Application of Low Frequency Band Tracking Signal Generator System in Spectrum Analyzer

For a long time, spectrum analyzer and tracking signal generator have measured the swept scalar frequency response of active and passive networks. Although many spectrum analyzers have the option of tracking signal generator, most of these signal generators can only provide sweep signals for the basic frequency band of the analyzer. The following method will also cover the first extended high frequency band of many spectrum analyzers. In addition, a method to allow scanning of the measured component with intermediate frequency conversion is also given.

Fig. 1 shows a common RF spectrum analysis structure with tracking source function. In this structure, the tracking signal generator is realized by a mixer, an amplifier and an RF signal generator set at the first fixed intermediate frequency (if). For this scheme, frequency scanning can be realized only when the analyzer scans the first local oscillator (LO). In addition, the analyzer must also provide output sampling of the first LO. If the resolution bandwidth (RBW) filter (whether analog or digital) of the analyzer is fixed and only the first LO is scanned, this scheme can output the correct signal.

In this configuration, the signal generator is set to the first if of the desired frequency band (which can be obtained from the analyzer manufacturer's data or experiments). The first if can be found by using the widest resolution bandwidth (RBW) filter and adjusting the signal generator until there is a rise in the noise base on the analyzer. By switching to a narrower RBW and adjusting the signal generator to the maximum amplitude, the if value can be estimated more accurately.

When the first LO signal of the analyzer is mixed with the first IF signal of the signal generator, the output of the mixer will contain a spectrum component, which is just the RF signal tuned by the analyzer at that point in the scanning (scanning signal generator signal). Then put the DUT between the mixer output and the RF input of the analyzer. The signal of the tracking signal generator will be scanned automatically during the scanning process, providing a fully synchronous signal for the swept scalar measurement.

Although it is a good idea to connect the lo output of the analyzer directly to the mixer, this will cause problems for most analyzers. Generally, there is not enough reverse isolation at the first LO output. As a result, the first IF signal of the signal generator will leak to the first if stage of the analyzer, so as to enter the first and subsequent if stages of the instrument, so as to improve the noise base.

A circulator can increase isolation (about 20dB), but reduce the first LO power sent to the mixer. A better way is to use a high isolation amplifier, which can provide up to 50dB isolation in the frequency band up to 6GHz. In addition, it can allow broadband operation, and can achieve a good lo driving level when only the first LO with low power can be obtained. The input power of this high isolation amplifier is 0dbm.

This improved method and configuration are shown in Figure 1, which is the basis of two tracking signal generator systems developed by DKD company. In one system (model tg100), the mixer and high isolation amplifier are integrated together (Fig. 2). In another system (model tg200), the amplifier is a separate module, so that different mixers can be used for specific analyzers in low and high frequency bands (Fig. 3). The former operates in the frequency band of 500KHz to 2.6GHz, and its first high frequency extension range is 2.0 to 4.5ghz. The latter uses one mixer for the low frequency band (to 2.6 GHz) and another mixer for the first high frequency extension band (to 6 GHz). The optimal input power of both systems is 0dbm. For many analyzers, adding a first LO frequency multiplier and an appropriate mixer can work to the second high frequency expansion band. For many instruments, the frequency band is above 10GHz.

Figure 4 shows the second tracking signal generator system, in which hp8566a / b spectrum analyzer from Agilent is used. The signal generator is configured to cover the first frequency band (DC to 2.5GHz) of the spectrum analyzer. The first if of the instrument in the low frequency band is 3621.4mhz, and the power of the signal generator at this frequency is about 6dbm. In this configuration, the signal generator inputs the radio frequency (R) port of the mixer, the sampling of the first LO is input to the local oscillator (L) port, and the intermediate frequency (I) port contains the difference signal generated by mixing, that is, the output signal of the tracking signal generator. The mixed sum product is also added to the input of the DUT, but its frequency is 2 × (3621.4mhz) = 7242.8mhz or higher. The 3dB attenuator on the I port improves the VSWR when looking back from the DUT to the tracking signal generator. The 6dB attenuator reduces the LO signal power from hp8566 to 0dbm more suitable for tracking signal generator.

