Tektronix merges scope and spectrum analyser functions

EDN Europe, 30 Aug 2011

By adding a spectrum analysis channel to its MSO4000 mixed signal oscilloscope chassis, Tektronix has created what it terms a new class of measurement instrument, the Mixed Domain Oscilloscope, or MDO. The spectrum analysis function is a completely separate channel – rather than simply producing spectral information by FFT processing of the data captured by the oscilloscope channel – and the two measurement sets are correlated. You can therefore look at time-correlated analogue, digital and spectral (RF) views of data collected from a single device-under-test – for example, to look at how an RF signal is behaving in response to baseband or logic stimuli.
Tek says that its marketing reveals that over 60% of scope users also employ a spectrum analyser, but that with separate instruments the task of correlating and interpreting signals is difficult and time-consuming; it anticipates that if you are working on any product design that has an RF aspect, the MDO will become a standard test tool.

A typical task that could be transformed by the MDO would be observing how a frequency-hopping RF output responds to the signals issuing from its control logic. Tek positions the spectrum analysis capability of the MDO4000 as equivalent to a “entry-level” stand-alone instrument, but with a capture bandwidth that is much wider than that of a conventional spectrum analyser, and with a dynamic range to -60 dBc.
The MDO4000s have four analogue input channels with 500 MHz or 1 GHz real-time bandwidth; 16 digital inputs with 60-psec resolution; and a single RF channel that has a capture bandwidth of 1 GHz, and a frequency range that extends to 6 GHz. In some configurations, the effective capture bandwidth can be as much a 3GHz, Tek adds.
When you use the mixed-domain capability of the instrument, you are working with a captured trace; as you view the time-domain traces (conventional scope traces) the MDO overlays a ‘window’ indicator on the display, that Tek calls the Spectrum Time display. The spectrum analysis capability, in common with all spectrum analysers, has trade-offs between parameters such as acquisition time and resolution bandwidth; The spectrum time indicator highlights a segment of a time-domain trace and tells you that the spectrum analysis data you see is the spectrum of everything that took place in that interval. From the RF point of view, for a given RF input setting, spectrum time is the time interval the instrument needs to deliver the respective view – you can “walk” that interval through a complete captured record. Each channel has a 20 Mpoint record length.
You can view amplitude, frequency and phase information, vs. time; and the spectrum analyser can generate spectrograms with signal amplitude represented by colour. In addition to conventional scope triggering you can trigger on RF power; and an optional module (MDO4TRIG) allows additional trigger types to use the RF power level as a source, allowing you to isolate an RF event of interest, looking for a specific pulse width, or a timeout event or runt, or to include the RF input in a logic pattern defined along with the analogue and digital channels. A single trigger from any source triggers the entire instrument, preserving the time-correlation. Tek attributes the quote that, “getting the timebase aligned is the biggest part [of the MDO’s offering]” to a beta-unit customer trial. Previously, Tek says, although you could trigger separate scopes and analysers via linked triggers, the traces – as well as being on different screens – were not time-correlated so were very difficult to interpret.
The RF channel has an N-type connector, allowing you to use 50-Ω active probes; and you can adapt that connection to Tek’s VPI probe system. The MDO4000 series comprises four models, all with four scope channels and 16 logic input channels. Two have analogue sampling rates of 2.5 Gsamples/sec and 500 MHz analogue bandwidth, and two with 5 Gsamples/sec and 1 GHz; each can have 3 or 6 GHz RF bandwidth. Prices start at €15,300. – by Graham Prophet
Tek has a “launch event” at www.scoperevolution.com; and there is a video here