Patent Description:
Products using RF isolators that are sold to consumers or industry must comply with Federal Communications Commission (FCC) part <NUM> limits on radiated RF emission. For the frequency band of <NUM> to <NUM>, there should be no frequency with an RF field strength greater than 200uV/meter when measured at a distance of <NUM> meters. It is often found that cables, long printed circuit board traces, and other wiring can form unintended antennas which cause excess radiated RF. In a system using RF digital isolators, such wiring could exist in the power and ground path at either side of the isolator. If a fixed frequency RF carrier is used, there can also be resonances at that frequency which can actually amplify emitted RF. It is thus beneficial for RF isolators to use spread spectrum RF, wherein the RF is not a single frequency but is spread out to multiple frequencies at lower amplitudes.

The abstract of <CIT> states: 'A spread-spectrum clock signal generator comprises a circuit loop receiving a reference signal (Fref) at a reference frequency and adapted to generate an output signal (Fout) at an output frequency dependent on and locked to the reference frequency, and a modulator circuit generating a modulation signal (Im) at a modulation frequency; the modulation signal is injected into the circuit loop to induce a modulation of the frequency of the output signal with respect to the frequency dependent on the reference frequency. The circuit loop is a frequency-locked loop and has a bandwidth sufficiently higher than the modulation frequency, so that the output frequency tracks the modulation signal. Frequency-offset correction means are further provided, for evaluating a frequency offset between an average frequency of the output signal and the frequency dependent on the reference frequency, and for generating a frequency-offset correction signal which is injected into the circuit loop for correcting the evaluated frequency offset.

The invention is defined as set out in the independent claim <NUM>. Further embodiments are depicted in the dependent claims.

In various embodiments, a self-oscillating spread spectrum frequency control loop is provided. The self-oscillating spread spectrum frequency control loop contains a gated voltage-controlled oscillator (VCO). Upon receiving a digital turn-on gate signal, the VCO generates a first frequency which continually decreases to a lower frequency. Upon reaching the desired lower frequency, the VCO output frequency then begins to continually increase. Upon reaching the desired upper frequency, the frequency begins to decrease again. The frequency "down-then-up" process is repeated for as long as the digital gate control signal is asserted. The VCO is able to generate the spread spectrum carrier by receiving a triangle wave signal into its Vcontrol input. The self-oscillating spread spectrum frequency control loop is suitable to be used within RF digital isolators, which benefit from transmitting and receiving a signal whose bandwidth is stretched as wide as possible or, in another embodiment, as wide as possible above a known minimum frequency.

A self-oscillating spread spectrum frequency control loop circuit is disclosed. The circuit includes a gated VCO which, when gated on, receives a digital signal that can start or stop its oscillation. When gated on, the VCO is able to generate a spread spectrum carrier by receiving a triangle wave signal that is self-generated by a delaying ramp generator when enclosed in a loop, where its ramp direction is controlled by a frequency comparator. The loop can be used to generate a spectrum spread as wide as possible above a known minimum frequency. The self-oscillating spread spectrum frequency control loop circuit produces a carrier signal whose spectrum is spread (bandwidth is increased). The circuit may be used within RF digital isolators, as RF digital isolators benefit from using spread spectrum carriers.

RF isolators that utilize low-pass filters in the transmitter and high-pass filters in the receiver, where the F-3dB cutoff frequencies of both filters vary in a correlated manner, can be used together in a manner that does not produce spread spectrum frequencies below a minimum frequency. Die from a given wafer lot, when designed such that the low-pass and high-pass cutoff frequencies track, can be used in any combination to form RF digital isolators whose minimum spread spectrum frequency does not go below the minimum frequency for operation required by that wafer lot.

The term RF, short for Radio Frequency, as used herein, refers to any signal or wave propagating at a frequency in the radio spectrum, that is, anywhere from <NUM> to <NUM>. RF signals can be transmitted wirelessly or through wires.

<FIG> is a representative drawing of a circuit <NUM> which generates a spread spectrum signal, according to exemplary embodiments. The circuit <NUM>, known herein also as a self-oscillating spread spectrum frequency control loop <NUM>, consists of a gated voltage-controlled oscillator (VCO) <NUM> which receives a digital input <NUM>. The digital gate signal <NUM> is received into the gated VCO as one of two values, denoted RF_onH or RF_offL.

