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Hello, I am French so sorry for my bad English.
How to measure the input impedance of an emitter lawmate 2.4Ghz 500mw?

Thanks

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Your question needs clarification. Do you want to [1] measure the video input's impedance (connection to camera) or [2] do you want to measure the RF output's drive impedance (connection to antenna)?

[1] Video Input Impedance: The vTx should be internally DC terminated with 75 ohms. This low input impedance allows practical measurements with a common ohmmeter.

[2] RF Output (drive) Impedance: Not a hobby friendly measurement, requires expensive test equipment. See "Load-Pull" discussion in this document:
http://www.nxp.com/assets/documents/data/en/application-notes/AN1526.pdf

Regarding RF Output impedance, it is more common to measure RF power at 50 ohms impedance. This measurement requires a good RF power meter. For most of us, this is better than knowing the precise RF drive impedance since the power meter will report the actual RF power the transmitter will deliver into a properly designed antenna.

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"This low input impedance allows practical measurements with a common ohmmeter."

How should I place my ohmetre?

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Disconnect camera and battery power. One meter lead on vTx's ground, the other lead on the vTx's Video Input. A properly designed vTx will be about 75 ohms, which is an industry standard.

Since your are interested in the video input impedance of a Lawmate vTx, I recommend reading this old discussion (posted in 2007):
https://www.rc-cam.com/forum/index.php?/topic/2344-lawmate-tx-input-impedance/

 

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Unfortunately that will adversely reduce the amplitude of the video signal. If the vTx has a video level POT, and you have the proper test equipment, then you can re-calibrate the video level to the industry standard 1Vpp amplitude.

Traditionally you would need a o-scope to check and calibrate the video level. But now there is an alternative -- This cool DiY Project was specifically created to check FPV video levels:
https://www.rc-cam.com/forum/index.php?/topic/4126-diy-fpv-video-calibration-tool-low-cost/

 

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So we put the resistance and then we set the level, right?

Edited by swake

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