter, as shown in Figure 2-15. The output of the IF chain in this example, an amplitude modulated sine wave, is applied to the detector. The response of the detector follows the changes in the envelope of the IF signal, but not the instantaneous value of the IF sine wave itself.

Figure 2-15. Envelope detector 

For most measurements, we choose a resolution bandwidth narrow enough to resolve the individual spectral components of the input signal. If we fix the frequency of the LO so that our analyzer is tuned to one of the spectral components of the signal, the output of the IF is a steady sine wave with a constant peak value. The output of the envelope detector will then be a constant (dc) voltage, and there is no variation for the detector to follow. 

However, there are times when we deliberately choose a resolution bandwidth wide enough to include two or more spectral components. At other times, we have no choice. The spectral components are closer in frequency than our narrowest bandwidth. Assuming only two spectral components within the passband, we hav two sine waves interacting to create a beat note, and the envelope of the IF signal varies, as shown in Figure 2-16, as the phase 

between the two sine waves varies.

Figure 2-16. Output of the envelope detector follows the peaks of the 

IF signal 

6. The envelope detector should not be confused with the display detectors. See Detector types later in this chapter. Additional information on envelope detectors can be found in Agilent Application Note 1303, Spectrum Analyzer Measurements and Noise, literature number 5966-4008E. 7. A signal whose frequency range extends from zero (dc) to some upper frequency determined by the circuit elements. Historically, spectrum analyzers 

with analog displays used this signal to drive the vertical deflection plates of the CRT directly. Hence it was known as the video signal.The width of the resolution (IF) filter determines the maximum rate at which the envelope of the IF signal can change. This bandwidth determines how far apart two input sinusoids can be so that after the mixing process they will both be within the filter at the same time. Let's assume a 21.4 MHz final IF and a 100 kHz bandwidth. Two input signals separated by 100 kHz would produce mixing products of 21.35 and 21.45 MHz and would meet the criterion. See Figure 2-16. The detector must be able to follow the changes in the envelope created by these two signals but not the 21.4 MHz IF signal itself. 

The envelope detector is what makes the spectrum analyzer a voltmeter. Let's duplicate the situation above and have two equal-amplitude signals in the passband of the IF at the same time. A power meter would indicate a power level 3 dB above either signal, that is, the total power of the two. Assume that the two signals are close enough so that, with the analyzer 

tuned half way between them, there is negligible attenuation due to the roll-off of the filter 8. Then the analyzer display will vary between a value that is twice the voltage of either (6 dB greater) and zero (minus infinity on the log scale). We must remember that the two signals are sine waves (vectors) at different frequencies, and so they continually change in phase 

with respect to each other. At some time they add exactly in phase; at another, exactly out of phase. 

So the envelope detector follows the changing amplitude values of the peaks of the signal from the IF chain but not the instantaneous values, resulting in the loss of phase information. This gives the analyzer its voltmeter characteristics. 

Digitally implemented resolution bandwidths do not have an analog envelope detector. Instead, the digital processing computes the root sum of the squares of the I and Q data, which is mathematically equivalent to an envelope detector. For more information on digital architecture, refer to Chapter 3. 

Displays

Up until the mid-1970s, spectrum analyzers were purely analog. The displayed trace presented a continuous indication of the signal envelope, and no information was lost. However, analog displays had drawbacks. The major problem was in handling the long sweep times required for narrow resolution bandwidths. In the extreme case, the display became a spot that moved slowly across the cathode ray tube (CRT), with no real trace 

on the display. So a meaningful display was not possible with the longer sweep times. 

Agilent Technologies (part of Hewlett-Packard at the time) pioneered a variable-persistence storage CRT in which we could adjust the fade rate of the display. When properly adjusted, the old trace would just fade out at the point where the new trace was updating the display. This display was continuous, had no flicker, and avoided confusing overwrites. It worked quit 

well, but the intensity and the fade rate had to be readjusted for each new measurement situation. When digital circuitry became affordable in the mid-1970s, it was quickly put to use in spectrum analyzers. Once a trace had been digitized and put into memory, it was permanently available for display. It became an easy matter to update the display at a flicker-free rate without blooming or fading. The data in memory was updated at the sweep rate, and 

since the contents of memory were written to the display at a flicker-free rate, we could follow the updating as the analyzer swept through its selected frequency span just as we could with analog systems. 8. For this discussion, we assume that the filter is perfectly rectangular.