VCOs are oscillators whose output can be varied over a range, as controlled by the control voltage, with the output frequency being directly related to the control voltage. VCOs may be designed to vary frequency only over a small range or over a very wide range. In one embodiment, useful for generating spread spectrum RF, the control voltage can vary the output frequency over a <NUM> to <NUM> range. By varying the control voltage, the output frequency of the signal produced by the VCO is adjusted. VCOs are used for a variety of applications, including frequency and phase modulation, and are also found in phased-locked loops. A gated VCO is a VCO that does not always turn on when power is applied but waits for a digital gate signal to be asserted before beginning to oscillate and ceases oscillating when the signal is de-asserted. In the gated VCO <NUM> of <FIG>, the digital input has one of two values, denoted RF_onH and RF_offL.

The gated VCO <NUM> generates differential signals 106a and 106b (collectively, "differential signal(s) <NUM>"), which are denoted as RF_outH or RF_outL , respectively. Differential signals are complementary signals in which the receiving circuit responds to the electrical difference between the two signals. The differential signals <NUM> are received into a frequency comparator <NUM>. The frequency comparator <NUM> detects whether an input frequency is higher or lower than a predefined frequency, Fnominal. If the frequency is lower than the Fnominal frequency, the output of the circuit is high, otherwise the output is low. In the circuit <NUM>, based on a comparison of the difference between the differential signals <NUM> and the Fnominal frequency, the frequency comparator <NUM> generates a digital signal <NUM>, which may have one of two values, denoted freq_is_lowH or freq_is_highL.

In the circuit <NUM>, the digital signal <NUM> from the frequency comparator <NUM> is received into a delaying ramp generator <NUM>. A ramp generator is a circuit that creates a linear rising or falling output with respect to time. Ramp generators typically produce a sawtooth waveform. Another way to describe the delaying ramp generator <NUM> is as a black box integrator with an Up/Down switch controlled by the digital input (freq_is_lowH/freq_is_highL) from the frequency comparator <NUM>.

In exemplary embodiments, the delaying ramp generator <NUM> generates a triangle waveform, which is a particular type of sawtooth waveform in which the rise time is equal to the fall time. The ramp generator <NUM> of <FIG> is denoted a delaying ramp generator because the frequency of the triangle wave at Vcontrol is largely determined by the time delay between a change of level at <NUM> and the change in Vcontrol at <NUM>. In other embodiments, the time required to ramp through any hysteresis within the frequency comparator adds to the loop <NUM> total delay. In the delaying ramp generator <NUM>, one value of the digital input is denoted as ramp_upH and the other value denoted as ramp_downL. The delaying ramp generator <NUM> generates a signal <NUM> received into the Vcontrol input of the gated VCO <NUM>. In exemplary embodiments, the signal <NUM> is a triangle waveform. When Vcontrol <NUM> is ramped high the output frequency at RF outputs <NUM> is high and when Vcontrol is ramped low the output frequency at <NUM> is low. Thus the self-generated wave at Vcontrol serves to ramp the frequency at <NUM> up and down, thus spreading its spectrum.

In exemplary embodiments, the triangle wave-shaped Vcontrol received into the VCO <NUM> ramps the RF frequency above, then below the nominal frequency (Fnominal), where Fnominal is set within the frequency comparator <NUM>.

There are multiple uses for spread spectrum RF in communication. In addition to the radiated noise reduction described above, spread spectrum communications are very resistant to jamming and are harder to intercept than fixed carrier modulation systems. Thus, the spread spectrum loop disclosed herein can be used for other purposes in addition to within RF isolators.

The circuit <NUM> of <FIG> is thus a circuit used to generate spread spectrum RF upon command. In one embodiment, the circuit <NUM> is used with an RF-passing isolation barrier. In another embodiment, the circuit <NUM> is used with a circuit that detects on-off keying.

