In telecommunications and signal processing , baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers , converting some other variable into an electrical signal. For example, the electronic output of a microphone is a baseband signal that is analogous to the applied voice audio. In conventional analog radio broadcasting , the baseband audio signal is used to modulate an RF carrier signal of a much higher frequency.
33-504: A baseband signal may have frequency components going all the way down to the DC bias , or at least it will have a high ratio bandwidth . A modulated baseband signal is called a passband signal . This occupies a higher range of frequencies and has a lower ratio and fractional bandwidth . A baseband signal or lowpass signal is a signal that can include frequencies that are very near zero, by comparison with its highest frequency (for example,
66-505: A line code and an unfiltered wire are used). A baseband processor also known as BP or BBP is used to process the down-converted digital signal to retrieve essential data for a wireless digital system. The baseband processing block in GNSS receivers is responsible for providing observable data: that is, code pseudo-ranges and carrier phase measurements, as well as navigation data. An equivalent baseband signal or equivalent lowpass signal
99-501: A narrowband signal model . A stream of information about how to amplitude-modulate the I and Q phases of a sine wave is known as the I/Q data . By just amplitude-modulating these two 90°-out-of-phase sine waves and adding them, it is possible to produce the effect of arbitrarily modulating some carrier: amplitude and phase. And if the IQ data itself has some frequency (e.g. a phasor ) then
132-505: A periodic function in the time domain , the DC bias , DC component , DC offset , or DC coefficient is the mean value of the waveform . A waveform with zero mean or no DC bias is known as a DC balanced or DC free waveform. The term originates in electronics, where DC refers to a direct current voltage. In contrast, various other non-DC frequencies are analogous to superimposed alternating current (AC) voltages or currents, hence called AC component or AC coefficients . In
165-454: A 0-level. In order to avoid these kinds of bit errors, most line codes are designed to produce DC-balanced signals. The most common classes of DC balanced line codes are constant-weight codes and paired-disparity codes . In audio recording , a DC offset is an undesirable characteristic. It occurs in the capturing of sound, before it reaches the recorder, and is normally caused by defective or low-quality equipment. It results in an offset of
198-401: A 2D vector , or as separate streams. When called "I/Q data" the information is likely digital. However, I/Q may be represented as analog signals. The concepts are applicable to both the analog and digital representations of IQ. This technique of using I/Q data to represent the modulations of a signal separate to the signal's frequency is known as equivalent baseband signal , supported by
231-416: A carrier sine wave. IQ data has extensive use in many signal processing contexts, including for radio modulation , software-defined radio , audio signal processing and electrical engineering . I/Q data is a two-dimensional stream. Some sources treat I/Q as a complex number ; with the I and Q components corresponding to the real and imaginary parts. Others treat it as distinct pairs of values, as
264-537: A digital bit stream over baseband channel, typically an unfiltered wire, contrary to passband transmission, also known as carrier-modulated transmission. Passband transmission makes communication possible over a bandpass filtered channel, such as the telephone network local-loop or a band-limited wireless channel. The word "BASE" in Ethernet physical layer standards, for example 10BASE5 , 100BASE-TX and 1000BASE-SX , implies baseband digital transmission (i.e. that
297-491: A sound waveform can be considered as a baseband signal, whereas a radio signal or any other modulated signal is not). A baseband bandwidth is equal to the highest frequency of a signal or system, or an upper bound on such frequencies, for example the upper cut-off frequency of a low-pass filter . By contrast, passband bandwidth is the difference between a highest frequency and a nonzero lowest frequency. A baseband channel or lowpass channel (or system , or network )
330-495: A transmitter, I/Q data is also a common means to represent the signal from some receiver. Designs such as the Digital down converter allow the input signal to be represented as streams of IQ data, likely for further processing and symbol extraction in a DSP . Analog systems may suffer from issues, such as IQ imbalance . I/Q data may also be used as a means to capture and store data used in spectrum monitoring. Since I/Q allows
363-410: A voltage vs. time function that is sinusoidal with a frequency f. When it is applied to a typical (linear time-invariant) circuit or device, it causes a current that is also sinusoidal. In general there is a constant phase difference φ between any two sinusoids. The input sinusoidal voltage is usually defined to have zero phase, meaning that it is arbitrarily chosen as a convenient time reference. So
