The Jitter Man is right. The
refsideband parameter is used to
determine the input frequency. It is used when computing either the
input referred noise or the noise figure. The PNoise analysis directly
computes the output noise, and to compute either the input referred
noise or the noise figure, it must divide through by the gain. To
compute the gain, it must know the location and frequency of both the
output and the input. To do the noise analysis you had to specify the
output, which you generally do by specifying a pair of output nodes. You
specify the output frequency when you specify the noise analysis
frequency sweep range. To compute the input referred noise or the noise
figure you also need to specify the input sources. If the source is
either a voltage or current source, you get the input referred noise. If
the source is a port, you get the noise figure. However, just
specifying the name of the source is not enough. It must also know the
input frequency, which you specify using
refsideband. The input frequency is related to
refsideband as follows ...
|
fin| = |
refsideband*
fPSS +
fout|
In other words,
refsideband
is the amount that the input frequency differs from that of the output
frequency and is measured in multiples of the PSS fundamental frequency
(the clock or LO frequency). It is constrained to be an integer.
Consider
a circuit that has a PSS fundamental frequency of 1GHz. Further
consider performing a PNoise analysis of that circuit and focus on one
point in that noise analysis, the point where the output frequency is
1MHz.
- If refsideband=0, then fin=1MHz.
This would be the case in circuits where the input and the output are
at the same frequency. This is generally the case in switched-capacitor
filters, chopper-stabilized amplifier, sample-and-holds, etc.
- If refsideband=1, then fin=1.001GHz. This would be the case in an high-side fundamental mixer.
- If refsideband=-1, then fin=999MHz. This would be the case in an low-side fundamental mixer.
- If refsideband=2, then fin=2.001GHz. This would be the case in an high-side second harmonic mixer.
- If refsideband=-2, then fin=1.999GHz. This would be the case in an low-side second harmonic mixer.
- Etc.
Consider
a frequency divider. If the input clock frequency is at 2GHz and it is a
divide-by-two divider, then the output frequency will be at 1GHz and
the PSS analysis will be performed with a fundamental frequency of 1GHz.
Now, if you want to know the output spot noise at 1.001GHz, you would
specify the noise analysis frequency to be 1.001GHz. If you also want to
know the input referred spot noise at 2.001GHz, then you would specify
refsideband=1 because the input frequency is one multiple of the PSS fundamental frequency above the output frequency.
In
this example, the quantity of interest is the single-sideband spot
noise. If you want to know the phase noise, you would use the procedure I
described in my paper on phase noise, which involves computing the
strobed noise and dividing through by the slew rate at the threshold
crossing. Here you are only using the ouput noise, which is unaffected
by
refsideband.
-Ken