<FIG> is a graph <NUM> showing the signal <NUM> generated by the delaying ramp generator <NUM>, according to exemplary embodiments. The graph plots time (x-axis) versus voltage (y-axis) at Vcontrol <NUM>. Note that the frequency output of most VCOs is quite linear with Vcontrol voltage and if the y-axis was re-dimensioned as frequency, the graph would display the RF output <NUM> frequency vs time. The delaying ramp generator <NUM> generates an output signal <NUM> that may be described as a triangle wave. In some embodiments, by receiving the triangle wave signal <NUM>, the gated VCO <NUM> is able to spread the bandwidth of the transmitted differential signal <NUM> to occupy the frequency spectrum available for transmission.

The digital (RF) isolators described herein are concerned with unintended/unexpected occurrences of RF in an electronic circuit. As explained above, to comply with FCC limits on radiated RF emission, for the frequency band of <NUM> to <NUM>, there should be no frequency with an RF field strength greater than 200uV/meter when measured at a distance of <NUM> meters. Wiring within a system, whether PCB traces or cables, can form unintended antennas that cause excess radiated RF in the power, ground, and signal paths at either side of a digital isolator. Systems with a fixed frequency RF carrier can experience amplified emitted RF due to resonances of wiring and PCB traces at that frequency.

It is thus useful for an RF isolator to use a spread spectrum carrier signal which reduces emission at any one frequency. If the spectrum can be spread over a wide frequency band, this reduction will apply even when used with wiring that would have failed if a single-frequency carrier was used. Similarly, when used with well-designed wiring, the spread spectrum carrier will still reduce the emissions and influence other RF communications less, which is the purpose of the FCC limits. Spreading the spectrum over an increasingly wide bandwidth will continually reduce the emission at any one frequency. It is thus useful to use a spectrum spread as wide as possible. The self-oscillating spread spectrum frequency control loop <NUM> may thus be useful for RF isolation applications.

<FIG> is a representative drawing of a circuit <NUM> which generates a spread spectrum carrier for an RF isolator, according to exemplary embodiments. The circuit <NUM> consists of the self-oscillating spread spectrum frequency control loop <NUM> of <FIG>, which receives the digital input signal <NUM> and generates the complementary differential signals RF_outH 106a and RF_outL 106b. The differential signals <NUM> are received into an RF-passing isolation barrier <NUM>, which forms an isolation barrier between the input circuit <NUM> and an output circuit <NUM>, which generates a digital output signal <NUM>. The RF-passing isolation barrier <NUM> is designed to protect both the input circuit <NUM> and the output circuit <NUM> from large common mode voltage differences and transients. By using the circuit <NUM>, the circuit <NUM> is able to generate a spread spectrum carrier for the RF-passing isolation barrier <NUM>.

RF digital isolators are used to convey digital information across potentially catastrophic common mode voltages and common mode voltage changes. A logic signal is applied at the input and is duplicated at the output with respect to the ground of that circuit. RF digital isolators often use capacitors to implement the RF-passing isolation barrier between the input and output systems, although this is not required. Isolation capacitors may be formed from lead frame traces, may be formed within the package by vertically stacking dice over a dielectric, may be on-chip capacitors or may be externally implemented from PCB traces. Similarly, transformer RF- passing isolation may be implemented with on-chip or external components.

High-pass filters are electronic circuits that pass signals with frequencies higher than the F-3dB cutoff frequency, and attenuate signals (that is, makes the amplitude smaller) of frequencies lower than the cutoff frequency. Low-pass filters are electronic circuits that pass signals with frequencies lower than the F-3dB cutoff frequency while attenuating signals with frequencies higher than the cutoff frequency.

It is opportune to design the first stage of an isolated receiver section of an RF digital isolator as a high pass filter. This presents particular characteristics that may enable optimization of the digital isolator and thus protect the entire circuit against larger common mode voltage differences and transients.

If, for example, a receiver coupling network is based upon a high pass filter, it is optimal to use the highest F-3dB cutoff frequency possible as this will optimize the frequency range and the transient edge speed over which the system will reject common mode transients. The F-3dB cutoff frequency of the high-pass filter is the frequency at which the amplitude of the incoming signal drops by 3dB or <NUM>%. A high pass filter greatly attenuates frequencies below the F-3dB cutoff frequency and thus will help to reject any common mode transients at those lower frequencies.

