All of these capabilities and more are manifested in the Tektronix RSA6120A spectrum analyzer (Fig. 3). It incorporates a feature set that directly targets digital-RF designers working in the Ku-band frequencies (12 to 18 GHz). The instrument also offers extremely high SFDR of 75 dB in the Ku-band and an enhanced set of radar measurements. Its switched-filter preselection overcomes the limitations of earlier spectrum analyzers.
Sporting a full 110 MHz of real-time bandwidth, the RSA6120A offers the industry’s best dynamic range in both wideband and narrowband measurements, according to the company. An important benchmark for narrowband performance is the third-order intercept spec (TOI).
The RSA6120A offers a TOI spec that’s 3 to 8 dB better than competing instruments, Tektronix says. The analyzer is specified for a TOI of +19 dB above 6 GHz. On the wideband side, it delivers impressive signal fidelity in the Ku-band.
The instrument’s roster of radar measurements includes new pulse statistics and histograms. It also features new pulse measurements, including overshoot, ripple, and droop. A total of 27 different automatic scalar and vector measurements is available on each radar pulse, all at the push of a single button. Its impulseresponse measurement provides automatic measurement of radar chirp quality. Users gain qualitative measurement of impedancemismatch problems in the signal chain.
A key to the RSA6120A analyzer’s performance is its application of microwave preselection in the signal-downconversion chain to narrow down to a band of interest from within a wideband window. While traditional preselector approaches use YIG-based (yttrium-iron-garnet) technology, this approach can cause phase and amplitude distortion by altering magnetic fields.
The RSA6120A also can introduce hysteresis when tuning across frequencies. Moreover, because YIG preselectors are inherently narrowband devices, wideband measurements cannot be made with the preselector inline. As a result, the instrument’s acquisition window is thrown open to images and other artifacts of upstream mixers.
Instead, the RSA6120A uses a switched-filter preselector, which offers wide frequency coverage and is always inline. Flatness across 100 MHz is specified at ±0.7 dB RMS and ±1.5° RMS. While switched-filter preselectors also introduce amplitude and phase errors, as do YIG filters, the errors introduced by a switchedfilter preselector are stable over time and temperature and can be characterized and compensated for in software.
So unlike the YIG preselector, the switched-filter preselector can be used for wideband signals. Because the preselector in the RSA6120A analyzer is always inline, radiation from its local oscillator (LO) into the RF signal chain is virtually eliminated.