In this article we will introduce how to measure a pulse RF signal with a spectrum analyzer. We will take S3302 handheld spectrum analyzer as an example to show detailed operation steps.
Definition of pulse RF signal
The pulse RF signal refers to a RF pulse string of the same repetition frequency and constant pulse width, shape and amplitude. This section introduces several methods of measuring pulse RF signal parameters, including how to measure the center frequency, pulse width and pulse repetition frequency. In addition, the measurement of peak pulse power is also discussed.
The resolution bandwidth has great influence on pulse RF signal measurement. You must understand the relationship between the resolution bandwidth and pulse repetition frequency. If the resolution bandwidth is narrower than the pulse repetition frequency, only individual frequency components of the pulse RF signal will appear on the screen. This is known as the narrow band mode. The mode in which the resolution bandwidth is broader than the pulse repetition frequency is known as the broad band mode. In this case, you can see the spectrum envelope formed by pulse segments that are equalized by the tested pulse repetition frequency.
Center frequency, side lobe ratio and pulse width measurement of pulse RF signal
1) Set the output signal of the signal generator:
Set the frequency of the signal generator as 1GHz and power as -20dBm. Connect the output of the signal generator to the input port of the spectrum analyzer, as shown in Fig. 1. Set the repetition frequency of pulse modulation as 1kHz and pulse width as 900ns. Enable the pulse modulation and RF output.
Fig.1 Typical Connections
2) Set the spectrum analyzer:
The pulse RF signal is generally measured in the broad band mode. In order to prevent the influence of the video filter on measurement results, the video bandwidth should be set as 3MHz.
• Press [Reset] key.
• Press [Frequency], [Center Frequency] and 1[GHz].
• Press [frequency], [Span], 10[MHz], [sweep], [Sweep Time Auto Man] and 60[ms].
• Press [BW], [RBW Auto Man], 100 [kHz], [VBW Auto Man] and 100 [kHz].
• Press [BW], [Detector] and [Peak] to activate the peak detector.
• Enable the center frequency function and adjust the span until the center side lobe and at least one pair of side lobes appear on the screen, as shown in Fig. 2.
Fig. 2 Main Lobe and Side Lobe
Increase the sweep time (decrease the sweeping speed) until the graph is filled into a solid line, as shown in Fig.3. If the spectrum line cannot be filled, it indicates that the instrument is not in the broad band mode. In this case, the following steps of measurement of the sidelobe ratio, pulse width and peak pulse power will not apply. The resolution bandwidth should be more than 1kHz.
Fig. 3 Trace Display in Solid Line Form
3) Read the center frequency of the pulse and amplitude of the main lobe.
• Press [Peak].
The marker reading is the center frequency of the pulse and amplitude of the main lobe.
4) Set the marker at the center frequency of the main lobe, and measure the side lobe ratio:
l Press [Peak], [Maker], [Delta], [Peak] and [Next Peak].
The amplitude difference between the main lobe and side lobe is the side lobe ratio, as shown in Fig. 4.
Fig. 4 Side Lobe Ratio Shown by the Marker
5) Measure the pulse width, which is equal to the reciprocal of the frequency difference between the peaks of two side lobe envelopes.
• Press [Maker], [Delta], [Peak], [Next Pk Right] and [Next Pk Right].
In this case, the reciprocal of the frequency difference indicated by the differential marker is the pulse width, as shown in Fig. 5. To obtain the most accurate pulse width, you can manually adjust the marker location and measure the distance between the zero crossing points of two adjacent side lobes. You can also reduce the resolution bandwidth to make the zero crossing point sharper and measurement accuracy higher.
Fig. 5 Pulse Width Shown by the Marker
Pulse repetition frequency (PRF) measurement
The pulse repetition interval (PRI) refers to the time interval between any two adjacent pulse responses.
1) Set the output signal of the signal generator:
Set the frequency of the signal generator as 1GHz and power as -20dBm. Connect the output of the signal generator to the input port of the spectrum analyzer. Set the repetition frequency of pulse modulation as 1kHz and pulse width as 900ns. Enable the pulse modulation and RF output.
2) Set the spectrum analyzer:
• Press [Reset] key.
• Press [frequency] and 1[GHz].
• Press [frequency], [Span], 10[MHz], [sweep], [Sweep Time Auto Man] and 1.705[s].
• Press [BW], [RBW Auto Man] and 1[kHz].
• Press [BW], [VBW Auto Man] and 3[MHz].
• Press [BW], [Detector] and [Peak] to activate the peak detector.
• Adjust the span until the main lobe and at least one side lobes appear on the screen.
• Readjust the output amplitude of the signal generator until it is shown in the screen. Reduce the sweep time (i.e. increase the sweeping speed) until the contents similar to those in Fig. 6 are displayed.
3) Measure the pulse repetition interval:
l Press [Sweep] and [ Sweep$Cont|Single]
l Press [Peak], [Maker][Delta] and [Peak] [Next Peak]. The difference of two markers is the pulse repetition interval (PRI), and its reciprocal is the pulse repetition frequency (PRF).
Fig. 6 Measurement of Pulse Repetition Frequency
Peak pulse power measurement
Now we have obtain the main lobe amplitude and pulse width. In addition, we can easily obtain the resolution bandwidth of the spectrum analyzer. Therefore, the peak pulse power can be obtained based on such parameters.
In the broad-band measurement mode of the spectrum analyzer:
Peak pulse power =(main lobe amplitude) -(20 log Teff x BWi)
Where:
Teff -pulse width, in second. Second
BWi – impact bandwidth, in Hz (equal to the resolution bandwidth applied in 1.5´ “Pulse width measurement�?
In the narrow-band measurement mode of the spectrum analyzer:
peak pulse power =(main lobe amplitude) -(20 log Teff/T)
Where:
Teff -pulse width, in second. Second
T——pulse repetition frequency
The phenomenon in which the peak pulse power is not equal to the main lobe amplitude is known as pulse desensitization. The sensitivity of the spectrum analyzer will not be decreased by the pulse signal. Accurately, pulse desensitization is caused by distribution of the CW carrier power of pulse modulation to a number of spectrum components (i.e. carrier and sideband). Therefore, each spectrum only contains part of the total power.
