The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention.
In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. Equation 1 describes a QAM signal.
One approach to solving the problem of the x 2 signal corrupting the phase error estimate would be to modify the FIG. This approach does improve channel separation. The amount of improvement is limited because the feed back loop is sensitive to latency.
Lowering the cut-off frequency of the low pass filter requires increasing the length of the filter to potentially thousands of taps, which increases latency. A preferred method described below solves both the problem of latency in the phase-locked loop and the problem of signal content on the quadrature-phase channel. First the in-phase and quadrature-phase baseband signals are decomposed into their zero frequency DC and higher frequency AC components. To avoid the problem of introducing latency into the feed back loop, the signal decomposition algorithm is based on a one-pole AC rejection filter.
The method may be used on fixed point Digital Signal Processors DSPs with noise shaping integrated into the signal decomposition algorithm. An incoming amplitude modulated RF signal 40 is filtered 42 and quantized 44 to form a quantized AM signal The quantized AM signal 46 is downconverted 48 to form an in-phase baseband signal The quantized AM signal 46 is also downconverted 54 to form a quadrature-phase baseband signal A phase lock loop PLL circuit uses a numerically controlled oscillator NCO 64 to generate a local carrier signal 66 that tracks the received carrier.
The local carrier signal 66 is used to downconvert the quantized AM signal 46 into the in-phase baseband signal The carrier signal 36 is phase shifted 68 by ninety degrees and used to downconvert the AM signal 46 into the quadrature phase baseband signal The in-phase baseband signal 50 is decomposed 70 into a DC component 72 and an AC component The quadrature-phase signal 56 is also decomposed 76 into a DC component 78 and AC component The AC components are the in-phase I and quadrature-phase Q data channels.
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DC components 72 , 78 are used to estimate phase and frequency errors 82 for every sample cycle. The phase error 86 and frequency error 84 are both filtered 88 , 90 , and one or the other selected 92 for control of the NCO The filtered frequency error may be used during pull-in or acquisition operation when the PLL is attempting to lock to the incoming carrier. The filtered phase error may be used after frequency lock has been achieved.
Signal decomposition 70 , 76 , error estimation 82 , and filtering 88 , 90 will be discussed in more detail below.
USB2 - Low latency analog QAM coherent demodulation algorithm - Google Patents
An input signal X may be an in-phase or quadrature-phase baseband signal 50 , 56 as illustrated in FIG. Output signal y yac is the AC component of input signal X. Output signal y dc is the DC component of the input signal X. Pole indicates a constant used to adjust the dynamic stability e. The output of the third tap is fed back after a delay to the second tap, which provides noise shaping. Such a filter has a latency of less than hundreds or even tens of samples, and in any event less than thousands of samples of a typical FIR filter.
It is expected the cut-off frequency will be lower than the pass-band of the signal of interest. For acoustic signals of interest, the cut-off frequency could be on the order of tens of hertz or ones of hertz, such as 2 hertz. Cut-off frequency may be described using any known filter characterization methods, such as determining the frequency at which the filter's amplitude response falls 3 decibels dB.
The sine and cosine of the phase angle can be calculated from the DC values I dc , Q dc. Symbology is the same as for FIG. An input signal x may be the frequency error 84 or phase error 86 from signal estimation 82 FIG.
Are You Being Affected by Phase Coherence?
An output signal y is a filtered version of the input signal x. C i and C p are filter coefficients. The transfer function is given by the equation:. Coefficients may be individually optimized to achieve desired pull-in and lock time responses.
In fact, multiple instances of these processes may be implemented on a single DSP chip. Where more than two channels of data require transmission, multiple demodulators may be implemented. Multiple demodulator instances may be synchronized by inserting a pilot signal, such as the carrier, into a QAM channel and further adjusting NCO's of different demodulators until pilot tones from all demodulators are synchronized.
Multi-channel synchronization is sometimes called coherent demodulation. The embodiments disclosed above downconverted information channels to baseband and derived an error signal from DC components of the baseband signals. Log In Sign Up.
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Coherent receiver based on a broadband optical phase-lock loop John Bowers. Coherent receiver based on a broadband optical phase-lock loop. Introduction Optical phase-locked loops have found renewed interest with the reemergence of coherent optical link technologies. For data transmission, a homodyne optical phase-lock loop can generate the highest sensitivity when sufficient loop bandwidth can be maintained .
This technique becomes particularly attractive at high bit-rates when alternative DSP-based solutions are not yet mature, or when low receiver power consumption is desirable. Alternative phase- lock loop applications are coherent synchronization of laser arrays  or frequency synthesis by offset locking . In this work, a broadband optical phase-lock loop is demonstrated for demodulation of analog phase-modulated optical links.
Phase modulation has attracted interest for application in linear optical links with the existence of very linear optical LiNOb3 phase modulation as an alternative to non-linear intensity modulators. The challenge is now transferred to the receiver side. In a standard optical interferometer based phase demodulation, there is a sinusoidal relation between the optical phase and the detected photocurrent.
This nonlinearity limits the available link dynamic range. In contrast, a high-gain optical phase-lock loop will provide linear demodulation provided phase feedback is supplied to a linear optical phase modulator. Figure 1 shows a schematic of the receiver architecture, previously demonstrated in a low-frequency proof-of- principle experiment . Here, a common source is split in two paths, each containing an optical phase modulator.
The input signal is applied to the first modulator. Upon recombination with the second part, the detected optical signal has a sinusoidal dependence on the optical phase.
The photocurrent is then measured, amplified and provided as negative feedback to the second reference modulator. It should be noted that the feedback cannot separate detector shot-noise from signal so that the shot-noise limited SNR remains unchanged despite the reduction in net received phase. Concept schematic of the demonstrated coherent receiver with feedback. Thick lines: optical link; thin lines: electrical link. Integrated Receiver The optical phase demodulator consists of two integrated chips — one photonic, one electronic — mounted on a common microwave carrier.
Figure 2a shows the photonic integrated circuit consisting of a balanced UTC photodetector , tracking phase modulators and a 2x2 waveguide MMI coupler. The wirebonds connecting to the hybrid integrated electronic IC not shown can be seen in the upper part of the figure. In quadrature, this type of balanced receiver discriminates against common-mode and second-order nonlinearities. The tracking optical phase modulators are driven differentially so as to add opposite-sign phase shifts to the incoming signal and LO resulting in a cancellation of even-order nonlinearities and common-mode noise.
Additionally, driving the modulators in a differential fashion doubles the drive voltage presented to the modulator thereby reducing the maximum voltage required.
The capacitances of the photodiodes and modulators are exploited as circuit elements rather than being parasitics that need to be eliminated. They perform the desired loop integrations and hence, can be much larger. The electronic chip that interfaces with the PIC is primarily a trans-conductance amplifier that converts the voltage generated by photodiode integration into a modulator drive current.
However, for multi-channel component characterization and beamforming test systems, you need to perform highly phase-aligned and phase-controllable multi-channel signal generation. You must correct these skews and ensure the measured differences come from the device under test, and not from the test system.
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Learn about how to correct these skews and adjust phase differences between multiple channels in my next post. Eric Hsu Signal Generators Wireless Device Test. See All Tags.