Programmable analog circuits yield single-chip sinusoidal oscillators
Internally programmed low-pass filters yield classic RC oscillator.
Stefano Salvatori and Paolo Lorenzi, University of Rome, Rome, Italy; Edited by Brad Thompson and Fran Granville -- EDN, January 19, 2006
Programmable-logic devices provide a popular method of implementing complex functions in digital designs. Although manufacturers don't yet offer analog circuits whose complexity compares to VLSI digital circuits, field-programmable analog circuits are enjoying extensive use in signal-conditioning and filtering applications. Based on CMOS-operational-transconductance and switched-capacitor amplifiers, these devices offer a convenient approach to relatively complex design problems. Lattice Semiconductor's (www.latticesemi.com) ispPAC10 in-system-programmable analog circuit and its accompanying PAC Designer software offer a convenient method of circuit design and verification (Reference 1). This Design Idea presents two simple sinusoidal oscillators based on the ispPAC10.
Resistors within the ispPAC10 are fixed at a nominal 250 kΩ, and all capacitors are user-selectable from 1.07 to 61.59 pF. Figure 1 shows an ispPAC10 with its internal blocks 1, 2, and 4 connected as a cascade of three first-order lowpass filters to form a classic phase-shift RC oscillator. Altering the capacitors' values produces oscillation frequencies over a range of 18 to 130 kHz. Each PAC block's gain is fixed at a factor of two to obtain a loop gain of –8, which Barkhausen's condition for oscillation requires (Reference 2). Configured from Block 3, a first-order lowpass filter reduces the THD (total harmonic distortion) on the oscillator's output. The values of capacitors in Block 3 are optimized for filtering performance and thus differ from those of the phase-shift stages.
The circuit in Figure 2 describes a two-integrator loop that forms a classic quadrature-RC oscillator. The circuit's oscillation frequency spans 12 to 126 kHz and depends on the time constants of the integrators that blocks 1 and 2 form. In theory, each integrator's gain should have an absolute value of unity, but, in practice, ispPAC allows specification only of inverting integrators, and producing a stable sinusoidal signal requires a gain of at least –4 in Block 1. The circuit uses a gain of –10. Two additional blocks of the ispPAC10 device form a second-order lowpass filter that decreases the output's THD. In both oscillator circuits, you can alter the lowpass filters' gain so that the circuit's outputs deliver specific voltages, such as 1V p-p, at all frequencies.
Table 1 and Table2, respectively, contain summaries of the phase-shift and quadrature oscillators' components and output characteristics. CN refers to the value of the capacitor used in the nth PAC block for oscillation at frequency f0. The design uses a Tektronix TDS1002 digital oscilloscope's FFT function to measure THD and the spectral line width of each output frequency at a level of –20 dB with respect to the central frequency, f0.
Figure 3 illustrates the application of a microcontroller to dynamically reconfigure an ispPAC-based oscillator for specific frequencies. The nonvolatile memory stores frequency-specific capacitance and gain values for each of the ispPAC10's circuit blocks. Data transfers occur using the IEEE 1149.1 JTAG-standard protocol through the ispPAC10's serial test-access-port interface.
| References |
|
-
Once upon a time, there were compositors and printers and proof readers and editors who worked together to ensure that what was published matched the author's intention.
It was time consuming and expensive, and mistakes were still made, but it did provide a kind of accountability and enhanced the credibility of the publication when it worked.
Computers have done away with these unnecessary human interventions. Now any fool can get an article printed, and as long as the reader gets the general idea, who cares if the details are wrong? Just don't actually build the heart monitor that you see published.
Peter Cusack - 2006-16-2 16:08:00 PST -
It's so amazing how an evident printing error could be a perfect reason for some critics to "shoot" against the authors of this design idea. I mean that it's not so necessary to be an expert in oscillator design to notice that such a rough mistake is not due to carelessness of the authors, but instead caused by a banal error of a bad interpretation of the original documents. Infact the results reported in the tables would sound like bizarre for a such wrong design: noticeable results for a non-sense circuit! However, a part from such impulsive and ready-to-attack comments, i think it is an interesting application for a device including so few elements.
Enea Braja - 2006-7-2 11:12:00 PST