While advanced processes may be able to generate synthetic inductors of suitable bandwidth and quality factor, cheaper larger geometry fabrication processes may not. It is thus useful to consider designs whose high-pass filter receiver coupling network is largely determined by passive resistor and capacitor values, known as an RC filter or RC high-pass filter. The F-3dB cutoff frequency for an RC high-pass filter is based on the product of the resistor (R) and capacitor (C) values at each stage of the filter. Note that the disclosed spectrum spreading techniques do not require RC filters, any high-pass and low-pass filters can be used. But the use of the same type of resistors and capacitors in both the high-pass and low-pass filters does generate additional benefits as discussed below.

<FIG> is a representative drawing of a circuit <NUM> which generates a spread spectrum carrier for an RF isolator, according to exemplary embodiments. The circuit <NUM> consists of an input circuit <NUM> to receive a digital signal <NUM>, an RF-passing isolation barrier <NUM>, and an output circuit <NUM> to generate a digital output <NUM>. The RF-passing isolation barrier <NUM> provides an isolation barrier between the input circuit <NUM> and the output circuit <NUM>.

The input circuit <NUM> includes a low-pass filter <NUM> while the output circuit <NUM> includes a high-pass filter <NUM>. Additionally, the input circuit <NUM> includes a gated VCO <NUM>, a demodulator <NUM>, and a bidirectional ramp generator <NUM> while the output circuit <NUM> further includes a high-frequency demodulator <NUM>. As in the circuits <NUM> and <NUM>, the gated VCO <NUM> generates complementary differential signals 414a and 414b (collectively, "differential signal(s) <NUM>"), which are denoted as RF_outH or RF_outL, respectively. The differential signals <NUM> are received by the low-pass filter <NUM>, which allows signals of longer wavelengths (lower frequencies) to pass through while attenuating signals of shorter wavelengths (higher frequencies). From the low-pass filter <NUM>, differential signals 418a and 418b (collectively, "differential signal(s) <NUM>") are received into a demodulator <NUM>. Demodulators are electronic circuits that recover the information content from a modulated carrier wave, where the modulated carrier wave is used to transmit the information, whether on a wire or wirelessly. The demodulator <NUM> receives the differential signals <NUM> and generates a single output signal <NUM>. The output signal <NUM> is received into a bidirectional ramp generator <NUM>, which generates a signal <NUM> to be received into the Vcontrol input of the gated VCO <NUM>. In exemplary embodiments, the signal <NUM> is a triangle wave, such as is illustrated in <FIG>.

At the output circuit <NUM>, differential signals 428a and 428b (collectively, "differential signal(s) <NUM>") from the RF-passing isolation barrier <NUM> are received into the high-pass filter <NUM>, which allows signals of shorter wavelengths (higher frequencies) to pass through while attenuating signals of longer wavelengths (lower frequencies). From the high-pass filter <NUM>, differential signals 432a and 432b (collectively, "differential signal(s) <NUM>") are received into the high-frequency demodulator <NUM>. The high-frequency demodulator <NUM>, which separates the information from the carrier signal, receives the differential signals <NUM> and generates the single output signal <NUM>.

The high-pass filter <NUM> at the isolated side of the RF-passing isolation barrier <NUM> (that is, the output circuit <NUM>) sets the amount of common mode transient interference that the isolator will withstand without error. The higher the F-3dB cutoff frequency of the high-pass filter <NUM>, the higher the frequency and higher the slew rate (change of voltage or current) of the common mode transient that the RF-passing isolation barrier <NUM> can withstand without error. Thus, it is useful to design the RF-passing isolation barrier <NUM> to use as high a frequency as can be reliably generated. In exemplary embodiments, the high-pass filter <NUM> is a passive RC high-pass filter.

In one embodiment, the RF-passing isolation barrier <NUM> is constructed using capacitors. In a second embodiment, the RF-passing isolation barrier <NUM> is made using a transformer. The principles of creating a spread spectrum by the gated VCO may be achieved with many different RF isolator configurations.

A problem arises if the RF carrier signal (e.g., the differential signal <NUM>), which can also be thought of as the spread spectrum, is of too low a frequency for the high-pass filter <NUM>. In this case, the output of the high-pass filter will be of insufficient amplitude and the demodulated output <NUM> will not follow the applied digital input signal <NUM>. Put another way, there will not be enough RF passed on to demodulate. An error will have occurred.

