For comparisons I use the lowest level where I can read all the OSD level as the defined sensitivity for the receiver. For this receiver I marked it as -95 dBm. Notice how the 1Khz reference audio signal drops out about the same time as the video.
By the way, it is not an easy thing to make accurate measurements at these low signal levels. I wanted to be sure that I could do this with an absolute accuracy of less than 1 dB. Here's a quick summary of the process:
1) A 60mw Video TX was used as the source. It was mounted inside an RF tight box with high quality chassis mounted bypass filters for the video, sound and power inputs. Copper Tape was used to reduce ground/RF leakage between the 60mw TX and the inside of the box. In-line attenuators were used inside the box to bring the signal down to -23 dBm at the SMA output of the box.
2) The -23 dBm level was verified with a well calibrated HP8594 Sectrum Analyzer. This was measured without modulation.
3) A PIC controller was used with a programmable step attentuator to provide the final -80 dBm to -100 dBm signal at the receiver input. Double shield cables were used for all RF connections. The PIC display was set to show the calibrated signal level at the receiver and setup so that the FPV camera could see it as the signal was reduced.
Notice that the video sync is maintained all the way down to -97 dBm where the signal where the image is just barely visible. To achieve this you must have the TX and RX video modulation and demodulations set to the correct levels. For that you need a good scope which can accurately show you the video levels.
One big discovery is that unlike the Lawmate receivers, all the Chinese transmitters I tested had a high input resistance rather than the standard 75 ohms for the video input. This really destroys weak signal reception because the TX will often be overdriven by the video camera expecting to see a 75 ohm load. That will cause the sync to drop out at higher signal levels well before major video degradation. For those of you that may have experience this, try adding a 75 ohm resistor in parallel with the video input of the transmitter.
Edited by Old Man Mike, 14 April 2010 - 10:17 AM.