Abstract:
A radiation hard design for oscillator circuits and circuits having differential outputs is described. The design includes connecting or otherwise coupling outputs of these circuits to a passive polyphase filter. The passive polyphase filter provides four quadrature outputs that are free of glitches that may have occurred at the filter input.

Description:
FIELD 
   The present invention relates generally to oscillator circuits and other circuits having a differential output, and more particularly, relates to a radiation hard design for these circuits. 
   BACKGROUND 
   Oscillator circuits are commonly used in electronic circuit designs to generate an oscillating output signal. These oscillator circuits may be found in clock generation circuits, phase-locked loop (PLL) circuits, timing circuits, and so on. For example, a ring oscillator, which generally includes an odd number of inverters connected in series, may be used as part of a PLL for clock and data recovery, frequency synthesis, and clock synchronization. 
   Differential oscillators are commonly used in order to suppress noise. For example, the differential oscillator may be based on a first Colpitts oscillator and a mirror image Colpitts oscillator that is coupled to the first Colpitts oscillator. The differential oscillator outputs differential output signals that are about 180 degrees out of phase. Colpitts oscillators are well-known in the art as are other differential oscillator designs. 
   With a differential scheme, any element in an electronic system that has to suppress noise is split into two branches. One of the branches contains the normal information, whereas the other branch contains the complementary (inverted) information. With proper matching of components, a highly symmetric design between the two branches can usually be obtained. Since the useful information is recovered or extracted from the difference in state between the two branches, common mode disturbances that affect both branches equally are ideally balanced out completely. Differential output stages are used in many conventional oscillator circuits and other circuit designs to provide balanced transmission properties with good noise immunity. 
   An oscillator circuit or other circuit providing a differential output may be used in applications in which the circuit is exposed to harsh environments, such as space and military applications. However, these circuits may be susceptible to Single Event Effects (SEE). SEE is a disturbance in an active semiconductor device caused by a single energetic particle. As semiconductor devices become smaller and smaller, transistor threshold voltages decrease. These lower thresholds reduce the charge per node needed to cause errors. As a result, the semiconductor devices become more and more susceptible to transient upsets. 
   One type of SEE is a single event upset (SEU). SEU is a radiation-induced error in a semiconductor device caused when charged particles lose energy by ionizing the medium through which they pass, leaving behind a wake of electron-hole pairs. The electron-hole pairs form a parasitic conduction path, which can cause a false transition on a node. The false transition, or glitch, can propagate through the semiconductor device and may ultimately result in the disturbance of a node containing state information, such as an output of a latch, register, or gate. 
   Typically, an SEU is caused by ionizing radiation components, such as neutrons, protons, and heavy ions. The ionizing radiation components are abundant in space and at commercial flight altitudes. Additionally, an SEU may be caused by alpha particles from the decay of trace concentrations of uranium and thorium present in some integrated circuit packaging. As another example, an SEU may be caused by detonating nuclear weapons. When a nuclear weapon is detonated, intense fluxes of gamma rays, x-rays, and other high energy particles are created, which may cause SEU. 
   For example, transistors in an oscillator circuit may be susceptible to SEU. As a result, the oscillator circuit may not provide a periodic output that can be reliably used as a clock signal or for other purposes. While others have used triple modular redundancy or other voting schemes to harden oscillator circuits against the effects of SEU, providing three oscillator circuits is typically prohibitive due to space and process variances. 
   Thus, it would be beneficial to harden an oscillator circuit or other differential output circuit so that these circuits may be used in applications that are susceptible to SEE without having to triplicate the circuit in a circuit design. 
   SUMMARY 
   A hardened oscillator circuit is described. The circuit includes a first oscillator circuit, a second oscillator circuit having a design substantially the same as the first oscillator circuit, and a filter that receives at least one input from the first and second oscillator circuits. The filter provides an output that is not impacted by a glitch on one of the filter inputs. 
   Preferably, the first and second oscillator circuits are differential oscillator circuits running in quadrature or ring oscillator circuits that provide four phases. However, the first and second oscillators may have other designs. Preferably, the filter is a passive polyphase filter. 
   A hardened circuit having differential outputs is also described. This hardened circuit includes a first circuit that provides a differential output, a second circuit having a design substantially the same as the first circuit, and a filter coupled to the differential output of the circuit. The filter provides an output that is not impacted by a glitch on one of the differential outputs. Preferably, the filter is a passive polyphase filter. 
   By running the oscillator outputs or the differential outputs through the filter, the filter negates any SEU effect seen in any one phase of an oscillator or differential output circuit. This as well as other aspects and advantages will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. Further, it is understood that this summary is merely an example and is not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Presently preferred embodiments are described below in conjunction with the appended drawing figures, wherein like reference numerals refer to like elements in the various figures, and wherein: 
       FIG. 1  is a block diagram of a hardened oscillator circuit, according to an example; 
       FIG. 2  is a circuit diagram of a passive polyphase filter for use in the hardened oscillator circuit depicted in  FIG. 1 , according to an example; 
       FIG. 3  is a circuit diagram of a hardened oscillator circuit, according to an example; and 
       FIG. 4  is a block diagram of a hardened differential circuit, according to an example. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram of a hardened oscillator circuit  100 . The circuit  100  includes two oscillator circuits  102 ,  104  and a filter  106 . The outputs of the two oscillator circuits  102 ,  104  are connected or otherwise coupled to the inputs of the filter  106 . As described in more detail below, the filter  106  provides an oscillator output that has been hardened against the effects of radiation-induced errors. 