Unfortunately, most integrated circuit processes do not have available resistors whose resistance is well-controlled from one wafer lot to another. The absolute resistance of semiconductor resistors varies from wafer lot to wafer lot. Some foundries, for example, may allow the variance to be up to a <NUM>:<NUM> range. Integrated capacitors similarly have an allowed range which may be +/-<NUM>%. Thus, the F-3dB cutoff frequency of an RC pair may vary by <NUM>:<NUM> from wafer lot to wafer lot.

While resistance can be laser trimmed or programmed with EPROM-controlled switches, a smaller and less expensive integrated circuit will result if a circuit can be designed that does not require trimming of any type.

In exemplary embodiments, the oscillator-side RC low-pass filter <NUM> of the input circuit <NUM>, in cascade with the demodulator <NUM> is used to generate the digital pulse <NUM> received into the bidirectional ramp generator <NUM>. In exemplary embodiments, the digital pulse <NUM> is high when the frequency at RF_out <NUM> is below a chosen reference frequency, Fref.

In an exemplary embodiment, the low-pass filter <NUM> at the input circuit <NUM> tracks the high-pass filter <NUM> at the output circuit <NUM>. That is, the F-3dB cutoff frequencies of the high- and low-pass filters may vary but always vary together. In one embodiment, the Fcutoff of low-pass filter <NUM> and high-pass filter <NUM> can be made to track by being fabricated from identical types of resistors and capacitors fabricated in the same wafer lot. The reference frequency, Fref is chosen to be above a minimum frequency required at the high-pass filter <NUM>.

The bidirectional ramp generator <NUM> of the input circuit <NUM> is not further illustrated in <FIG>. However, similar to the delaying ramp generator <NUM> of the circuit <NUM> (<FIG>), the bidirectional ramp generator <NUM> receives a digital input signal and generates a triangle wave that is received into a gated VCO. As shown in <FIG>, the digital input signal <NUM> is received from the demodulator <NUM> into the bidirectional ramp generator <NUM>, which produces the analog signal <NUM> to be received into the Vcontrol input of the gated VCO <NUM>. In exemplary embodiments, the analog signal <NUM> is a triangle wave similar to the one illustrated in <FIG>.

In some embodiments, the input circuit <NUM>, which may be a transmitter, and the output circuit <NUM>, which may be a receiver, have a voltage supply of <NUM>. However, the common mode voltage between them may be thousands of volts. Or, more commonly, the common mode voltage could be zero volts, glitching up to thousands of volts. The RF-passing isolation barrier <NUM> is designed to keep these common mode voltages from damaging either circuit.

The design features of the circuit <NUM>, namely, having a triangle wave feed into Vcontrol of the gated VCO and having a low-pass filter on the transmitter side (input circuit) of an RF isolator that tracks the high-pass filter at the receiver side (output circuit), the circuit <NUM> ensures that the transmitter does not make a frequency so low that the high-pass filter cannot see it. The low-pass filter matches the high-pass filter at the F-3dB cutoff frequency and sets the lowest frequency that the VCO can go to, such that there is enough coming out of the high-pass filter so that the high-frequency demodulator at the output circuit is at the correct logic level.

<FIG> is a representative drawing of a circuit <NUM> which generates a spread spectrum carrier for an RF isolator, according to exemplary embodiments. This circuit <NUM> provides an implementation of a bidirectional integrator <NUM>. The bidirectional integrator consists of a single bit digital-to-analog converter at the input of an integrator, which integrator produces a triangle wave to be received into the Vcontrol input of a gated VCO. Further, the circuit <NUM> provides an implementation of an RF-passing isolation barrier that protects input and output circuits having low- and high-pass filters, respectively.

The circuit <NUM> consists of an input circuit <NUM> to receive a digital signal <NUM>, an RF-passing isolation barrier <NUM>, and an output circuit <NUM> to generate a digital output <NUM>. The RF isolation barrier <NUM> between the input circuit <NUM> and the output circuit <NUM> must be designed to withstand all the possible common mode voltage difference between input <NUM> and output <NUM>.