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#1732793082415396-529: Is a communication channel that can transfer frequencies that are very near zero. Examples are serial cables and local area networks (LANs), as opposed to passband channels such as radio frequency channels and passband filtered wires of the analog telephone network. Frequency division multiplexing (FDM) allows an analog telephone wire to carry a baseband telephone call, concurrently as one or several carrier-modulated telephone calls. Digital baseband transmission, also known as line coding , aims at transferring
429-522: Is a complex valued representation of the modulated physical signal (the so-called passband signal or RF signal). It is a concept within analog and digital modulation methods for (passband) signals with constant or varying carrier frequency (for example ASK , PSK QAM , and FSK ). The equivalent baseband signal is Z ( t ) = I ( t ) + j Q ( t ) {\displaystyle Z(t)=I(t)+jQ(t)\,} where I ( t ) {\displaystyle I(t)}
462-473: Is designed to preserve the applied AC signal. Similarly, amplifiers using field-effect transistors or vacuum tubes also have bias circuits. The operating point of an amplifier greatly affects its characteristics of distortion and efficiency; power amplifier classes are distinguished by the operating point set by the DC bias. DC offset is usually undesirable when it causes clipping or other undesirable change in
495-441: Is that the modulations in some signal can be treated separately from the carrier wave of the signal. This has extensive use in many radio and signal processing applications. I/Q data is used to represent the modulations of some carrier, independent of that carrier's frequency. In vector analysis, a vector with polar coordinates A , φ and Cartesian coordinates x = A cos( φ ), y = A sin( φ ), can be represented as
528-1729: Is the in-phase amplitude modulation, which explains why some authors refer to it as the actual in-phase component. In an angle modulation application, with carrier frequency f, φ is also a time-variant function, giving : A ( t ) ⋅ cos [ 2 π f t + φ ( t ) ] = cos ( 2 π f t ) ⋅ A ( t ) cos [ φ ( t ) ] + cos ( 2 π f t + π 2 ) ⋅ A ( t ) sin [ φ ( t ) ] = cos ( 2 π f t ) ⋅ A ( t ) cos [ φ ( t ) ] ⏟ in-phase − sin ( 2 π f t ) ⋅ A ( t ) sin [ φ ( t ) ] ⏟ quadrature . {\displaystyle {\begin{aligned}A(t)\cdot \cos[2\pi ft+\varphi (t)]\ &=\cos(2\pi ft)\cdot A(t)\cos[\varphi (t)]\ +\ \cos \left(2\pi ft+{\tfrac {\pi }{2}}\right)\cdot A(t)\sin[\varphi (t)]\\[8pt]&=\underbrace {\cos(2\pi ft)\cdot A(t)\cos[\varphi (t)]} _{\text{in-phase}}\ \underbrace {\ -\ \sin(2\pi ft)\cdot A(t)\sin[\varphi (t)]} _{\text{quadrature}}.\end{aligned}}} When all three terms above are multiplied by an optional amplitude function, A ( t ) > 0,
561-450: Is the inphase signal, Q ( t ) {\displaystyle Q(t)} the quadrature phase signal, and j {\displaystyle j} the imaginary unit. This signal is sometimes called IQ data . In a digital modulation method, the I ( t ) {\displaystyle I(t)} and Q ( t ) {\displaystyle Q(t)} signals of each modulation symbol are evident from
594-438: The constellation diagram . The frequency spectrum of this signal includes negative as well as positive frequencies. The physical passband signal corresponds to where ω {\displaystyle \omega } is the carrier angular frequency in rad/s. A signal at baseband is often used to modulate a higher frequency carrier signal in order that it may be transmitted via radio. Modulation results in shifting
627-505: The § Narrowband signal model . It is sometimes referred to as vector modulation . The data rate of I/Q is largely independent to the frequency of the signal being modulated. I/Q data can be generated at a relatively slow rate (e.g. millions of bits per second), perhaps generated by software in part of the physical layer of a protocol stack. I/Q data is used to modulate a carrier frequency, which may be faster (e.g. Gigahertz , perhaps an intermediate frequency ). As well as within
660-421: The 10,000 million samples per second required to sample directly at 5 GHz. A vector signal generator will typically use I/Q data alongside some programmed frequency to generate its signal. And similarly a vector signal analyser can provide a stream of I/Q data in its output. Many modulation schemes, e.g. quadrature amplitude modulation rely heavily on I/Q. The term alternating current applies to
693-492: The DC bias. The concept has been extended to any representation of a waveform and to two-dimensional transformations like the discrete cosine transform used in JPEG . IQ data A sinusoid with modulation can be decomposed into, or synthesized from, two amplitude-modulated sinusoids that are in quadrature phase , i.e., with a phase offset of one-quarter cycle (90 degrees or π /2 radians). All three sinusoids have
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#1732793082415726-413: The carrier also can be frequency modulated. So I/Q data is a complete representation of how a carrier is modulated: amplitude, phase and frequency. For received signals, by determining how much in-phase carrier and how much quadrature carrier is present in the signal it is possible to represent that signal using in-phase and quadrature components, so IQ data can get generated from a signal with reference to
