Center frequency: usually defined asBandpass filter(orBand stop filter）The geometric mean value of frequency is the midpoint between two 3dB points under the logarithmic coordinate, which is generally expressed by the arithmetic mean of two 3dB points.The frequency in the middle of the filter passband is subject to the center frequency, which is higher than the center frequency until the frequency voltage decays to 0.707 times. It is the upper side frequency, on the contrary, it is the lower side frequency. The passband is between the upper side frequency and the lower side frequency.
The frequency of RF pulse used by MR is not uniform, including a range of frequencies from low to high, which is usually expressed by its center frequency.When B0 is 1.0T, the Larmorr frequency is 42.5MHz. If the bandwidth of a certain RF pulse is 20kHz (i.e. 0.02MHz), the RF pulse actually contains the frequency in the range of 42.49-42.51MHz. Its center frequency is 42.5MHz. The wider the bandwidth, the thicker the fault thickness.When the bandwidth is unchanged, the higher the gradient magnetic field intensity in the direction selected for the layer, the thinner the layer thickness. Therefore, different layers can be obtained by adjusting the bandwidth of the RF pulse or changing the intensity of the gradient magnetic field. In clinical practice, the purpose of changing the layer thickness is mainly achieved by changing the intensity of the gradient magnetic field.[1]
Application selection of center frequency
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The selection of antenna center frequency needs to consider whether the detection depth, resolution and antenna size meet the site needs.In general, the higher the center frequency is, the shorter the electromagnetic wave length in the medium is, and the higher the detection resolution is, but the shallower the detection depth is, and vice versa.Therefore, when the resolution is satisfied and the site conditions permit, the antenna center frequency should be reduced as far as possible to maximize the detection depth.If the required spatial resolution is x (m), the surrounding environmentRelative permittivityIs εrThe following formula can be used as the basis for the initial selection of the center frequency f of the radar antenna, namely:
Generally, the thickness of the base course is 10~60 cm, and the thickness of the surface course is less than 20 cm. In the detection, it is also necessary to select a reasonable antenna center frequency according to the buried depth of the detection target interface to achieve the best effect.In addition to the working center frequency, the main factors affecting the detection depth include the dielectric absorption coefficient or conductivity, background power, transmission power and directivity, the shape and electrical characteristics of the target, etc. It can be seen that the factors affecting the detection depth are complex.Therefore, different materials of different pavement structure layers shall be tested and selected.
In order to study the technical index of the best test center frequency for GPR detection of pavement base course, we used the following formula to estimate the sections with metal plates buried at the top of the lower base course, the top of the subbase course, the top of the cushion course, the middle of the cushion course and the bottom of the cushion course in Jiaozuo Xiuwu section of Jiyuan Dongming Expressway: 400 MHZ, 900 MHZ, 1.0 GHZ,For the 2.0 GHZ antenna, a longitudinal section passing the ground projection position of five embedded plates is arranged for field test, and a transverse section across the pavement is arranged at each embedded plate. The test results of K11+400 section are shown in Figure 2-4-1, 2-4-2, 2-4-3, 2-4-4.In addition, the same field tests were carried out at K4+900 and K9+800 of Lot I.
From the measured data, we can draw the following conclusions:
(1) With the increase of the center frequency, the detection depth decreases significantly.
(2) The thickness and defects of the pavement base and subbase structure layer buried within 60.0cm can be accurately identified by using the 400 MHz and 900 MHz central working frequencies;Under ideal conditions (low humidity), the thickness and defects of the base layer, subbase layer and cushion layer within the depth of 150 cm and 100 cm can be detected respectively.
(3) The 1000 MHz and 2000 MHz center frequencies can only accurately detect the thickness and defects of the base layer buried within 40.0 cm. Under ideal conditions (low humidity), the structural layers within 80 cm and 60 cm below can be detected respectively.[2]
RF frequency modulation and center frequency correction
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Considering that FM-UWB has very low requirements for phase noise (- 80dBc/Hz@1MHZFrequency offset), in order to achieve ultra wideband spectrum, RF FM is usually realized by high gain RF VCO;Both LC-VCO and Ring VCO can be used in advanced processes such as 65nm.There are two methods for center frequency correction: digital frequency deviation pre compensation and frequency locked loop (FLL).
Figure 5-6 shows the center frequency correction circuit based on digital frequency deviation pre compensation.RF VCO first works in closed loop, when its center frequency is fcPLL negative feedback keeps the input control voltage of VCO at VCM+△m, frequency offset △ fcCorresponding analog voltage △mStored in memory (such as ROM) via ADC;Then the VCO works in the open loop state and is stored in the frequency offset voltage △ of the ROM pad.After DAC, it is directly added to the subcarrier with common mode level of KM, and the sum of the two is sent to VCO for RF frequency modulation.Thus, the RF center frequency is maintained at fcOn.
