HB Noise Analysis (hbnoise)
Description
The Periodic or Quasi-Periodic Noise (HBNOISE) analysis is similar to the conventional noise analysis, except that HBNOISE analysis includes frequency conversion effects. Hence, it is useful for predicting the noise behavior of mixers, switched-capacitor filters, and other periodically or quasi-periodically driven circuits. It is particularly useful for predicting the phase noise of autonomous circuits, such as oscillators.
HBNOISE analysis linearizes the circuit about the periodic or quasi-periodic operating point computed in the prerequisite HB analysis. It is the periodically or quasi-periodically time-varying nature of the linearized circuit that accounts for the frequency conversion. In addition, the effect of a periodically or quasi-periodically time-varying bias point on the noise generated by the various components in the circuit is also included.
The time-average of the noise at the output of the circuit is computed in the form of a spectral density versus frequency. The output of the circuit is specified with either a pair of nodes or a probe component. To specify the output of a circuit with a probe, specify it using the oprobe parameter. If the output is voltage (or potential), choose a resistor or port as the output probe. If the output is current (or flow), choose a vsource or iprobe as the output probe.
If the input-referred noise or noise figure is desired, specify the input source using the iprobe parameter. For input-referred noise, use either a vsource or isource as the input probe; for noise figure, use a port as the probe. Currently, only a vsource, an isource, or a port can be used as an input probe. If the input source is noisy, as is a port, the noise analysis computes the noise factor (F) and noise figure (NF). To match the IEEE definition of noise figure, the input probe must be a port with no excess noise and its noisetemp must be set to 16.85C (290K). In addition, the output load must be a resistor or port and must be identified as the oprobe.
If port is specified as the input probe, both input-referred noise and gain are referred back to the equivalent voltage source inside the port. S-parameter analysis calculates those values in traditional sense.
The reference sideband (refsideband) specifies which conversion gain is used when computing input-referred noise, noise factor, and noise figure. The reference sideband specifies the input frequency relative to the output frequency with:
|f(input)| = |f(out) + refsideband frequency shift|.
For periodic noise (only one tone in HB analysis), 'refsideband' is a number. Use refsideband=0 when the input and output of the circuit are at the same frequency, such as with amplifiers and filters. When refsideband differs from 0, the single side-band noise figure is computed.
While for quasi-periodic noise (multiple tones in HB analysis), reference sidebands are vectors. Assume that there is one large tone and one moderate tone in HB. A sideband Ki is a vector [Ki_1 Ki_2]. It gives the frequency at
Ki_1 * fund(large tone of HB) + Ki_2 * fund(moderate tone of HB)
Use refsideband=[0 0 ...] when the input and output of the circuit are at the same frequency, such as with amplifiers and filters.
The reference sideband option ('refsidebandoption') specifies whether to consider the input at the frequency or the input at the individual quasi-periodic sideband specified. Note that different sidebands can lead to the same frequency.
The noise analysis always computes the total noise at the output, which includes contributions from the input source and the output load. The amount of the output noise that is attributable to each noise source in the circuit is also computed and output individually. If the input source is identified (using iprobe) and is a vsource or isource, the input-referred noise is computed, which includes the noise from the input source itself. Finally, if the input source is identified (using iprobe) and is noisy, as is the case with ports, the noise factor and noise figure are computed. Therefore, if:
Ns = noise at the output due to the input probe (the source)
Nsi = noise at the output due to the image harmonic at the source
Nso = noise at the output due to harmonics other than input at the source
Nl = noise at the output due to the output probe (the load)
Fdsb = double sideband noise factor
NFdsb = double sideband noise figure
Fieee = IEEE single sideband noise factor
NFieee = IEEE single sideband noise figure
Fdsb = (No^2 - Nl^2)/(Ns^2+Nsi^2)
Fieee = (No^2 - Nl^2 - Nso^2)/Ns^2
When the results are output, No is named out, IRN is named in, G is named gain, F, NF, Fdsb, NFdsb, Fieee, and NFieee are named F, NF, Fdsb, NFdsb, Fieee, and NFieee respectively.