759-454: The center of the recording waveform that can cause two main problems. Either the loudest parts of the signal will be clipped prematurely since the base of the waveform has been moved up, or inaudible low-frequency distortion will occur. Low-frequency distortion may not be audible in the initial recording, but if the waveform is resampled to a compressed or lossy digital format, such as an MP3, those corruptions may become audible. A DC tape bias
792-418: The design of electronic amplifier circuits, every active device has biasing to set its operating point , the steady state current and voltage on the device when no signal is applied. In bipolar transistor biasing , for example, a network of resistors is used to apply a small amount of DC to the base terminal of the transistor. The AC signal is applied at the same terminal and is amplified. The bias network
825-421: The left-hand side of the equality is known as the amplitude/phase form, and the right-hand side is the quadrature-carrier or IQ form. Because of the modulation, the components are no longer completely orthogonal functions. But when A ( t ) and φ ( t ) are slowly varying functions compared to 2 π ft , the assumption of orthogonality is a common one. Authors often call it a narrowband assumption , or
858-411: The offset. Very low frequencies can look like DC bias but are called "slowly changing DC" or "baseline wander". DC-balanced signals are used in communications systems to prevent bit errors when passing through circuits with capacitive coupling or transformers . Bit errors can occur when a series of 1's create a DC level that charges the coupling capacitor, bringing the signal input down incorrectly to
891-489: The operating point of an amplifier. An electrical DC bias will not pass through a transformer or capacitor ; thus a simple isolation transformer or series-wired capacitor can be used to block or remove it, leaving only the AC component on the other side. In signal processing terms, DC offset can be reduced in real-time by a high-pass filter . For stored digital signals, subtracting the mean amplitude from each sample will remove
924-430: The phase difference is attributed to the current function, e.g. sin(2 π ft + φ ), whose orthogonal components are sin(2 π ft ) cos( φ ) and sin(2 π ft + π /2) sin( φ ), as we have seen. When φ happens to be such that the in-phase component is zero, the current and voltage sinusoids are said to be in quadrature , which means they are orthogonal to each other. In that case, no average (active) electrical power
957-461: The representation of the modulation separate to the actual carrier frequency, it is possible to represent a capture of all the radio traffic in some RF band or section thereof, with a reasonable amount of data, irrespective of the frequency being monitored. E.g. if there is a capture of 100 MHz of Wi-Fi channels within the 5 GHz U-NII band , that IQ capture can be sampled at 200 million samples per second (according to Nyquist ) as opposed to
990-408: The same center frequency . The two amplitude-modulated sinusoids are known as the in-phase (I) and quadrature (Q) components, which describes their relationships with the amplitude- and phase-modulated carrier. Or in other words, it is possible to create an arbitrarily phase-shifted sine wave, by mixing together two sine waves that are 90° out of phase in different proportions. The implication
1023-712: The signal up to much higher frequencies (radio frequencies, or RF) than it originally spanned. A key consequence of the usual double-sideband amplitude modulation (AM) is that the range of frequencies the signal spans (its spectral bandwidth ) is doubled. Thus, the RF bandwidth of a signal (measured from the lowest frequency as opposed to 0 Hz) is twice its baseband bandwidth. Steps may be taken to reduce this effect, such as single-sideband modulation . Conversely, some transmission schemes such as frequency modulation use even more bandwidth. The figure below shows AM modulation: DC bias In signal processing , when describing
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1056-440: The sum of orthogonal components: [ x , 0] + [0, y ]. Similarly in trigonometry, the angle sum identity expresses: And in functional analysis, when x is a linear function of some variable, such as time, these components are sinusoids , and they are orthogonal functions . A phase-shift of x → x + π /2 changes the identity to: in which case cos( x ) cos( φ ) is the in-phase component. In both conventions cos( φ )
1089-493: Was used in early tape recorders to reduce distortion. A DC bias is applied to the control grid of vacuum tubes in power amplifiers in order to regulate power. On a voltage-controlled oscillator (VCO), such as in a radio transmitter , selection of the center frequency of the carrier wave is done with a DC bias. For frequency modulation (FM), the AC component is the baseband audio signal plus any subcarriers . Frequency-shift keying can be done solely by changing
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