The input circuit <NUM> includes a low-pass filter <NUM> while the output circuit <NUM> includes a high-pass filter <NUM>. Additionally, the input circuit <NUM> includes a gated VCO <NUM>, a demodulator <NUM>, and a bidirectional integrator <NUM> while the output circuit <NUM> further includes a high-frequency demodulator <NUM>. Like the circuits <NUM>, <NUM>, and <NUM>, the gated VCO <NUM> generates differential signals 514a and 514b (collectively, "differential signal(s) <NUM>"), which are denoted as RF_outH or RF_outL, respectively. In the input circuit <NUM>, the differential signals <NUM> are received by the low-pass filter <NUM>, which allows signals of longer wavelengths (lower frequencies) to pass through while attenuating signals of shorter wavelengths (higher frequencies). From the low-pass filter <NUM>, differential signals 518a and 518b (collectively, "differential signal(s) <NUM>") are received into a low-frequency demodulator <NUM>. Further, the low-frequency demodulator <NUM> receives the differential signals <NUM> and generates a single output signal <NUM>, shown also as Freq_lowH. The output signal <NUM> is received at bidirectional integrator <NUM>, which generates a signal <NUM> to be received into the Vcontrol input of the gated VCO <NUM>. In exemplary embodiments, the signal <NUM> is a triangle wave, such as is illustrated in <FIG>.

At the output circuit <NUM>, differential signals 528a and 528b (collectively, "differential signal(s) <NUM>") from the digital isolator <NUM> are received into the high-pass filter <NUM>, which allows signals of shorter wavelengths (higher frequencies) to pass through while attenuating signals of longer wavelengths (lower frequencies). From the high-pass filter <NUM>, differential signals 532a and 532b (collectively, "differential signal(s) <NUM>") are received into the high-frequency demodulator <NUM>. The high-frequency demodulator <NUM> receives the differential signals <NUM> and generates the single output signal <NUM>.

The bidirectional integrator <NUM> includes a switch <NUM>, a DC current source <NUM>, an amplifier <NUM>, a capacitor <NUM>, and a second DC current source <NUM>. The DC current source <NUM> attached to the switch <NUM> which controls DC current source <NUM> is a simple implementation of a <NUM>-bit digital to analog converter. The output from the low-frequency demodulator <NUM> is the digital signal <NUM>. The amplifier <NUM> combined with the capacitor <NUM> is an integrator. The signal <NUM> will be high if the frequency of the differential signal <NUM> is below some predetermined frequency and low if the frequency of the differential signal is above the predetermined frequency. The digital signal <NUM> controls the switch <NUM> within the bidirectional integrator <NUM>.

In an exemplary embodiment, the cascade of low-pass filter <NUM> and demodulator <NUM> form a frequency comparator. The demodulated signal, Freq_lowH, is applied to a bidirectional integrator <NUM> and its output is applied to the control voltage input, Vcontrol, on the gated VCO <NUM>. This results in a first order sigma delta idle pattern at Vcontrol <NUM>. The differential implementation of the RF_out <NUM> and filters combined with the continuous modulation of the frequency above and below Fref greatly reduces electromagnetic interference. Under typical conditions the frequency modulation reduces peak electromagnetic interference (EMI) by 20dB when measured on an analyzer with <NUM> resolution bandwidth.

In exemplary embodiments, the amplifier with the capacitor as feedback in the bidirectional integrator <NUM> is configured as an integrator. In one embodiment, the capacitor is a <NUM>. 4pF capacitor, the DC current source <NUM> is a 4uA (DC) current source, and the DC current source <NUM> is an 8uA (DC) current source. When the switch <NUM> is opened, the DC current source <NUM> "pushes" (sources) <NUM> uA of current into the negative input of the amplifier <NUM>. That current causes the Vcontrol signal <NUM> to ramp in a negative direction at a rate given by dV/dt = (4uA)/(<NUM>. 4pF) = -10V/us. However, when the switch <NUM> is closed, the second DC current source <NUM> tries to "pull" (sink) 8uA of current from the negative terminal of the amplifier <NUM>. The amplifier <NUM> gets 4uA of that current from the DC current source <NUM>. The other 4uA thus must come through the capacitor <NUM>. This forces the voltage at Vcontrol to ramp into a positive direction at +10V/us.

