диафрагмированные волноводные фильтры / d0b6eab4-ee66-48f9-8028-3068c251b6eb

The isolation must be as great as possible between port 1 and 3 to reduce the leaking noise from transmitter to receiver. In Figure (4.20) isolation between ports 1 and 3 is given. Insertion losses from port 1 to 2 and from port 2 to 3 are less than 1 dB. Isolation has maximum with a value of 57 dB at 885 MHz from port 1 to 3.

4.5.5Switch

Switches are operated such to determine the transmitting and receiving modes of the radar. There are two switches operating reversely. Thus the noise from VCO cancelled when the radar is on receiving (listening) mode. The isolation of the switches when they are off is the important parameter to reduce or cancel the leakage noise from VCO. Mini-Circuits switch KSWHA-1-20 chosen, the photograph of the evaluation board of the switch given in Figure (4.21).

Figure 4.21 Photograph of KSWHA-1-20

Insertion loss of the switch is less than 1 dB and the isolations towards input to output and output to input are approximately 50 dB.

4.5.6Antenna

Antenna is one of the important parts of the radar system. Antenna determines where the generated power will be transmitted in free space. The antenna used has a very large

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beamwidth on both and axes. The normalized field pattern of the antenna is given in Figure (4.4). Photograph of the mounted antenna is given in Figure (4.22).

Figure 4.22 Photograph of the antenna

Figure 4.23 Antenna Return Loss

Mismatch of the antenna will result to loss of power thus the antenna must have a return loss (S11) as large as possible.

Antenna is aligned for a center frequency of 9.06 MHz. After mounting a radome to the antenna the center frequency will be shifted to a lower level that is the operating frequency. E and H normalized directivity patterns are given in Appendix D and E respectively.

4.6Radar Test Board

In the laboratory environment the system tests for the radar must be conducted. The range of the radar is greater than 1000 meters. To perform the tests and simulating the target suitable delay lines may be used. Using a coaxial delay line length of 1000 meters will result a very high attenuation on the transmitted signal. The transmitted

signal power will decay to the noise signal levels at the

end of the transmission line therefore receiving of the transmitted signal will be unavailable. Another choice is the using fiber optics for the transmission line. But there must be additional encoder and decoder circuitry for converting carrier signal to the visible light. In either coaxial line or fiber optic the length of the transmission line is 1000 meters and for different distance of the target different lengths of the transmission lines must be provided.

Another cheap and easy method of target simulation and providing delay to the transmitted signal is to use SAW filters discussed in Chapter 3. The SAW filter actually a delay line. The SAW velocity ( s) of LiNbO3 is approximately 3500 m/s thus for a delay of 2.85 microseconds only a 1 cm length of LiNbO3 substrate is sufficient. Also the practical

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bandwidth for LiNbO3 is very large so for higher frequency deviations ( F) of the radar SAW delay line can be used as well. For the center frequency of 850 MHz a SAW delay line fabricated on LiNbO3 substrate can be used for frequency deviation up to 200 MHz. But for SAW delay lines fabricated on GaAs substrate the maximum frequency deviation allowed is about 25 MHz at the same center frequency.

SAW delay line SF0850DL01825T from Micro Networks chosen for a fixed delay of 2.5 microseconds. Manufacturer’s specification for SAW delay line is given in Table (4.6).

Table 4.6 SAW Delay line specifications

Insertion Loss of the delay line is typical 27 dB. Due to high insertion loss SAW delay line can be used without any additional attenuator. Photograph of the radar test board given in Figure (4.24).

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Figure 4.24 Photographs of radar test board

Test board’s S-parameters and associated delay was measured. The frequency response (S21) is given in Figure (4.25) and the phase response given in Figure (4.26).

Figure 4.25 Test board frequency response

Insertion loss of the test board varies between -26.5 to -27.5 dB. Center frequency is 850 MHz and the 3 dB bandwidth is equal to 250 MHz. Delay of the test board calculated from the phase of the system.

If we add a delay to the reference signal as long as the delay of the test board phase of the system will be calculated as zero. To determine this delay marker delay method of the network analyzer HP 8720D was used. Network analyzer was automatically calculated the electrical length and the associated delay of the network. Measured electrical

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delay and associated electrical length and simulated target

distance given in Table (4.7).

Figure 4.26 Phase of the test board

Table 4.7 Delay of the test board

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Two delay lines cascaded will result a delay of approximately 5 sec. The insertion loss of the two delay line system is given in Figure (4.27).

S21 (dB)

Frequency (Hz)

Figure 4.27 Insertion loss of cascaded two delay lines

Total delay associated the two delay lines was measured by marker delay method of the network analyzer as 5.0035 sec. This time delay corresponds to a distance of 749.5 meters.

4.7Alternative Test Method

Test board with SAW delay line focused on adding a suitable time delay to the reference signal. Radar determines the distance to target by measuring the beat frequency corresponding to time delay. Time delay results a shift between the reference signal and transmitted signal this frequency difference is the beat frequency. Instead of delaying the transmitted signal a beat frequency injection to the transmitted signal at the transmission time (t=0)

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will also result a beat frequency in the IF side of the radar receiver.

In the alternative method we add an artificial time delay by adding a beat frequency to the carrier signal. Schematic view of the alternative test board is given in Figure (4.27).

Figure 4.28 Alternative test board

If the transmitted signal is Tx, delayed signal is Tx’, shifted signals at the output of the mixer1 and mixer2 are

Txs1 and Txs2 and the signal send back to the radar is Rx and k is being the mixer constant then;

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Thus final signal Rx which is send back to the radar with a suitable attenuation (L) is equal to Equation (4.55).

In the radar Rx is the received signal which will proceed through the RF port of the mixer shown in Figure (4.9). The signal at the LO port of the mixer (VLO’) is

IF signal at the receiver side of the radar is filtered by the beat frequency filters. Beat frequency filter is a

series of low pass (fcutoff=10 KHz) and high pass filter (fcutoff=150 KHz). We adjust LO- VCO to be the desired beat angular frequency for example; fLO=10 MHz and fVCO=9.9 MHz for a beat frequency for a beat frequency of 100 KHz. After the filtering the IF signal VIF the resultant signal at the input port of the digital signal processor VDSP is:

then VDSP can be expressed by Equation (4.64).

Delay line used in the alternative test method has a delay (Td) corresponding to an electrical length of minimum 30 meters. This delay must be greater than 100 nanoseconds which is the pulse duration of the transmitted signal. When the pulse radar is in transmission mode which lasts for 100 nanoseconds the receiver switch is on open position. Thus

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