dB - Logarithmic Quantities

dB calculations can also be performed using the dBQuantity class:

>>> from PhysicalQuantities.dBQuantity import dBQuantity
>>> a = dBQuantity(1, 'dBm')
>>> a
1 dBm

The IPython extension provides an easy way to do dB calculations:

>>> %load_ext PhysicalQuantities.ipython
>>> p = 0 dBm
>>> g = 20 dB
>>> a = 10 dB
>>> p + g - a
10 dBm

The list of supported dBUnits is small:

>>> from PhysicalQuantities.dBQuantity import dB_unit_table
>>> list(dB_unit_table.keys())
['dBnV',
 'dB',
 'dBV',
 'dBi',
 'dBnA',
 'dBuA',
 'dBc',
 'dBA',
 'dBW',
 'dBuV',
 'dBd',
 'dBm',
 'dBmA',
 'dBsm',
 'dBmV']

Name	Unit	Description
dBnV	V	dB Nanovolt
dBuV	V	dB Microvolt
dBV	V	dB Volt
dBnA	A	dB Nanoampere
dBuA	A	dB Microampere
dBA	A	dB Ampere
dBm	W	dB Milliwatt
dBW	W	dB Watt
dBsm	m^2	dB Squaremeter

Converting to other units is easy:

>>> p = 0 dBm
>>> print(p)
0 dBm
>>> print(p.dBW)
-30.0 dBW
>>> print(p.W)
0.001 W

Absolute values (dBm or dBW) are handled differently compared to unitless dB:

>>> p = 0 dBW
>>> p+p
3.0102999566398121 dBW
>>> g = 3 dB
>>> p+g
3 dBW

The _ attribute to remove units is available too:

>>> p = 0 dBm
>>> print(p.dBW)
-30.0 dBW
>>> print(p.dBW_)
-30.0
>>> print(p._)
0

Internal representation

Calling the dBQuantity() constructor creates the desired object:

>>> a = dBQuantity(0.1,'dBm')
>>> a
0.1 dBm

The information is stored in two attributes:

>>> a.value, a.unitname
(0.1, 'dBm')

The unit itself is represented as dBUnit object:

>>> u = a.unit
>>> u
<PhysicalQuantities.dBQuantity.dBUnit at 0x10baccf60>

The unit contains the name, the conversion factor (10 or 20), the reference impedance and the underlying PhysicalUnit:

>>> u.name, u.factor, u.offset, u.z0, u.physicalunit
('dBm', 10, 0, 50 Ohm, <PhysicalUnit mW>)

For relative dB values, offset and physicalunit are unknown:

>>> a = 0dB
>>> u = a.unit
>>> u.name, u.factor, u.offset, u.z0, u.physicalunit
('dB', 0, 0, 50 Ohm, None)

Calling with islog=False converts the value considering the target unit to log:

>>> a = dBQuantity(0.1,'dBm', islog=False)
>>> a
-10.0 dBm

Conversion from and to dB

Most linear units can be converted to dB using the dB attribute:

>>> p = 1 mW
>>> p.dB
0.0 dBm
>>> p = 10 dBm
>>> p.lin
10.0 mW

For relative dB values, the conversion factor has to be specified. There are two helper functions to calculate \(10*log10(value)\) or \(20*log10(value)\):

>>> from PhysicalQuantities.dBQuantity import dB10, dB20
>>> dB10(10)
10.0 dB
>>> dB20(10)
20.0 dB

Conversion back to linear values can be either \(10^{(value/10)}\) or \(10^{(value/20)}\). This can be simplified by using the properties lin10 or lin20:

>>> a = 10 dB
>>> a.lin10
10.0
>>> a.lin20
3.1622776601683795