<FIG> thus illustrates a spread spectrum oscillator in the input circuit <NUM> whose minimum frequency is also determined by the RC product of the same type of resistor and capacitor used within the output circuit <NUM>. When an input circuit <NUM> die from a particular wafer lot is paired with an output circuit <NUM> die from the same wafer lot, the input circuit die will keep the RF frequency above the minimum required for the values of R and C that were fabricated. Because the input circuit spectrum is held above the minimum frequency receivable, there will be no error. By identifying the minimum no error frequency within the transmitter, the spectrum can be spread as far as possible from there, thus reducing emissions further by spreading the spectrum further.

In exemplary embodiments, the circuit <NUM> includes an RF-passing isolation barrier <NUM> where the output circuit <NUM> coupling high-pass filter <NUM> F-3dB cutoff frequency, Fcutoff, is largely determined by the RC product of high-resistance polysilicon resistors and metal-to-metal (MIM) capacitors. A buffered replica RF signal is generated on input circuit <NUM> chip and applied to the low-pass filter <NUM> of the input circuit <NUM> whose initial F-3dB cutoff frequency is also determined by the RC product of high resistance poly resistors and MIM capacitors. The demodulator <NUM> converts the presence of RF at the low-pass output into a digital value (the signal <NUM>, Freq_lowH). This digital value <NUM> is used to switch a reference current at an integrator configured within the bidirectional integrator <NUM>.

In an exemplary embodiment, the frequency of the input circuit <NUM> is started at Vcontrol fully high (frequency of the gated VCO <NUM> at Fmax). At this frequency, there is not enough RF at the low-pass filter <NUM> output (differential signals <NUM>), so freqLowH will be low. The fixed reference current source <NUM> of 4uA slowly ramps Vcontrol lower. Thus, the frequency of the gated VCO <NUM> decreases until there is enough RF at the low-pass filter <NUM> output to set Freq_lowH high. At this point, the additional 8uA of sunk reference current forces an equivalent 4uA of current sink into the integration capacitor <NUM>. This forces Vcontrol to start to go high which will increase the frequency at the output of the gated VCO <NUM>.

Thus, if the F-3dB cutoff frequency of the low-pass filter <NUM> is set by choosing RC values to just barely provide enough RF for the low-pass demodulator <NUM> to decode a logic high, this will set the frequency where the loop will always start to increase frequency. This must be done by measuring the loop using the expected value of Cintegrate.

The highest frequency from the gated VCO <NUM> will be set by the time constants in the loop, dominated by Cintegrate. Smaller Cintegrate will cause larger peak-to-peak triangle voltage swing at Vcontrol (more swing at Vcontrol causes wider spectrum spread) with a faster repetition frequency. Larger Cintegrate will cause a smaller peak-to-peak swing at a lower triangle frequency. For optimal spectrum spread Cintegrate should be chosen as small as possible without allowing Vcontrol to clip. If Vcontrol clips, the frequency stops changing while Vcontrol is clipped and the emissions become more correlated, thus losing the advantages of spreading the spectrum.

Sigma delta ADC loops will form a repetitive idle pattern when presented with a DC input. For a first order loop, the idle pattern is a triangle wave. This is because the integrator will integrate positive for <NUM>% of the time and negative for <NUM>%. The loop will keep a <NUM>% duty cycle at Freq_lowH. The process of picking a F-3dB cutoff frequency Fcutoff with a given Cintegrate for a prescribed minimum frequency is identical to picking a higher frequency Fmiddle around which an idle pattern of amplitude set by Cintegrate will change.

When the frequency initially gets low enough to set freqLowH, the integration capacitor <NUM> had already been integrating for longer than a usual cycle. In fact, the integration capacitor <NUM> had integrated to exactly the voltage that is the lower voltage of the limit cycle that the loop will generate. That is why picking Fmin at a given Cintegrate is identical to picking a higher Fmiddle which will have a deviation at Vcontrol of Vidle_pattern peak-to-peak.

