The Growing Importance of Switched-Filter Preselection
By Tektronix

An irreversible trend in nearly all digital RF applications is toward greater use of wideband signals. Ever increasing amounts of data are being sent at faster rates and the only way to accommodate the demand is with wideband signals. Even the military, which often gets by with narrowband communication technologies, can hop its signals across a 200 MHz of spectrum or more.

For RF engineers, this means that test and measurement tools must allow them to measure wideband signals as efficiently as narrowband signals. Yet, until now, measuring signals wider than 40 MHz has been challenging due to the lack of spectrum analyzers with preselection filters capable of handling today’s wideband signals without significant distortion.

Why Preselection in Spectrum Analyzers
Before looking at how the new Tektronix RSA6120A realtime spectrum analyzer solves this problem, let’s first look at the importance of preselection in the design and test of RF and microwave products, and some of the challenges with traditional filtering technology.

Ever since E.H. Armstrong’s invention of the super heterodyne in 1918, radio receivers of all types, including spectrum analyzers, have adopted the same approach to signal down conversion. Although the benefits of this architecture include design simplicity and use of lower cost components, one disadvantage is that the act of harmonic mixing simultaneously opens a multitude of spectral windows. This means that the signal present at these windows will convert in the spectrum analyzer and distort the desired signal. In addition, spectrum analyzers that utilize harmonic mixing can create numerous false signals to appear on the instrument display, making it difficult or impossible to tell the real signal from the false ones.

The most common solution to this problem is to use a tunable preselector filter that removes the unwanted mixer images and responses to local-oscillator harmonics. In effect, the preselector closes all the spectral windows except for the one of interest by the spectrum analyzer. At microwave frequencies, the preselector filter in traditional spectrum analyzers is based on yttrium-iron-garnet (YIG) technology. The action of YIG spheres controlled within a magnetic field creates the filter passband resonances needed to remove unwanted images and responses from the spectrum analyzer’s signal path. For narrowband signals, YIG filters can be effective, as shown in Figure 1.

An additional challenge results in the spectrum analyzer’s choice of Intermediate Frequency (IF). For obvious reasons, the spectrum analyzer chooses an IF frequency that is either above or below the range of frequencies to be swept. Most spectrum analyzers choose a low frequency IF for microwave coverage. It is common for spectrum analyzers which cover more than about 3 to 4 GHz to choose an IF at 300 to 400 MHz. This affords the designer the chance to use lower cost components while realizing excellent RF performance. The trade-off is that harmonics of the local-oscillator open a number of these spectral windows, necessitating the need for preselection.

The screen capture, in Figure 1, on the left is representative of a single signal that can be represented across the microwave frequency range in a traditional spectrum analyzer when the YIG preselection filter is removed. On the right, the preselection filter is restored and the single signal of interest is shown.

YIG Filters – Narrowband Only
As shown, YIG preselector filters are effective within certain parameters, and have the advantage for the instrument manufacturer of being comparatively easy and inexpensive to implement. YIGs can also be designed to provide excellent rejection for out of band signals. For the user, however, YIGs have a number of significant limitations and problems.

First, YIG preselector filters are inherently narrowband. There are significant phase variations across the filter passband, which get worse as the signal approaches the edges of the filter. As a result, the YIG preselector must be bypassed when measuring signals wider than 35 to 40 MHz. Their bandwidth also varies according to center frequency. The same YIG filter at 18 GHz will have a different 3 dB bandwidth at a lower center frequency of 3 or 4 GHz.

The problems don’t stop there. Even if an attempt can be made to correct these variations with calibration, the tuning mechanism itself defeats such calibrations. The tuning is done by varying a magnetic field imposed on the YIG crystal. When this magnetic field is tuned to a different frequency and back to the first one, the magnetic hysteresis in the magnet structure causes an inability to return to the precise frequency used previously.

This causes variations in the phase calibration with each tuning change. And if this were not enough, there are small variations in amplitude and phase that sweep through the passband as the tuning is swept across large frequency bands. What’s more, these small variations can change somewhat unpredictably with temperature.

As a consequence of these limitations, the solution employed by traditional spectrum analyzers is to simply bypass the YIG preselector filter when measuring wideband signals. This may work fine if the measurement is being taken in a shielded hangar in Roswell, New Mexico, but in the real world, multiple signals almost always exist in the vicinity of the signal being measured.

It should also be noted that preselection serves another important function: it isolates internal signals from radiating out the RF connector. More specifically, preselection isolates the internal local oscillator (LO) from leaking out the front panel connector. This energy potentially could damage the device under test, or in the case of covert surveillance operations, could reveal the operator’s location.

A Better Preselection Filter
Clearly the use of preselection filters is vital and YIG filters fall short of what’s needed to accurately measure wideband signals in the real world. With the RSA6000A series spectrum analyzers, Tektronix has taken a different approach that addresses the growing need for a microwave spectrum analyzer with preselection for wideband and fast risetime pulses, as well as narrowband signals that can hop over a wide spectrum. This is accomplished through the use of a switched-filter preselector.

Using switched-filter preselector technology allows the RSA6000A series to evaluate a broad range of modern signals, even those up to 110 MHz wide. Switched-filter preselectors do introduce some amplitude and phase errors, but the critical difference compared to YIG filters is that these errors are stable over time and temperature. As such, the errors can be largely characterized and corrected in software. As signals are already fully corrected, this type of preselector can be used for wideband signals and fast risetime pulses without the need for continued external calibration procedures that must be employed using other spectrum analyzers. And, because the RSA6000A is always preselected, LO to RF radiation is virtually eliminated.

As noted, in the real world, multiple signals often exist in the vicinity of a signal being measured. In the wideband QPSK example shown in Figure 3, a second signal exists in a spectral window that is causing the traditional spectrum analyzer on the left to convert both signals at the same time, thus preventing it from demodulating the intended signal. This second signal happens to exist at a frequency that is twice the IF away from the first. The RSA6000A’s preselection filter rejects this signal and all other spectral windows that could interfere with the analysis of the signal of interest. The result is the accurate Error Vector Magnitude (EVM) measurement shown on the right.

Radiated LO Emissions
It’s also worth exploring how effective the RSA6000’s switched-filter preselection is at reducing radiated LO emissions compared to a spectrum analyzer where preselection is bypassed. In Figure 4, a third Tektronix spectrum analyzer with DPX display was used to compare a traditional spectrum analyzer on the left to the RSA6000. The LO radiation is indicated by the upper blue band. The largest portion of radiated signals (left portion of display) stem from the LO’s fundamental, while the higher-ordered LO harmonics extend all the way through 20 GHz. As this shows, the spectrum analyzer on the left has practically become an RF beacon. The RSA6000A displays minimal LO feed radiation and would have virtually no impact on the device under test and be very difficult to detect using scanning receivers.

Conclusion
The traditional swept spectrum analyzer has long employed YIG tuner technology that has served a purpose for measuring narrowband signal powers. However, as the bandwidth of modern communications has increased and the proliferation of microwave signals infiltrated the RF spectrum, the traditional architecture now limits the growing needs for the RF and microwave designer.

The modern approach to an always on wideband switched-filtered preselection, available in the Tektronix RSA6000 Series Spectrum Analyzer, provides the measurement confidence needed to measure the wideband signals of interest and fast risetime pulses. This can be done without the need for external calibration equipment, chasing mysterious responses in unpreselected sweeps, or worrying that the LO emissions from the analyzer are going to radiate back into a product or become a beacon for your enemy.

Tektronix
www.tek.com
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