SPECTRUM ANALYZER; Digital filters

Digital filters
Some spectrum analyzers use digital techniques to realize their resolution bandwidth filters. Digital filters can provide important benefits, such as dramatically improved bandwidth selectivity. The Agilent PSA Series spectrum analyzers implement all resolution bandwidths digitally. Other analyzers, such as the Agilent ESA-E Series, take a hybrid approach, using
analog filters for the wider bandwidths and digital filters for bandwidths of 300 Hz and below. Refer to Chapter 3 for more information on digital filters.

Residual FM
Filter bandwidth is not the only factor that affects the resolution of a spectrum analyzer. The stability of the LOs in the analyzer, particularly the first LO, also affects resolution. The first LO is typically a YIG-tuned oscillator ( tuning somewhere in the 3 to 7 GHz range) . In early spectrum analyzer designs, these oscillators had residual FM of 1 kHz or more. This instability was transferred to any mixing products resulting from the LO and incoming signals, and it was not possible to determine whether the input signal or the LO was the source of this instability.

The minimum resolution bandwidth is determined, at least in part, by the stability of the first LO. Analyzers where no steps are taken to improve upon the inherent residual FM of the YIG oscillators typically have a minimum bandwidth of 1 kHz. However, modern analyzers have dramatically improved residual FM. For example, Agilent PSA Series analyzers have residual FM of 1 to 4 Hz and ESA Series analyzers have 2 to 8 Hz residual FM. This allows bandwidths as low as 1 Hz. So any instability we see on a spectrum analyzer today is due to the incoming signal.

Phase noise
Even though w may not be able to see the actual frequency jitter of a spectrum analyzer LO system, there is still a manifestation of the LO frequency or phase instability that can be observed. This is known as phase noise ( sometimes called sideband noise) . No oscillator is perfectly stable. All are frequency or phase modulated by random noise to some extent. As
previously noted, any instability in the LO is transferred to any mixing products resulting from the LO and input signals. So the LO phase-noise modulation sidebands appear around any spectral component on the display that is far enough above the broadband noise floor of the system ( Figure 2-11) . The amplitude difference between a displayed spectral component and the phase noise is a function of the stability of the LO. The more stable the LO, the farther down the phase noise. The amplitude difference is also a function of the resolution bandwidth. If w reduce the resolution bandwidth by a factor of ten, the level of the displayed phase noise decreases by 10 dB 5 .

Figure 2-11. Phase noise is displayed only when a signal is displayed far
enough above the system noise floor

The shape of the phase noise spectrum is a function of analyzer design, in particular, the sophistication of the phase lock loops employed to stabilized the LO. In some analyzers, the phase noise is a relatively flat pedestal out to the bandwidth of the stabilizing loop. In others, the phase noise may fall away as a function of frequency offset from the signal. Phase noise is specified in terms of dBc ( dB relative to a carrier) and normalized to a 1 Hz noise power
bandwidth. It is sometimes specified at specific frequency offsets. At other times, a curve is given to show the phase noise characteristics over a range of offsets.

Generally, we can see the inherent phase noise of a spectrum analyzer only in the narrower resolution filters, when it obscures the lower skirts of these filters. The use of the digital filters previously described does not change this effect. For wider filters, the phase noise is hidden under the filter skirt, just as in the case of two unequal sinusoids discussed earlier.

5. The effect is the same for the broadband noise floor (or any broadband noise signal) . See Chapter 5, Sensitivity and Noise.

PSA Series Spectrum Analyzer
Some modern spectrum analyzers allow the user to select different LO stabilization modes to optimize the phase noise for different measurement conditions. For example, the PSA Series spectrum analyzers offer three different modes:

Optimize phase noise for frequency offsets < 50 kHz from the carrier In this mode, the LO phase noise is optimized for the area close in to the carrier at the expense of phase noise beyond 50 kHz offset. Optimize phase noise for frequency offsets > 50 kHz from the carrier
This mode optimizes phase noise for offsets above 50 kHz away from the carrier, especially those from 70 kHz to 300 kHz. Closer offsets are compromised and the throughput of measurements is reduced. Optimize LO for fast tuning When this mode is selected, LO behavior compromises phase noise at all offsets from the carrier below approximately 2 MHz. This minimizes measurement time and allows the maximum measurement throughput when changing the center frequency or span.

The PSA spectrum analyzer phase noise optimization can also be set to auto mode, which automatically sets the instrument s behavior to optimize speed or dynamic range for various operating conditions. When the span is >_10.5 MHz or the RBW is > 200 kHz, the PSA selects fast tuning mode. For spans > 141.4 kHz and RBWs > 9. 1 kHz, the auto mode optimizes for offsets

> 50 kHz. For all other cases, the spectrum analyzer optimizes for offsets < 50 kHz. These three modes are shown in Figure 2-12a.

The ESA spectrum analyzer uses a simpler optimization scheme than the PSA, offering two user-selectable modes, optimize for best phase noise and optimize LO for fast tuning, as well as an auto mode.

In any case, phase noise becomes the ultimate limitation in an analyzer's ability to resolve signals of unequal amplitude. As shown in Figure 2-13, we may have determined that we can resolve two signals based on the 3 dB bandwidth and selectivity, only to find that the phase noise covers up the smaller signal.