“Tektronix’s new generation oscilloscope MSO64 adopts the new TEK049 platform, which not only achieves a high sampling rate of 25GS/s when 4 channels are turned on at the same time, but also achieves a high vertical resolution of 12-bit hardware. At the same time, due to the adoption of a new low-noise front-end amplifier ASIC-TEK061, the noise level is greatly reduced. At 1mv/div, the measured RSM value of the noise floor is only 58uV, which is far lower than similar oscilloscopes in the market. These characteristics are the strong guarantee of high dynamics and low noise floor in MSO64 spectral mode – Spectrum View.
Tektronix’s new generation oscilloscope MSO64 adopts the new TEK049 platform, which not only achieves a high sampling rate of 25GS/s when 4 channels are turned on at the same time, but also achieves a high vertical resolution of 12-bit hardware. At the same time, due to the adoption of a new low-noise front-end amplifier ASIC-TEK061, the noise level is greatly reduced. At 1mv/div, the measured RSM value of the noise floor is only 58uV, which is far lower than similar oscilloscopes in the market. These characteristics are the strong guarantee of high dynamics and low noise floor in MSO64 spectral mode – Spectrum View.
Recently, Spectrum View has added the RF_vs_Time Waveform test function, which can be used to analyze the transient change process of the signal, including the transient change trend of signal amplitude, frequency and phase, so it is usually called the transient process analysis of the signal. Typical signal transient process analysis application scenarios include: pulse signal envelope and intrapulse modulation analysis, frequency hopping signal analysis, PLL frequency lock time test, RF switch switching time test, pulse modulator rise time test, RF Module and analog IQ Modulator absolute delay test, etc. This article will focus on the use of transient analysis capabilities for pulse, frequency hopping, and PLL frequency lock-time testing.
Figure 1. MSO64 features the new TEK049 platform and ultra-low noise front end TEK061
Transient Process Analysis Fundamentals
The analysis of the transient process of the signal is actually the analysis of the three elements of the signal – the change process of the amplitude, frequency and phase over time. Different signals pay attention to different parameters. For example, the frequency hopping signal pays special attention to the frequency change law, and the pulse signal pays more attention. Signal envelope and its time parameters, etc. But no matter what parameters are concerned, always get the amplitude, frequency and phase waveform first. How does Spectrum View get these waveforms?
Spectrum View adopts the DDC (digital down-conversion) architecture shown in Figure 2. After processing the original sampling points, the digital IQ data of the signal can be obtained, and the signal amplitude, frequency and phase characteristics are included in the IQ data. When each group of IQ samples corresponds to the amplitude, frequency and phase, their variation trend with time can be obtained, so as to complete the analysis of the signal transient process.
Figure 2. IQ data after digital downconversion
Transient Process Analysis Application Scenarios
(1) Pulse and frequency hopping signal test
For engineers engaged in RF pulse signal analysis and testing, it is usually necessary to test the pulse rise/fall time, pulse width and period and other time parameters, as well as the average and maximum value of the power within the pulse. Only after the envelope of the RF pulse signal is obtained, the test of these parameters can be carried out more conveniently. In the past it was common to use an external envelope detector, extract the envelope and then test with an oscilloscope. Using the transient analysis function of Spectrum View, the envelope of the signal can be easily obtained without any external accessories. The “C1-M” curve shown in Figure 3 is the envelope.
It is worth mentioning that the automatic measurement function of the oscilloscope can also be applied to the time domain envelope, so as to automatically complete the test of the time parameters and power parameters of the pulse signal, without the need to use the cursor test, thus improving the test accuracy. More and more modern radars use pulse compression technology to ensure the detection range and at the same time improve the range resolution, among which the chirp pulse is more common. In the test of the chirp signal, in addition to observing the above-mentioned time and power parameters, demodulation analysis of the frequency modulation in the pulse is also required to check the FM bandwidth, FM slope and linearity. In the transient mode of Spectrum View, demodulation analysis can be completed, such as the “C1-f” curve shown in Figure 3, and the test results can be saved for further analysis.