While the RC products at the low-pass filter <NUM> and the high-pass filter <NUM> track each other, Cintegrate tracks only the capacitors used within these filters. Thus, the offset frequency from the <NUM>% duty cycle (set by the peak swing at Vcontrol) does not track the RC product. When capacitors are <NUM>% higher in value, the gated VCO <NUM> theoretically could go as much as <NUM>% lower in frequency. Because the integration cap is also <NUM>% higher, Vcontrol goes <NUM>% less negative and the gated VCO <NUM> doesn't go as slow as it would if Cintegrate hadn't changed. In practice, the difference in frequency is negligible, the gated VCO <NUM> minimum frequency changes from <NUM> to <NUM>, in one embodiment. Thus, the change in Cintegrate actually further prevents the gated VCO <NUM> from generating a too-low frequency.

If the capacitors are <NUM>% lower in value, Vcontrol will swing <NUM>% lower than it would have, which means the gated VCO frequency will be in error in an unsafe direction. In practice, this change is <NUM> , from <NUM> to <NUM>, in some embodiments, which is not very significant. The <NUM>% frequency of the loop, set by the RC values of the low-pass filter <NUM>, are thus chosen to be just slightly higher in frequency than would have been necessary, in one embodiment.

<FIG> shows simulation result using the circuit <NUM>, in accordance with exemplary embodiments. The simulation result shows voltage over a <NUM> microsecond time period. A <NUM> digital input pulse (not shown) is received into the circuit <NUM>, and a <NUM>. 4ns digital output pulse <NUM> is generated (at top). The Vcontrol input <NUM> to the gated VCO shows the triangle wave. In the first <NUM> ns, the spectrum is being spread twice as far as usual. A modulated low-pass filter output <NUM> and a modulated high-pass filter output <NUM> are also shown. The low-pass filter output <NUM> tracks the triangle wave <NUM>. When the wave is low, the low-pass filter RF has a slightly bigger amplitude; when the wave is high, the low-pass filter RF has a smaller amplitude. Thus, the frequency that's being transmitted to the output circuit is controlled so that there won't be an error. The pulse width is distorted by less than <NUM> nanosecond, a result that is not easily obtained with an opto-isolator.

<FIG> is a diagram <NUM> showing a discrete Fourier transform (DFT) of the differential spread spectrum RF at the high-pass filter output and demodulator input (e.g., differential signals <NUM>) of the circuit <NUM>. Instead of a single line 10X taller at exactly <NUM> the spectrum has been spread into <NUM> bins between <NUM> and <NUM>. The second harmonics spread into <NUM> times <NUM> or <NUM> bins, the third harmonics spread into <NUM> times <NUM> o <NUM> bins and that pattern repeats for any higher harmonics present.

Claim 1:
A system to generate a gated spread spectrum carrier in a radio frequency (RF) isolated circuit, the system comprising:
an input circuit (<NUM>, <NUM>, <NUM>) comprising:
a gated voltage-controlled oscillator, VCO, (<NUM>, <NUM>) to receive a digital gate signal (<NUM>, <NUM>) and, when gated on upon assertion of the digital gate signal, to generate complementary first and second differential signals forming the spread spectrum signal to be transmitted across an RF-passing isolation barrier (<NUM>);
a low-pass filter (<NUM>) to receive the first and second differential signals (414a, 414b) from the gated VCO (<NUM>) and generate low-pass filtered differential signals (418a, 418b);
a demodulator (<NUM>) to receive the low-pass filtered differential signals (418a, 418b) and to generate an output signal (<NUM>) that is high if the frequency of the low-pass differential signal formed by the low-pass filtered differential signals is below a predetermined frequency and low if said frequency is above the predetermined frequency;
a delaying bidirectional ramp generator (<NUM>) to receive the output signal (<NUM>), the output signal (<NUM>) to ramp a voltage control signal in either a positive or a negative direction and to generate a sawtooth waveform as the voltage control signal to be received into a voltage control input of the gated VCO; and
an output circuit (<NUM>) comprising a high-pass filter (<NUM>) to receive third and fourth differential signals (428a, 428b) corresponding to the transmitted first and second differential signals from the RF-passing isolation barrier (<NUM>);
wherein the RF-passing isolation barrier (<NUM>) disposed between the input circuit (<NUM>) and the output circuit (<NUM>) protects the input circuit (<NUM>) and the output circuit (<NUM>) from common mode voltage differences and transients.