For a long time, the spectrum analyzer with the tracking signal generator, the frequency response measurement of the active and passive networks for the active and passive network. Although many spectrum analyzers have a tracking signal generator option, most of this signal generator can only provide a sweep signal for the basic frequency band of the analyzer. With the following methods, many spectrum analyzers will also cover high frequency bands. In addition, a method of scanning a measured component with an intermediate frequency transform is given.
Figure 1 shows a common RF spectrum analysis structure with a tracking source function. In this configuration, the tracking signal generator is implemented by a mixer, an amplifier, and an RF signal generator set in a fixed intermediate frequency (IF). . For this solution, frequency scan can be achieved only when the analyzer scans the vibration (LO). In addition, the analyzer must also provide the output sampling of the vibration. If the analyzer's resolved bandwidth RBW) filter (whether analog or digital) is fixed, only the LO is scanned, and this solution can output the correct signal.
In this configuration, the signal generator is set to the desired frequency band (which can be obtained from the analyzer manufacturer's data or experiment). IF can be found in the following method, even if the signal generator rises until the noise base is raised by a wide resolved bandwidth (RBW) filter. The IF value can be more estimated by switching to a narrower RBW and adjusting the signal generator to the amplitude.
When the analyzer's LO signal and the signal generator IF signal mix, the output of the mixer will contain a spectral component, which is just the point tunable RF signal (scanning signal generator signal) in the scan in the scan. ). The DUT is then placed between the mixer output and the analyzer's RF input. The tracking signal generator signal automatically scans during the scanning process, providing a fully synchronized signal for the sweep scalar measurement.
Although it is good to connect the analyzer's LO output to the mixer, this will bring problems to most analyzers. Typically, there is no sufficient reverse isolation in the LO output port. As a result, the IF signal of the signal generator will leak to the IF level of the analyzer, thereby enters the instrument and subsequent IF levels, so that the noise substrate is improved.
An annular device can increase isolation (approximately 20 dB), but will reduce the LO power sent to the mixer. A better way is to adopt a high isolation amplifier that provides up to 50 dB isolated to the 6 GHz band. In addition, it can allow broadband operations, and it can achieve a good LO drive when only low power LO can be obtained. The input power of this high isolate amplifier is designed to be 0 dBm.
This improvement method and configuration are shown in Figure 1. It is the basis for the two tracking signal generator systems developed by DKD. In one of the systems (model TG100), the mixer and high-isolation amplifier are integrated (Fig. 2). In another system (Model TG200), the amplifier is a separate module, so that different mixers can be used for a particular analyzer of the low band and high frequency bands (Fig. 3). The former working band is 500kHz to 2.6GHz, and its high frequency expansion range is 2.0 to 4.5 GHz. The latter uses a mixer for a low frequency band (to 2.6GHz), and another mixer is used for high frequency spread segments (to 6 GHz). The input power of the two systems is 0dBm. For many analyzers, an LO multiplier and appropriate mixer can be operated to work to the second high-frequency spread segment, and for many instruments, this band is 10GHz.
A second tracking signal generator system is given, in which the HP8566A / B spectrum analyzer from Agilent is used. This signal generator configuration can cover the frequency band of the spectrum analyzer (DC to 2.5 GHz). The IF of the instrument at the low frequency section is 3621.4 MHz, and the power of the signal generator is approximately + 6dBm in this frequency. In this configuration, the signal generator inputs the radio frequency (R) port of the mixer, the sample input to the local vibration (L) port, the intermediate frequency (i) port contains a difference signal generated by the mixed frequency, ie tracking Signal generator output signal. Mixed and products are also added to the input of the DUT, but their frequency is 2 × (3621.4 MHz) = 7242.8 MHz or higher. The 3DB attenuator of the i-port improves the voltage stationer ratio that looks back from the DUT to the tracking signal generator. The 6dB attenuator reduces the LO signal power from HP8566 to a more suitable for tracking signal generators.