Figure 3. Spectrum, waveform, envelope, frequency and phase curves of RF Chirp Pulse
Similarly, Spectrum View can also be applied to the analysis of frequency hopping signals, and the observation is still the frequency demodulation curve. After the frequency hopping pattern is obtained, parameters such as the dwell time of each frequency point and the switching time between adjacent frequency points can be further analyzed.
Figure 4. Spectrum View’s transient mode can directly demodulate frequency hopping patterns
(2) PLL frequency lock time test
Frequency synthesizers based on PLL technology are widely used, and PLL frequency synthesizers are essential in both communication and radar systems. Because PLL frequency synthesis has very high frequency stability and excellent phase noise performance, these are important factors to ensure the performance of communication and radar systems. PLL is a negative feedback control system. Figure 5 shows a brief schematic diagram of the architecture. From the perspective of closed-loop transmission characteristics, PLL has a certain loop bandwidth, which mainly depends on the low-pass filter on the loop-Loop filter. The loop bandwidth determines not only the phase noise performance of the output signal, but also the speed at which the PLL locks. Phase noise performance and locking speed are two parameters that PLL frequency synthesis development engineers must consider as a compromise, so they are also two parameters that must be tested in the debugging stage.
Figure 5. Schematic diagram of PLL frequency synthesizer architecture
For the lock time test, the traditional test method is to directly feed the RF signal output by the PLL into the spectrum analyzer, and then set the trigger to observe the envelope of the RF signal in the zero span mode. However, this method has two disadvantages: ① The trigger position is used as the time reference point, and the PLL has started to work before the trigger time, so the locking time cannot be accurately calibrated; ② Since this method determines whether the lock is completed from the envelope, the test The error will be huge. Because the envelope of the signal has a great relationship with the RBW set by the spectrum analyzer, there is such a situation – even if the frequency is not completely locked, the signal can still pass through the RBW filter completely, so as to obtain a normal envelope signal. At this time, the calibrated lock time will be too small, which cannot correctly reflect the performance of the PLL.
Using the transient analysis function of Spectrum View can easily solve this problem. The test connection is shown in Figure 6. In addition to connecting the RF output to the oscilloscope, the PLL circuit under test also provides a synchronous trigger signal as a time reference. In the transient analysis mode, call up the Frequency_vs_Time waveform. After the frequency is locked, it will be close to a straight line, and observe at which time the frequency is locked successfully (for example, define that the frequency error is within ±5% of the nominal frequency, it is considered that the locking is successful), so Accurately test lock time.
Figure 6. PLL Frequency Lock Time Test Connection Diagram
Figure 7. Measured results of PLL frequency lock time
(3) RF switch switching time test
As a commonly used device in radio frequency circuits, switches are usually used for switching between multiple radio frequency links, so as to realize time-sharing work. For example, smart phones basically support multiple wireless communication systems, and the switching between various systems is realized through the switch of the radio frequency front end. This type of RF switch is a single-pole multi-throw switch. Usually, in addition to the insertion loss, isolation, standing wave ratio and other parameters of the switch, it is also necessary to pay attention to the switching time of the switch to ensure a strict timing relationship between each link.
How to test the switching time of the switch? Figure 8 shows the schematic diagram of the test connection. The oscilloscope is the core device of the whole test. In addition, a signal source is required to provide the RF excitation signal to the switch. During the test, the signal source provides a CW signal to feed the switch, and the control circuit also provides a trigger signal to the oscilloscope as a time reference while controlling the switch. In order to accurately test the switching time, it is necessary to obtain the envelope of the RF signal output by the switch. On the oscilloscope side, the switching time can be determined by comparing the delay between the external trigger signal and the envelope signal.
The oscilloscope usually uses an external envelope detector to test the envelope of the signal, but this will introduce additional delay, which will affect the test accuracy. In contrast, Spectrum View can directly Display the RF signal envelope (Magnitude_vs_Time), which is more accurate in testing and more convenient in application.