   The first oscillator circuit  102  is an un-hardened oscillator circuit. The first oscillator circuit  102  may be duplicated providing the second oscillator circuit  104 . The two oscillator circuits  102 ,  104  may have substantially the same design. 
   Preferably, the two oscillator circuits  102 ,  104  are differential oscillator circuits running in quadrature or a ring oscillator circuit that can provide four phases. However, other circuit designs may be used for the two oscillator circuits  102 ,  104 . For example, the oscillator circuits may have any quad-phase oscillator design, such as a voltage controlled oscillator (VCO), a current controlled oscillator (CCO), or any other resonator or periodic output device producing similar waveforms in amplitude and frequency. 
     FIG. 1  depicts two outputs of each of the oscillator circuits  102 ,  104  connected to the filter  106 . The two outputs indicate that the oscillator circuits  102 ,  104  are differential oscillator circuits. However, oscillator circuits having a single output or a differential oscillator with just one output may be connected to the filter  106 . In the case of a differential oscillator circuit, one of the outputs may be unconnected to another device, otherwise referred to as “floating.” 
   When a single oscillator output from each of the oscillator circuits  102 ,  104  is connected to the filter  106  the oscillators circuits  102 ,  104  may have a particular phase relationship. For example, when the output of the first oscillator circuit  102  has a phase of about 0°, the output of the second oscillator circuit  104  may be substantially 90°, 180°, or 270° depending on how the oscillator circuits  102 ,  104  are connected to the filter  106 . 
   Preferably, the filter  106  is a passive polyphase filter. Generally, a passive polyphase filter comprises a multistage resistor and capacitor (RC) network that provides phase shifts, such as 90° phase differentials. The filter  106  is not limited to any particular passive polyphase filter design. Because the passive polyphase filter is typically formed using passive components (i.e., resistors and capacitors), the passive polyphase filter is unlikely to be impacted by SEU. 
     FIG. 2  depicts an example passive polyphase filter  200  that may be used for the filter  106 . The filter  200  is a multistage passive polyphase filter with two stages shown and ellipses indicating that more than two stages may be in the filter  200 . Multiple stages in the filter  200  may increase the frequency range over which the filter  200  operates. The number of stages used in filter  200  may depend on the application in which the filter  200  is used. The filter  200  is not limited to any particular number of stages. 
     FIG. 3  is a circuit diagram of a hardened oscillator circuit  300 . In this example, differential oscillators  302 ,  304  are operating in quadrature. The oscillators  302 ,  304  are connected to the inputs of the passive polyphase filter  200  depicted in  FIG. 2 . The filter  200  provides a four phase output that is sustained during short disruptions of one of the oscillators  302 ,  304 . 
   By running the outputs of the oscillator circuits  102 ,  104  through the filter  106 , the filter  106  negates any SEU effect seen in any one phase of an oscillator circuit  102 ,  104 . This hardening effect may be the result of one oscillator&#39;s output being sufficient to maintain drive through the passive polyphase filter  106 , producing the four quadrature outputs during the absence of the combined oscillator driving signals. During the SEU effect and subsequent oscillator recovery time, a continuous phase quadrature signal may be maintained with minimal jitter and amplitude variations. As a result, the filter  106  may harden the first oscillator circuit  102  against the effects of radiation with only a duplication of the oscillator circuit  104 . 
   This hardening effect may also be used to harden other circuits that provide a differential output.  FIG. 4  is a block diagram of a hardened differential circuit  400 . The circuit  400  includes a first differential circuit  402 , a second differential circuit  404  that is substantially the same as the first differential circuit  402 , and a filter  406 . The differential outputs of the differential circuits  402 ,  404  are connected or otherwise coupled to the inputs of the filter  406 . The filter  406  provides four outputs in phase quadrature that has been hardened against the effects of radiation-induced errors. 
   The differential circuits  402 ,  404  may be any type of circuit that provides a differential output signal. For example, the circuits  402 ,  404  may be a differential output amplifier, a power amplifier, a line driver, a buffer, and so on. The differential circuits  402 ,  404  are designed to provide quadrature phase outputs of substantially 0°, 90°, 180°, and 270°. For example, the outputs of the first differential circuit  402  may be at 0° and 180°, while the outputs of the second differential circuit  404  may be at 90° and 270°. 
   Like the filter  106 , the filter  406  is preferably a passive polyphase filter. For example, the filter  406  may be similar to the passive polyphase filter  200  depicted in  FIG. 2 . However, the hardened differential circuit  400  is not limited to any particular passive polyphase filter design. 
   By running the differential outputs of the circuits  402 ,  404  through the filter  406 , the filter  406  negates any SEU effect seen in any one phase of the circuits  402 ,  404 . This hardening effect may be the result of one circuit&#39;s output being sufficient to maintain drive through the filter  406 . During the SEU effect and subsequent circuit recovery time, a continuous phase quadrature signal may be maintained with minimal jitter and amplitude variations. As a result, the filter  406  may harden the first differential circuit  402  against the effects of radiation with only a duplication of the differential circuit  404 . 
   By adding a passive polyphase filter to the outputs of a pair of oscillators or differential circuits, the oscillator circuit or differential circuit may be hardened against the effects of radiation. As a result, these circuits may be used in applications that expose these circuits to radiation, such as space and military applications. Additionally, these circuits may be used without triple redundancy modulation or other voting techniques, which take more real estate in a circuit design than the techniques described herein. 
   It should be understood that the illustrated embodiments are examples only and should not be taken as limiting the scope of the present invention. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.