Understanding dynamic range

Dynamic range

Dynamic range is the difference between the highest and lowest amplitude signals that can be accurately measured at the same time. A higher dynamic range means better ability to simultaneously measure high- and low-level signals.

Most modern spectrum analyzers have a dynamic range of over 100 dB. Several factors influence the lower and upper limits of an analyzer’s dynamic range:

• Lower limit is largely determined by the analyzer’s displayed average noise level (DANL) and, in some cases, by phase noise.
• Upper limit is often determined by maximum safe input power and compression. The presence of spurious products generated within the analyzer also plays an important role.

Displayed average noise level (DANL)

The absolute lower limit of dynamic range is DANL. This is the internal noise of the analyzer and is often referred to as the noise floor. Signals with amplitudes below the DANL cannot be seen or measured.

DANL is a function both the hardware and user-configured settings. The hardware determines the analyzer’s noise figure, which is the internal noise the analyzer adds to the input signal. However, it is possible to change the DANL by adjusting the settings: lowering the resolution bandwidth and/or input attenuation will decrease an analyzer’s DANL and thus increase the dynamic range.

• Learn more about DANL

Phase noise

Phase noise is short-term variation in a signal’s frequency stability, and it can sometimes also contribute to the lower limit of the dynamic range. Instability in signal frequency creates noise in the form of sidebands both above and below a carrier. In many cases, this phase noise is created by an input signal mixing with a “noisy” oscillator. This phenomenon is often called reciprocal mixing.

• Learn more about measuring phase noise

Phase noise is usually greater the closer you are to the carrier. The noise power in these phase noise sidebands can increase DANL and thus decrease dynamic range.

Maximum input power

The upper limit of dynamic range is determined first by maximum input power. All spectrum analyzers have a specified maximum safe input power, and in modern analyzers, this is typically 1W. If this input power is exceeded, the analyzer’s front end may be permanently damaged or destroyed.

If signals with a power greater than the maximum safe input power must be measured, external attenuators are a simple and reliable way to reduce the input power to a safe level. However, this attenuation can also push lower amplitude signals beneath the noise floor.

Compression

Internal active components, such as mixers and amplifiers, also limit the upper limit of the dynamic range. In an ideal active device with gain, the device input power is linearly scaled by the gain. However, if the input signal amplitude becomes large enough, a real device will no longer be able to produce this constant gain, and the output power will no longer follow this linear relationship. This situation is called compression.

Compression is usually quantified as the 1dB compression point, which is where the expected and actual output powers differ by 1dB. Compression limits how accurately you can measure high amplitude signals and thus affects the upper limit of dynamic range.

Spurious signals

Compression doesn’t just cause measurement accuracy to drop; it can also create distortion or spurious products. These include both harmonics and intermodulation products that are internally generated by the analyzer. The frequencies of both harmonics and intermodulation products are usually related to the frequency and amplitude of the input signal and thus can be predicted. In some cases, however, spurious signals may also be unrelated to input signal frequency. You can see these on the analyzer display even when no input signal is present.

It is typically difficult to filter out or otherwise “work around” spurious signals. As such, the presence of spurious signals limits the range over which you can accurately measure signals.

Spurious free dynamic range (SFDR)

Spurious free dynamic range (SFDR) quantifies the effect of spurious signals. As defined above, dynamic range is the distance between the largest and smallest signals that can be simultaneously and accurately measured. SFDR is the difference between the fundamental signal and the largest spurious signal. Unlike dynamic range, which is measured in dB, SFDR is reported in units of dB-C. This is the dB relative to the power of the fundamental or carrier. This makes sense because the amplitude of spurious signals produced by an active device is a function of the fundamentals or input signal. In other words, the further a device is pushed into compression, the greater the amplitude of its spurious signals.

SPFR is a more appropriate measurement than dynamic range if the dynamic range is primarily limited by strong spurious signals rather than noise or compression.