Figure 8. RF Switch Switching Time Test Connection Diagram
(4) Pulse Modulator Rise Time Test
The pulse modulator is a key component in the pulse system radar system, which can generate RF pulse signals with fast rising/falling edges and high switching ratio through external control. Pulse modulators are often implemented with single-pole single-throw RF switches, which determine the rise/fall times and on-off ratios of the RF pulses that can be generated. In practical applications, it is often desirable to generate RF pulses with the fastest possible edges, so that narrower pulses can be generated and the range resolution can be improved.
It is worth mentioning that although the RF switch can be used as a pulse modulator, its rise time is not the switching time described above. The switching time of a switch is limited by the response time of its control circuit, while the rise time is limited by the bandwidth supported by the switch.
The pulse modulator rise time test connection is shown in Figure 4. The system needs to provide a radio frequency CW signal and a baseband pulse signal for controlling the modulator. In order to accurately test the rise time, it is recommended to use an arbitrary wave generator (AWG) to generate the baseband pulse signal, because the bandwidth of the AWG is large enough, and the rise time of the generated pulse signal is much shorter than that of the pulse modulator.
The CW signal is converted into an RF pulse signal by a pulse modulator, and then fed into an oscilloscope for testing. In Spectrum View mode, directly call up “Magnitude_vs_Time”, and use the automatic measurement function of the oscilloscope to accurately measure the rise time of 10%~90% or 20%~80%.
Figure 9. Pulse Modulator Rise Time Test Connection Diagram
(5) RF module absolute delay test
In some coherent multi-channel applications, in order to ensure the time synchronization between channels, higher requirements are placed on the absolute delay of the RF modules/components on the channel, such as power amplifiers, up and down converters, analog IQ modulators, etc. , so the absolute delay of these modules needs to be calibrated.
As we all know, the vector network analyzer has the function of testing the group delay, but the group delay is not an absolute delay. Only when the phase-frequency characteristics exhibit an ideal linear relationship, the group delay is an absolute delay. Obviously, such an ideal device does not exist. In addition to the absolute delay, the actual test may also involve parameter tests such as the rise/fall time of the RF pulse signal after passing through this type of device. Therefore, the oscilloscope is an ideal choice for this type of test.
During the absolute delay test, the system feeds a radio frequency pulse signal to the DUT, and outputs a synchronous trigger signal as a time reference. After the envelope of the pulse signal is extracted in the Spectrum View mode, the automatic measurement function of the oscilloscope can be used to measure it. Determine the absolute delay. For high-bandwidth applications, the channels used are also broadband RF modules. In order to test the parameters in this occasion, it is recommended to use broadband signals. Figure 10 uses Tektronix’s arbitrary wave generator to provide high-frequency Bandwidth chirp signal.
Figure 10. Schematic diagram of RF module absolute delay test connection
The absolute delay test of the analog IQ modulator is similar to the above test method, except that the analog I signal and Q signal need to be provided to the DUT. The test connection is shown in Figure 11. In order to accurately test the time delay, the radio frequency pulse signal is still used. The simplest radio frequency pulse is a constant carrier wave in the pulse, and the corresponding baseband IQ signal only has a signal in the I channel, and the Q channel signal is 0. It is recommended to use a linear frequency modulation pulse signal during the test. Both I and Q channels have signals, which can make the I and Q branches of the modulator work separately to simulate their real working status.
Similar to the absolute delay test of RF modules such as power amplifiers, the delay test of the analog IQ modulator also requires a time reference signal, which is provided by the arbitrary wave signal generator shown in Figure 11. After Spectrum View measures the envelope of the RF pulse signal, the automatic measurement function can be used to measure the time difference between the envelope signal and the reference signal, so as to accurately calibrate the absolute delay. Figure 12 shows the delay of the analog IQ modulator. results of testing.
Figure 11. Analog IQ Modulator Absolute Delay Test Connection Diagram
Figure 12. Measured Absolute Delay Results of Analog IQ Modulators
Join the new time-frequency analysis technology of Tektronix Live Lecture at 14:30-15:30 on March 27th, and you will hear about Spectrum View features and applications