Different RF Parameters that can be measured in Spectrum Analyzer

Channel Power :

• This measures the total power present in the channel bandwidth.
• To measure power of a CW signal, a marker need to be added at the peak of the signal.
• Unlike a continuous-wave (CW) signal, the power of a modulated signal like WCDMA is spread across a wide bandwidth. As a result, a complex integration is needed to calculate the total power of the channel.
• Using a Spectrum Analyzer, one can use the “Channel Power” measurement to calculate the total channel power and power spectral density by specifying the center frequency and integration bandwidth of the signal.

Adjacent Channel Power (ACP):

• ACP is the measure of the Average Power transmitter generates in channel next to the intended channel.
• For all transmitting devices, device should only transmit in its designated channel and not interfere with devices operating in adjacent channels where device is not licensed to transmit.
• ACP measurement is important for helping the device avoid interference with other devices.
• ACPR is the ratio of the Power in the intended channel compared to the power in the adjacent channel.

• ACP measurement use the reference level of 0 dBm and it is required ACPR to be low as possible.
• Poor ACPR is an indication of spectral spreading or switching transients for DUT which is a definite problem.
• Spectrum Analyzer has built in ACP measurement , one need to specify the channel frequency , bandwidth and channel offset of the signal.
• The results of ACP measurement are displayed as bar graph or as spectrum data or as the combination of the two.

Measuring a signal’s “width”:

• Measuring signal bandwidth is important.
• Signals that are too wide often indicate problems somewhere along the signal path.
• Regulatory requirements : Many radio communications systems assign signals to defined frequency ranges(channels).
• The most common way of quantifying a signal’s width is by making an occupied bandwidth measurement.

Occupied Bandwidth:

• Occupied Bandwidth (OBW) is the measure of the bandwidth (in Hz) which contains a given percentage of the total signal power- usually 99%.
• Occupied bandwidth is usually less than channel nominal channel width.

• The Spectrum Analyzer is used with Occupied Bandwidth mode for measuring Occupied Bandwidth.
• One need to specify the Center frequency , % Power and Channel Bandwidth. Channel Bandwidth is needed as its help Analyzer correctly set span, Resolution Bandwidth etc.
• The OBW measurement also indicates the difference between the analyzer center frequency and the channel center frequency, thereby helping in determining if the modulation is symmetrical about the carrier frequency entered. This difference is referred to as the transmitter frequency error .

Peak –to- Average Power Ratio (PAPR):

• Modern communication system uses digital modulation instead of analog modulation and have a higher peak to average signal power ratio(PAPR).

Signals with high PAPRs are challenging due to:

• High Dynamic range in A/D and D/A converters
• Highly linear amplifier with high peak power.

Statistics Measurements:

• Many digitally modulated signals are noise-like in both the time and frequency domains. As a result, statistical measurements of these signals can provide useful information.

Three statistical power sensor measurement –

• PDF(Probability Density Function)
• CDF(Cumulative Distribution Function)
• CCDF(Complementary Cumulative Distribution Function)

CCDF:

• A CCDF curve is defined by how much time the waveform spends at or above a given power level. The percent of time the signal spends in that region defines the probability for that particular power level. This is of great value to design engineers when testing and troubleshooting the nonlinearity of power amplifiers.
• CCDF emphasizes the maximum (or peak) power values and shows how often it is reached.
• As the CCDF moves to the right, the signal becomes more stressful.
• With the information from the CCDF curve it helps in visualizing the effect of modulation formats or the effect of combining multiple system components , evaluating spread spectrum systems and design and test discrete RF components.
• Signals with similar modulation may have very different CCDF curve.
• Device behaviour can be quantified by measuring CCDF at both the input and the output.

BURST POWER :

• The burst power (transmit power) measurement is used to determine the power delivered to the antenna system on the radio-frequency channel under test.
• The burst power measurement verifies the accuracy of the mean transmitted RF carrier power. This can be done across the frequency range and at each power step.
• This measurement is primarily for use with time domain modulated signals (Bluetooth™, GSM/Edge, and NADC).
• Mobile stations and base stations must transmit enough power, with sufficient modulation accuracy, to maintain a call of acceptable quality without leaking power into frequency channels or timeslots allocated for others.
• The burst power measurement determines the average power for an RF signal burst at or above a specified threshold value or during the detected burst width. The threshold value may be absolute, or relative to the peak value of the signal.
• The mean carrier power is calculated based on the burst width and the threshold level. Burst width can be set manually or automatically.

SPURIOUS EMISSIONS :

• Spurs outside the device’s assigned channel can be problematic and can even cause the device to fail regulatory compliance tests.
• Oscillators and transmitters generate non-harmonic, low-level spurs. Those unwanted spurs outside of the assigned channel are collectively called spurious emissions.
• Unwanted, stray frequency content can appear both inside and outside of our device’s designated bandwidth. But, with spurious emissions testing, one really only care about the once outside of our device’s designated bandwidth.
• Agency like FCC in US and ETSI in Europe put strict limits on spurious emissions.
• Signal Analyzer have built-in Spurious Emission mode. If any of the spur fail there is an indicator for that.
• When measuring spurious emissions noise floor of the signal analyzer should also be considered. If it is too high, low level spurs cannot be seen.
• Knowing what spurious emission coming from the device will help in identifying any potential design or regulatory issues.

SPECTRUM EMISSION MASK (SEM):

• When working with spread-spectrum signals such W-CDMA or LTE, one can characterize the transmitters by measuring the power of in-band and out-of-band emissions in a chosen frequency band at specific offsets.
• Communication Standard stipulate the maximum power a device can generate in adjacent channels. A quick way to see the device is compliant with these standards is to use SEM.
• It measures the levels of spurious signals in up to six pairs of offset frequencies and relates them to the carrier power.
• Select the radio standard of interest and the analyzer will automatically setup a pre-defined mask. It also provides a pass/fail indication of whether or not the transmitter meets the standard.

THIRD ORDER INTERCEPT (TOI):

• When two or more signals are modulated, they produce a form of distortion called “intermodulation products”. Devices in their non-linear region generate harmonics and intermodulation products.
• The combination of the fundamental signals and second harmonics creates third order intermodulation products, some of which is very close to the fundamental signals and can’t be filter out.
• Third-order intercept(or IP3) is a way to quantify intermodulation distortion(IMD) by extending linearity of the device under test.

• A high TOI is a sign of healthy system and a low TOI is symptom that our designs may need refinement.
• TOI measurement begin by taking a sweep of the incoming signal and then it measures the two fundamental tones followed by third-order intermodulation products .
• With the measurement table one can see the absolute power of each fundamental tone and absolute and relative power of both the upper and lower intermodulation product .
• When measuring third-order intercept, it is important to keep the sources (generators) isolated.
• Attenuation can be used to verify that the IMD is being created external to the analyzer.

Total Harmonic Distortion (THD):

• From simple continuous wave to complex digitally modulated signals, every real signal has some amount of distortion.
• It is important to characterize this in the system because distortion results in energy at unintended frequencies, which could cause interference.
• Total Harmonic Distortion is the ratio of the sum of powers of all surrounding harmonic components to the power of the fundamental signal .
• THD should be as low as possible.
• THD of the signal can be used to evaluate whether or not the signal will cause any interference with system operating in other channels.