Attenuator Test & Noise

Sometimes third-order performance is given as TOI (third-order intercept).
This is the mixer level at which the internally generated third-order distortion
would be equal to the fundamental(s), or 0 dBc. This situation cannot be
realized in practice because the mixer would be well into saturation.
However, from a mathematical standpoint, TOI is a perfectly good data
point because we know the slope of the line. So even with TOI as a starting
point, we can still determine the degree of internally generated distortion
at a given mixer level.

We can calculate TOI from data sheet information. Because third-order
dynamic range changes 2 dB for very dB change in the level of the
fundamental tone(s) at the mixer, we get TOI by subtracting half of the
specified dynamic range in dBc from the level of the fundamental(s) :

Using the values from the previous discussion:
TOI = 30 dBm ( 85 dBc) / 2 = + 12.5 dBm

Attenuator test
Understanding the distortion graph is important, but we can use a simple
test to determine whether displayed distortion components are true input
signals or internally generated signals. Change the input attenuator. If
the displayed value of the distortion components remains the same, the
components are part of the input signal. If the displayed value changes, the
distortion components are generated internally or are the sum of external
and internally generated signals. We continue changing the attenuator until
the displayed distortion does not change and then complete the measurement.

Noise
There is another constraint on dynamic range, and that is the noise floor of
our spectrum analyzer. Going back to our definition of dynamic range as the
ratio of the largest to the smallest signal that we can measure, the average
noise of our spectrum analyzer puts the limit on the smaller signal. So
dynamic range versus noise becomes signal-to-noise ratio in which the
signal is the fundamental whose distortion we wish to measure.

We can easily plot noise on our dynamic range chart. For example, suppose
that the data sheet for our spectrum analyzer specifies a displayed average
noise level of 110 dBm in a 10 kHz resolution bandwidth. If our signal
fundamental has a level of 40 dBm at the mixer, it is 70 dB above the
average noise, so we have a 70 dB signal-to-noise ratio. For very dB that
we reduce the signal level at the mixer, we lose 1 dB of signal-to-noise ratio.
Our noise curve is a straight line having a slope of 1, as shown in Figure 6-2.

If we ignore measurement accuracy considerations for a moment, the best
dynamic range will occur at the intersection of the appropriat distortion
curve and the noise curve. Figure 6-2 tells us that our maximum dynamic
range for second-order distortion is 72.5 dB; for third-order distortion,
81.7 dB. In practice, the intersection of the noise and distortion graphs is
not a sharply defined point, because noise adds to the CW-like distortion
products, reducing dynamic range by 2 dB when using the log power scale
with log scale averaging.