The computation of gain and IRN for quasi-periodic noise in HBNOISE assumes that the circuit under test is impedance-matched to the input source. This can introduce inaccuracy into the gain and IRN computation.
An HBNOISE analysis must follow an HB analysis.
Syntax
Name [p] [n] ... hbnoise parameter=value ...
The optional terminals (p and n) specify the output of the circuit. If you do not specify the terminals, you must specify the output with a probe component.
Parameters
In practice, noise can mix with each of the harmonics of the periodic drive signal applied in the HB analysis and end up at the output frequency. However, the HBNOISE analysis includes only the noise that mixes with a finite set of harmonics that are typically specified using the maxsideband parameter.
If Ki represents sideband i, then for periodic noise:
f(noise_source) = f(out) + Ki * fund(hb)
For quasi-periodic noise with multi-tone in HB analysis, assuming that there is one large tone and one moderate tone, Ki is represented as [Ki_1 Ki_2]. Corresponding frequency shift is as follows:
Ki_1 * fund(large tone of HB) + Ki_2 * fund(moderate tone of HB)
If there are L large and moderate tones in HB analysis and a set of n integer vectors representing the sidebands:
f(noise_source) = f(out) + SUM_j=1_to_L{ Ki_j * fund_j(hb) }
The maxsideband parameter specifies the maximum |Ki| included in the HBNOISE calculation. For quasi-periodic noise, only the large tone, which is the first fundamental, is affected by this entry. All the other tones, which are the moderate tones, are limited by maxharms specified for an HB analysis.
The number of requested sidebands changes the simulation time substantially.
You can designate a voltage to be the output by specifying a pair of nodes on the HBNOISE analysis statement or by using the 'oprobe' parameter. Any component with two or more terminals can be a voltage probe. When there are more than two terminals, they are grouped in pairs, and you use the portv parameter to select the appropriate pair.
Any component that naturally computes current as an internal variable can be a current probe. If the probe component computes more than one current, you use the porti parameter to select the appropriate current. You must not specify both portv and porti. If you specify neither, the probe component provides a reasonable default.
You can use the stimuli parameter to specify what serves as the inputs for the transfer functions. There are two choices: stimuli=sources and stimuli=nodes_and_terminals.
stimuli=sources indicates that the sources present in the circuit are to be used. You can use the xfmag parameters provided by the sources to adjust the computed gain to compensate for gains or losses in a test fixture. You can limit the number of sources in hierarchical netlists by using the save and nestlvl parameters.
stimuli=nodes_and_terminals indicates that all possible transfer functions are to be computed. This is useful when it is not known in advance which transfer functions are interesting. Transfer functions for nodes are computed assuming that a unit magnitude flow (current) source is connected from the node to ground. Transfer functions for terminals are computed assuming that a unit magnitude value (voltage) source is connected in series with the terminal. By default, the transfer functions from a small set of terminals are computed. If you want transfer functions from specific terminals, specify the terminals in the save statement. You must use the :probe modifier (for example, Rout:1:probe) or specify useprobes=yes on the options statement. If you want transfer functions from all terminals, specify currents=all and useprobes=yes on the options statement.
You can specify sweep limits by specifying the end points, or the center value and span of the sweep. Steps can be linear or logarithmic, and you can specify the number of steps or the size of each step. You can specify a step size parameter (step, lin, log, or dec) to determine whether the sweep is linear or logarithmic. If you do not specify a step size parameter, the sweep is linear when the ratio of stop to start values is less than 10 and logarithmic when this ratio is 10 or greater. Alternatively, you can use the values parameter to specify the values that the sweep parameter should take. If you provide both a specific set of values and a set specified using a sweep range, the two sets are merged and collated before being used. All frequencies are in Hertz.
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