Which measurement instruments have yet to be integrated? In my previous article, I did not include an emerging instrument: the waveform generator (WG), a partial successor to function generators (FGs). They digitally synthesize and process waveforms that are then converted to continuous (analog) form by a fast DAC.
Analog Devices Inc. makes the AD9833, applying direct digital synthesis (DDS) followed by a DAC. It outputs the classic three FG waveforms (triangle, sine, and square) at rather low amplitudes.
More WG ICs with more integration can be expected. To drive the usual 50Ω cable from the 50Ω source, an on-chip output amplifier needs to source and sink at least 100mA to produce the usual ±10V output. The power dissipation works against the TC tracking of the precision R-2R dividers of the DAC, though it should not be a problem for the DDS. Inclusion of the output amplifier is possible for a WG and would be much worse for a classic FG, where the four-decade range on frequency sweep for a given full-scale frequency corresponds to a voltage range within the triangle-wave generator of four decades. A 1V full-scale control of frequency has a zero-scale voltage of 100μV. Trans-chip thermals can interfere with such circuitry.
Now consider a complicated instrument: the oscilloscope, the mainstay of electronics benches. Pop the lid on a scope, and you will see nonmonolithic subsystems. The human-machine interface, or front panel, is of course a basic limitation. The entire waveform path of a scope is never built from either CRT or LCD technology, though active devices are integrated into the better LCD panels. For instance, input attenuators have sheet-metal shields — without which the volts/division knob would go down only to something like 0.1V/div. Any lower range would encounter too much radiated noise. The power supply also has large magnetic components.
Before the digital storage oscilloscope (DSO), analog scope integration was increasing. Vertical amplifiers were reduced in the 1970s to preamps, channel switches, and CRT-driver amplifiers with considerable off-chip support. Thermal distortion was compensated by external RC networks. Silicon integration of the high-bandwidth delay line remains a formidable challenge, and its frequency compensation is non-exponential. The horizontal axis was also capable of significant integration, with a trigger generator IC followed by a sweep generator IC and then a horizontal output amplifier that drove the CRT plates. An additional IC handled sweep control logic.
The functions contained in a DSO lend themselves to integration by the very nature of the scope — they are digital. The horizontal functions are replaced by digital circuitry. The trigger input waveform, routed from the vertical amplifier, is continuous and is turned into an edge by a fast comparator and a DAC trigger level. The sweep generator (or time-base) is replaced by fast digital counter-timer circuits, with count modulus and sampling rate set by a μC. One grand benefit of DSOs is that the display need not be written to in real-time.
Most of the analog in a DSO is in the analog front end — the analog circuits before the digitizer. Much of this circuitry can be and now is integrated. What remains is the attenuator, though it too shows possibilities for greater integration. Long ago, attenuators were completely passive networks. They were voltage dividers made up of precision resistors and capacitors (the capacitors for AC compensation) and housed in a sheet-metal shield (enclosure). The shields, intended to minimize interference from electrical fields, are hard to integrate, but they could be reduced to the point where all they enclose is an IC.
In the 1970s, H-P scope engineers began combining multi-path amplifiers with attenuators. That will be a topic for another article.
Not much circuitry is left in a DSO that is not in ICs. That makes it easier for new companies to bring DSOs to the market. Tektronix built a leading-edge IC fab facility in the 1970s, with bipolar junction transistors having fT values of a few gigahertz. That was leading-edge technology at the time — ahead of other semiconductor companies.
Eventually, it was not feasible for Tek to cloister all this technological capability only for scope manufacturing (though it put the company in the driver's seat competing with H-P in analog scopes). Now DSOs are being made in China. For its low-end DSO line, Agilent rebrands scopes from Rigol. It sells two-channel, 100MHz DSOs for $399. This would not be possible without extensive, nonproprietary analog integration. Two more rising stars in China making DSOs are Owon and Atten. Both are selling competitive measurement instruments.