The disadvantages of this pre compensation method are obvious: ① too many modules are required, such as ADC, DAC, ROM and PLL, which is not conducive to CMOS integration, and the design is complex and power consumption is high; ②It is necessary to switch between two operating modes, which inevitably introduces switching noise; ③When VCO performs frequency modulation, its center frequency cannot be corrected in real time.
When the RF VCO performs frequency modulation, although its instantaneous frequency changes rapidly, its average value or center frequency changes slowly. A frequency negative feedback loop such as FLL can be used to correct it in real time.Figure 5-7 shows the center frequency real-time correction circuit based on FLL.It uses dual channel RF VCO and FLL with very narrow loop bandwidth to realize fast frequency modulation and slow frequency correction.
The frequency discriminator uses the clock counting method to detect the center frequency deviation, and controls the subsequent self addition and subtraction counter and △ - ∑ DAC to obtain the correction voltage VCZLThe latter adjusts the VCO correction path to reverse correct the center frequency deviation.FLL loop bandwidth varies from △ - ∑ DAC toRC filterThe cut-off frequency of shall be as small as possible, but it shall be able to keep up with the drift rate of the center frequency with the power supply voltage and temperature.In order to eliminate the kickback noise fed by the LC VCO variable capacitor tube to the △ - ∑ DAC, an isolation drive (Buffer) is introduced into the circuit.In order to suppress the common mode noise of power supply and ground, differential DAC is used.In order to reduce the power consumption of the correction loop, the high current module such as the high frequency current mode (CML) divider is controlled by the duty cycle clock and operates in the sub continuous state.When the high frequency divider does not work, the output of the counter should remain unchanged. At this time, the center frequency of the VCO will be locked until the next duty cycle cycle comes, so that the correction can be continued. In this way, the total power consumption of the FLL loop can be reduced on the premise of ensuring real-time correction of the core frequency,
In the traditional LC VCO or Ring VCO, add a MOS variable capacitor tube, or add a V-to-I conversion circuit to achieve a dual path VCO.Considering that the center frequency of the RF VCO has a deviation of ± 20% under PVT, and at the same time, the ultra wideband spectrum ≥ 500 MHz should be obtained, which requires the modulation gain and correction gain of the VCO to meet the 100 MHz/V level.Therefore, for LC VCO, the modules such as DC isolation capacitor and voltage bias in its traditional structure must be removed;The digital segmented tuning module is also unavailable.Figure 5-8 shows the dual channel LC VCO for FM-UWB transmitters.FM-UWB uses VCO to convert the amplitude information of triangular wave subcarrier into RF frequency information. The amplitude frequency conversion process is best linear, while the gain of LC VCO is nonlinear, because the voltage capacitance curve of single ended cumulative MOS varactor is nonlinear.In order to improve the linearity of frequency modulation, a differential tuned variable capacitor structure is used. The traditional linearity optimization scheme based on distributed bias is not suitable for such VCOs.[3]
Center frequency fault handling
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(1) The center frequency cannot be measured.At this time, first check whether the modulator oscillator tube starts to vibrate by using a multimeter to check whether the - 19V power supply is introduced and whether the DC working state of the oscillator tube and emitter output device is normal.Then measure whether there is AC voltage at the output terminals of the collector and emitter of the oscillator tube with an ultra-high frequency millivoltmeter.If there is no voltage at any level, it is more likely that the level will fail. At this time, the triode or suspicious components at this level can be welded down and checked with a multimeter to determine whether it is good or bad.
(2) The center frequency is high or low.If adjust WfourIf the center frequency is still high or low, check from the following aspects:
① Whether the power supply voltage is normal, and how about the performance of the voltage regulator diode 2CW3: first change the power supply voltage within ± 1V, and see how much the frequency reading on the frequency counter changes. If the voltage regulator tube has good performance, the frequency should not change more than dozens of KHz. If the change is large, it indicates that the voltage regulator tube has a problem.
If you think the performance of 2CW3 is not good, you can open the cover plate of the modulator, and use a multimeter to measure the voltage at both ends of the voltage stabilizing tube with a DC voltage of 10V. Normally, the voltage should be 9~10.5V. If the measured voltage is lower or higher than this range, you need to replace the voltage stabilizing tube.
② The components of oscillation channel are damaged.In the modulator, the easily deteriorated component is the polyester vinyl capacitor (the oscillating circuit capacitor and the coupling capacitor of the oscillating tube) that fails (most of them are open circuit). At this time, the capacitor should be replaced with a new one.
③ Check whether the contact of oscillating channel elements is good.
In addition, since there is no automatic frequency trimming and thermostatic device in the modulator, the center frequency of the modulator will be affected by the change of ambient temperature, and the frequency will decrease when the temperature rises. The center frequency will change about 100KHz between 15 and 35 ℃.
In addition to the influence of ambient temperature, the center frequency may vary by several hundred KHz with the change of slot load and the influence of various distributed capacitors in the circuit.[4]
