Specifications

The range of the log amplifier can be another limitation for spectrum

analyzers with analog IF circuitry. For example, ESA-L Series spectrum

analyzers use an 85 dB log amplifier. Thus, only measurements that are within

85 dB below the reference level are calibrated.

The question is, can the full display range be used? From the previous

discussion of dynamic range, we know that the answer is generally yes. In fact,

dynamic range often exceeds display range or log amplifier range. To bring

the smaller signals into the calibrated area of the display, we must increase

IF gain. But in so doing, we may move the larger signals off the top of the

display, above the reference level. Some Agilent analyzers, such as the

PSA Series, allow measurements of signals above the reference level without

affecting the accuracy with which the smaller signals are displayed. This is

shown in Figure 6-8. So we can indeed take advantage of the full dynamic

range of an analyzer even when the dynamic range exceeds the display range.

In Figure 6-8, even though the reference level has changed from –8 dBm

to –53 dBm, driving the signal far above the top of the screen, the marker

readout remains unchanged.

Measurement range is the ratio of the largest to the smallest signal that

can be measured under any circumstances. The maximum safe input level,

typically +30 dBm (1 watt) for most analyzers, determines the upper limit.

These analyzers have input attenuators settable to 60 or 70 dB, so we can

reduce +30 dBm signals to levels well below the compression point of the

input mixer and measure them accurately. The displayed average noise

level sets the other end of the range. Depending on the minimum resolution

bandwidth of the particular analyzer and whether or not a preamplifier is

being used, DANL typically ranges from –115 to –170 dBm. Measurement

range, then, can vary from 145 to 200 dB. Of course, we cannot view a

–170 dBm signal while a +30 dBm signal is also present at the input.

Figure 6-8. Display range and measurement range on the PSA Series