Clever Geek Handbook

Transmitter for FMCW GPR

Hello everyone! I would like to share my history of developing a transmitter for a chirp continuous radiation radar with a signal bandwidth of 1000 MHz and a nonlinearity of the frequency change of 10 -4 %. Such a transmitter will later be used in FMCW georadar (Frequency-Modulated Continuous Wave).





The introduction provides brief information about FMCW GPR and formulates (substantiates) the requirements for the transmitter, the subsequent sections describe the stages of development, modeling and experimental verification of the transmitter. The georadar itself has also been developed and is undergoing trial operation. If there is a keen interest in the article, similar articles on the development of antennas and GPR in general will be posted.





Introduction

Subsurface sensing radars (georadars) on the market are mostly impulse radars. Recently, however, there have been a number of reports [1, 2, 3] on the development of georadars using continuous signal radiation. At the same time, both theoretically and practically, the advantages of continuous radiation radars are shown [1, 4]:





  1. the dynamic range of a georadar with continuous radiation is more than 20 dB higher than the dynamic range of impulse analogs (all other things being equal). In practice, this can mean an increase in the detection depth by 3 times for point targets and 4-5 times for linear extended targets;





  2. In GPR with continuous radiation, it is possible to use various types of antennas (not only dipoles or a bow tie), including shielded antennas with a high gain, with circular polarization (for example, the Archimedes spiral). The use of a shielded antenna of the Archimedes spiral type allows the radiation to be concentrated strictly downward (towards the ground), the absence of side and rear lobes reduces the GPR's susceptibility to the presence of trees, metal fences and other "interference" objects. It is worth noting here that in conjunction with some antennas it is necessary to apply deconvolution to increase the depth resolution (due to antenna ringing), sacrificing the signal-to-noise ratio.





  3. ( ), (, ). . , . FMCW , .





() .





FMCW FMCW (. 1). , . , . , ( ). , .   .





Fig. 1 - Functional diagram of FMCW chirp radar
. 1 – FMCW

FMCW ( , ). β€” FMCW , . 





, FMCW .





, , . , , . , FMCW RIMFAX Perseverance (150-1200) 10 [3]. Orfeus [2] (100-1000) ( ) 0,521,042.





RIMFAX, ORFEUS 1000 . 5 ( 9).





, , (100-1100).





FMCW , . 2 , 0% 0,5 %. . , 0,1 % [1].





Fig. 2 - Beat frequencies with transmitter frequency nonlinearity of 0% and 0.5%
. 2 – 0% 0,5%

, :





  1. – ;





  2. – (100-1100)





  3. – 0,1 %;





  4. 100 ( );





  5. – 10;





  6. – ;





  7. β€” 400 β€” 500;





  8. , , , , , ;





  9. β€” .





( 3) () . , , , , (), . F F , F>F>1100, (F-F)=1000. . ,  6,67 (150 ), (10-4-10-1).





, , , , , . , , ( ) :





(1-1001);





(10-1010);





(100-1100)  β€” ;





(1000-2000);





(2000-3000).





4   , . , 121 ( – 51 ), 0,0001% (0,1%).





Fig. 3 - Functional diagram of the developed transmitter
. 3 –
Fig. 4 - Changing the frequency of the output signal of the transmitter in time
. 4 –
Fig. 5 - Phase noise of the transmitter at a carrier frequency of 1100 MHz at an offset (1-10000) kHz
. 5 – 1100 (1-10000)

5 , 1100 , (1-10000).





, , , , RG405 1 ( 6).





Fig. 6 - Diagram of the experiment for evaluating the quality of the signal generated by the transmitter
. 6 – ,

- () - 100 48 . , – . GnuRadio.





320 (48*6,67), , , .





Fig. 7 - Signal spectrum corresponding to the delay in a cable length of 1 m
. 7 – , 1

7 , . , . 





7 , , , 100 . , 100 . , , ( FMCW β€” ), . - , , .





, . .





10010025 3, – 0,2, 30 . ., . , , FMCW , .





1,5 . . . , .





  1. D.J. Daniels, Ground penetrating radar, 2nd edition. The Institution of Electrical Engineers. London. 2004. 752 p.





  2. F. Parrini et. al., Β«ORFEUS GPR: a very large bandwidth and high dynamic range CWSF radarΒ»//Proceedings of the 13 International Conference on Ground Penetrating Radar, Lecce. Italy. 2010. pp. 1-5.





  3. . Hamran et al, Β«RIMFAX: A GPR for the Mars 2020 rover missionΒ» // 2015 8 th International Workshop on Advanced Ground Penetrating Radar (IWAGPR). Florence. Italy. 2015. pp. 1-4.





  4. M. Pieraccini, "Noise performance Comparison Between Continuous Wave and Stroboscopic Pulse Ground Penetrating Radar" // IEEE Geoscience and Remote Sensing Letters. vol. 15, no. 2. Feb. 2018. pp. 222-226.







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