Using this test configuration for measurement, the DUT is a straight through transmission line (Fig. 5) with a frequency range from DC to 2.5GHz. Manually set the resolution bandwidth filter to 3kHz, and manually set the video bandwidth and scanning time. At this time, the uncalibrated results show that there is about 7dB attenuation (roll off) from low end to high end. When the analyzer input and tracking signal generator output are 50? When the terminal replaces the tested part of the straight line, the noise base can be seen over the dynamic range of more than 80dB (Fig. 6). Subtract (graphically) the attenuation corresponding to the straight line in Figure 1 from the analyzer results. The result of video subtraction is a flat trace when connecting the through line. Because the correction is through video storage [vidmem_ A - (vidmem_bdl)], due to roll down, the noise base at the high end will rise. Therefore, using 3kHz RBW, 3621.4mhz signal power of 8dbm, 10dB internal attenuation and i-port 3dB attenuator, the dynamic range of about 80dB can be realized.

The choice of resolution bandwidth (RBW) of spectrum analyzer is mainly determined by the quality of signal generator. For unstable signal sources, such as non phase locked cavity signal generator, the narrowest RBW filter available is about 100kHz. The relatively stable signal source allows the use of narrowband RBW filter. Of course, with the widening of RBW filter band, the base noise of the system will also increase and the dynamic range will decrease.

The low frequency band tracking signal generator system can be used to measure a band-pass filter with a center frequency of 1445mhz (Fig. 7). The start frequency is 1345mhz and the cut-off frequency is 1545mhz. The ordinate scale is 10dB / div, the attenuation of the analyzer is set to 10dB, 1s frequency sweep period, 10kHz RBW and 3khzvbw. Before scanning, the reference line is set by using image subtraction to remove the change in the output line of the tracking oscillator.

As shown in Fig. 8, the tg200 is configured to provide a tracking generator scanning signal for the first extended high frequency band of the hp8566 spectrum analyzer. For this high band measurement, port 1 of the mixer is connected to the signal generator. The frequency corresponding to the first if end of the full band (2 to 22 GHz) analyzer is 321.4 MHz (the signal generator is tuned at this frequency). At this time, the R port of the mixer contains an effective tracking generator signal and a harmful image. The YIG tracking filter at the input of the analyzer can effectively eliminate this harmful mirror signal.

Figure 9 shows the tg100 system for low frequency measurement of hp8568a / b spectrum analyzer (DC to 1.5GHz). For the fixed second LO, the rated working frequency of the first if end of the analyzer is 2050.300mhz (the signal generator is tuned at this frequency), but the second LO has a certain amount of scanning range, which is less than 2MHz. The scanning function of the second LO port signal can be disabled by using the "shiftt" instruction sequence without affecting the performance of the tracking signal generator. At this time, make the analyzer work in the mode that only the frequency sweep of the first LO port and the signal of the first if port are fixed. When 50 loads are terminated, the results show that the dynamic range of the test device is about 90dB.

When scanning and analyzing frequency conversion devices (such as mixers), most existing tracking generation oscillators and network analyzers do not allow bias to the input signal. However, such a measurement can be made using the tg100 / 200 system, provided that the frequency offset introduced by the DUT does not exceed the limit of the first if of the spectrum analyzer. By inputting the offset compensation signal on the signal generator connected to the tg100 / 200 system, the frequency conversion tested component can be swept and analyzed.

For example, a down converter is scanned and analyzed using tg100 module and hp8566 (FIG. 10). Generally, in order to apply tg100 and hp8566 in the first frequency band, the signal generator frequency is set to 3621.4mhz. However, since the DUT in this example uses a mixer with LO frequency of 232.6mhz, the signal must be offset compensated, otherwise it will be wrong to track the output signal of the oscillator. The solution is to set the offset compensation frequency of the signal generator to 232.6mhz, so that the output frequency of the signal generator is 3388.8mhz. In this way, the tracking generator outputs the correct frequency that can compensate for the internal frequency offset of the DUT.

Application of Low Frequency Band Tracking Signal Generator System in Spectrum Analyzer 1

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