With this test configuration, the measurement is performed, and the measured component (DUT) is a straight-through transmission line (Fig. 5), the frequency range is from DC to 2.5 GHz. Manually set the resolved bandwidth filter to 3kHz and manually set the video bandwidth and scan time. At this time, the unclear result is displayed, from the low end to the high end with an attenuation (roll drop). Use 50 in the input and tracking signal generator output? When the terminal replaces the straight line, the noise substrate can be seen on a dynamic range of more than 80 dB (Fig. 6). Subtract (from the graphic) in the analyzer results (from the graphic) Figure 1. The result of the video subtraction is that a flat trail occurs when connecting straight lines. Because the correction is completed by video storage [VIDMEM_A- (Vidmem_BDL)], due to the rise, the high-end noise base will rise, so the 3621.4 MHz frequency signal power is + 8dBm, 10 dB internal attenuation, and I ports. The 3DB attenuator can achieve approximately 80 dB dynamic range.
The selection of the spectrum analyzer resolved bandwidth (RBW) is mainly determined by the quality of the signal generator. For unstable signal sources, such as non-locked cavity signal generators, the available narrow RBW filters are about 100 kHz. The relatively stable source allows the use of narrowband RBW filters. Of course, as the RBW filter frequency band is widened, the base noise of the system will increase, while the dynamic range will fall.
The low frequency band tracking signal generator system can be used to measure the band pass filter 1445 MHz (Fig. 7). The start frequency is 1345MHz, and the cutoff frequency is 1545MHz. The longitudinal lacquered scale is 10 dB / div, the analysis of the analyzer is set to 10 dB, 1S sweep cycle, 10 kHz RBW and 3kHzvbw. Before the start of the scan, the reference line is set by using the image subtraction to remove changes in the tracking of the oscillator output line.
As shown, the TG200 is set to extend the high frequency band for the HP8566 spectrometer to provide a tracking generator scan signal. For this high frequency band measurement, the port 1 of the mixer is connected to the signal generator. The frequency of the IF end corresponding to the full-band (2 to 22 GHz) analyzer is 321.4 MHz (the signal generator is tuned here). At this time, the R ports of the mixer include a valid tracking generator signal and a harmful image. The YIG trace filter of the analyzer input can effectively eliminate this harmful mirror signal.
The TG100 system is shown for the HP8568A / B spectrometer (DC to 1.5 GHz) low frequency band measurement. For a fixed second LO, the IF terminal of the analyzer is 2050.300 MHz (the signal generator tuning at this frequency), but the second LO has a certain amount of scanning range, this range is less than 2 MHz. The scanning function of the second LO port signal can be disabled by using the "Shiftt" instruction sequence, and the performance of the tracking signal generator can not be affected. At this time, the analyzer works in: only the LO port sweep and the fixed mode of the IF port signal are fixed. When the 50 load is terminated, the result shows that the dynamic range of the test device is approximately 90 dB.
When scanning frequency transform devices (such as mixers), most existing tracking generators and network analyzers do not allow input signals to be biased. However, such measurements can be performed using TG100 / 200 systems, the condition is the frequency bias introduced by the measurement component (DUT) does not exceed the limitations of the spectrum analyzer IF. By inputting the bias compensation signal on the signal generator connected to the TG100 / 200 system, the frequency transform type is measured by sweep analysis.
For example, the TG100 module and HP8566 are used to scan analysis (Figure 10). Typically, in order to apply TG100 and HP8566 in the frequency band, the signal generator frequency is set to 3621.4 MHz. However, since the DUT in this example uses a mixer having a LO frequency of 232.6MHz, the signal must be biased to compensate, otherwise the output signal that tracks the oscillator will be erroneous. The solution is to set the bias compensation frequency of the signal generator to 232.6MHz, so that the output frequency of the signal generator is 3388.8 MHz. Thus, the tracking of the oscillator will compensate for the correct frequency of the DUT internal frequency offset.
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