FROM EDN EUROPE: SiGe gives 8-GHz scope inputs
By Graham Prophet -- EDN, May 13, 2004
The long-running digital-scope wars are back on familiar ground, that on which each participant periodically beats its opponents around the head with a rolled-up data sheet specifying the parameter or parameters in which it has managed to edge ahead. This time, it is Tektronix, choosing as its battle ground one of the most fundamental spec points: real-time amplifier bandwidth. In the TDS 6000B series scopes, Tek is offering bandwidth up to 8 GHz and is differentiating between the intrinsic analogue bandwidth of the amplifiers and the "extended" bandwidth achieved by applying correction with DSP, in both magnitude and phase.
As with any other amplifier, the response of the input amplifiers rolls off at the top of its frequency response to a –3-dB point at approximately 7 GHz. As in other "digital" scopes, you can extend this response by applying some extra amplification to lift the signal in the region where the response is rolling off and then shaping the response with a FIR filter optimised for flat passband. In this way, you get a flat response out to a quoted 8 GHz.
There being no free lunch, you lose out in some other areas; for example, the extra amplification lifts the noise floor, although Tek does not specify by how much. To offer maximum signal fidelity, the 6000B series allows you to turn this DSP-based correction on and off. Tek is making a competitive issue of the assertion that, in some competing scopes, you can achieve the quoted bandwidth with DSP applied, and you don't get the option of disabling it.
The instruments will come with cables matched to 1 psec for direct connection to a target pc board. Tek will later introduce a probing technology to match the new amplifiers. The company bases a new probe module, the P7380-SMA, which has 8-GHz bandwidth, on SiGe (silicon-germanium) components.
As important as the bandwidth, according to a Tek spokesman, is the Pinpoint trigger system, which provides a full set of triggering facilities on two channels, A and B (delayed), with the ability to combine any set of features between the two. You get the full 7-GHz analogue bandwidth in all of the trigger modes, not just on rising-edge triggers. Trigger accuracy is specified with a jitter figure of less than 1.5 psec. You can, for example, set up a trigger sequence that includes a reset; if you set up an "A-followed-by-B" event sequence in which the "B" event never arrives, the scope rearms instead of waiting indefinitely.
Other specifications include up to 20G samples/sec on each of the four channels and 32M-sample record length per channel. The first release will contain two versions: The 6804 has 7-GHz analogue bandwidth extended to 8 GHz with processing, for around $79,000; you can opt for a slightly lower 6-GHz analogue bandwidth on the 6604 for $69,000.
Tektronix, +44 1344 392000, www.tektronix.com.
