ch27_am-fm_met355

27AM/FM Metrics (Option 355)

03-NOTE

The FieldFox assumes you have manually set up the CF, span, and amplitude settings. Refer to Chapter 9, “SA (Spectrum Analyzer) Mode (Option 233–Mixed Analyzers).”

In this Chapter

“Using AM/FM Metrics” on page 706

“SINAD and THD Calculations” on page 712

“Interpreting AM/FM Metrics Error Messages” on page 715

Using AM/FM Metrics

Using FieldFox’s analog demodulation, users maintaining AM/FM radio systems can demodulate and characterize AM and FM signals. They can tune to the signal and listen to the audio tones using FieldFox’s built-in speakers or a headphone. They can also display the RF spectrum, the demodulated waveform and AM/FM metrics such as carrier power, modulation rate, and SINAD.

AM/FM Metrics enables you to make the following measurements:

AM Measurements:		—AM Measurements: Carrier power, AM rate, AM modulation index
FM Measurements:		—FM deviation (peak and RMS), Modulation Rate, Carrier power, Demodulated voltage, SINAD, THD (total harmonic distortion)

And, can be enabled/disabled simultaneously with Tune and Listen.

Overview of Measurement Types

If you are familiar with the AM/FM measurements and terms, skip to “How to Set Up AM/FM Metrics” on page 708.

Table 27-1 Measurement Details
Measurement details
Modulation Rate		The modulation rate quantifies how fast the modulation is and equates to the frequency of the baseband signal that modulates the carrier.
SINAD (Signal to Noise and Distortion Ratio)		Another commonly used distortion measure is signal to noise and distortion ratio (SINAD). SINAD, in nature, is the reciprocal of the modulation distortion provided by the “distortion/total Vrms,” but the SINAD is usually expressed in a logarithmic term as follows: Where: S = Signal Power D = N = Noise SINAD = [(S + D + N) / (D + N)] OR Refer to “SINAD and THD Calculations” on page 712.
THD (Total Harmonic Distortion)		The total harmonic distortion (THD) measurement further isolates the total harmonic distortion component from the noise component in the total unwanted signal. It helps the user to troubleshoot the root causes of modulation distortion. S = Signal Power D = N = Noise THD = [(D + N) / (S + D + N)]
RF spectrum of the modulated signal		This is the most traditional spectrum analyzer measurement viewing the modulated carrier signal power in frequency domain. Prior to being modulated, the signal power of a sinusoidal carrier concentrates at the carrier frequency. By contrast, modulation causes sidebands indicating the power redistribution over frequencies. The pattern of the sidebands depends upon the modulation format.
Demod waveform		This measurement retrieves the baseband signal from the modulated signal via the demodulation process, and displays the baseband signal in a pattern of modulation depth/deviation versus time. Since the modulation depth/deviation is directly proportional to the instantaneous amplitude of the baseband signal, the measurement result helps to intuitively evaluate the quality of the baseband signal.
AM modulation depth		AM modulation depth = (Peak-to-Peak / 2)


Modulation Metrics
FM deviation		The FM deviation reflects the amount of the frequency modulation to which the baseband signal modulates the carrier signal. The quantity being measured is the peak frequency deviation that is the maximum frequency excursion from the carrier frequency.
Carrier power		This measures the power of the carrier signal without the modulation. Ideally, power for a sinusoidal carrier signal is concentrated around its carrier frequency.

How to Set Up AM/FM Metrics

—Connect a test tone to the FieldFox SA Port 2 (Port 2)

—Press Measure > Analog Demod Tune & Listen > Analog Demod

Selecting FM Wide vs. FM Narrow does not effect the displayed waveforms or metrics. But, if you are using the Audio Demod feature and listening to the demodulated waveform, then select the appropriate FM type, because the tones do differ based on the FM type selected. Refer to Table 27-1 on page 706 .

Chapter 9, “SA (Spectrum Analyzer) Mode (Option 233–Mixed Analyzers)” and to “Analog Demod” on page 180.

Left display - Frequency Spectrum (Power vs. Frequency)

Right display - Demodulated waveform vs. time where Demod SwpT sets the width of the demod window.

—Then Freq/Dist > Center to set the carrier frequency.

—Set Freq Span so that all sidebands above -60 dB are displayed in the left window.

—Optionally, adjust the AF Res BW to better iden side bands. Le Chapter 9, “SA (Spectrum Analyzer) Mode (Option 233–Mixed Analyzers)” and “AF RBW (Audio-Frequency Resolution BW)” on page 198.

—Optionally, Freq/Dist > Tune & Listen OFF. Tune & Listen can be set when you are in Analog Demodulation mode, so you can enable Tune & Listen sweeps in between your Analog Demod sweeps

Metrics are displayed in the AM/FM Modulation Metrics table on the FieldFox. For AM and FM Metrics examples, refer to Figure 27-1 through Figure 27-8 on page 717.

Figure 27-1 AM Metrics - Example 1 AM-FM_Metrics-AM_Ex1

Figure 27-2 AM Metrics - Example 2 AM-FM_Metrics-AM_Ex2NarwBnd

Figure 27-3 FM Metrics - Wide band Example FM Metrics- WideBand FMex1

SINAD and THD Calculations

—FieldFox converts the audio waveform to the audio frequency spectrum using an FFT (i.e., The FFT result is not displayed). Refer to Figure 27-4 on page 712.

—The FFT waveform is used to calculate SINAD and THD distortion metrics

—The fundamental signal, harmonics, and noise from 0 Hz to 24 kHz are applied to the formulas. Refer to Figure 27-5 on page 713.

—For more on calculations, refer to “Overview of Measurement Types” on page 706.

—See also, https://www.keysight.com/us/en/assets/7018-04493/application-notes/5991-4913.pdf.

Figure 27-4 Audio Waveform with FFT Calculations

Figure 27-5 SINAD and THD Results - Example 1 AudioWFM_Disp1

Figure 27-6 SINAD and THD Results - Example 2 AudioWFM_Disp2

Interpreting AM/FM Metrics Error Messages

This section discusses possible AM/FM Metrics error messages and how to correct them.

Test Tone Not Found

A Test Tone Not Found message is displayed under the following circumstances:

—FieldFox frequency span is too wide

—No side lobes are displayed in the results

Refer to the examples in “Test Tone Not Found - Example” on page 716 and “Test Tone Not Found - Corrected Example” on page 717.

Test Tone Not Found - Example

The following response had the following settings:

—Source CF: 140 MHz

—FM Deviation: 18 kHz

—Modulation Rate: 1 kHz

Reasons for error:

—Frequency span of 1.0 MHz is too wide

—No side lobes displayed, so test tone not found

Figure 27-7 Test Tone Not Found - Example TestToneNF_Mess

Test Tone Not Found - Corrected Example

The following response was corrected with these setting changes:

—Frequency set to narrower frequency span of 50 kHz

—Ideally the frequency span should show the modulation envelope down to –60 dB below the RF power level

Response corrected, because:

—Narrowing the frequency span enables the modulation envelope’s side lobes to be identified

—FM algorithm identifies the side lobes

—The algorithm matches the modulation envelope to the mathematically calculated FM modulation or distortion patterns

Figure 27-8 Test Tone Not Found - Corrected Example TestToneNF_MessCorrected