Vtx and Vrx
VTX & VRX? Sounds like a model of car, what are they ???
A Vtx is a video transmitter, the camera output and if present the audio connects to it. It transmits on the selected channel over the airwaves to the receiver.
A Vrx is a video receiver, the receiver is set to the same channel as the transmitter and receives the video and audio transmitted and provides them as separate outputs.
There are a wide variety of each available with lots of different features and frequency band. Lets take a look at a typical example.
A 2.4 Ghz Vtx
This is a typical 2.4 Ghz Video Transmitter. It determines how far and what channel the video and audio are transmitted. Transmitters can get very hot when cool air is not flowing across them. You should not have your Vtx powered up in a room for more than a few minutes or you risk the possibility of damaging the unit.
Terminology of a VTX
When you purchase a Vtx you have to decide which type you want, there’s a bit of jargon in the specs this is what they mean :
Switching regulator :
A switching regulator regulates voltage by turning the supply on and off thousands of times a second – so that the actual flow is reduced. (No power is wasted – but they can emit RF noise unless it is be carefully suppressed).
A linear regulator regulates voltage by converting the excess voltage into heat (excess power is wasted – but there is no possibility of RF noise).
5V camera power :
This means it is equipped with a output at 5v from the transmitter. Its for powering a 5 volt camera.
The Vtx normally is supplied with a 12 volt supply. Some FPV cameras are 12 volt so you can use the same wire. 5 volt operation FPV cameras however need a 5 volt supply, either off a Bec Ubec or the Rc receiver.
This transmitter however has a 5 volt supply out so the camera can be powered straight off that instead of installing extra circuits and complexity.
High bandwidth stereo-audio channels :
The higher the bandwidth transmitter the more information can be cleanly sent to the receiver. This is especially important for antenna trackers as the audio is used as the telemetry data stream.
CH1 : 2414
CH2 : 2432
CH3 : 2450
This is the channels the VTX can transmit on the 2.4 Ghz band.The channels are 2.414 Ghz, 2.432 Ghz and 2.450 Ghz.
As stated this is a 3 channel transmitter, more channels are available on other units, some have 4 others up to 8 channels are selectable to the user.
Its important to make sure your channels available on the Vtx match up to the channels on your receiver before purchase.
+27dBm typical output power :
The rated output in real world terms. Dbm can be used to calculate range capabilities of each component. From the figure +27dbm we can determine this is a 500mw transmitter.
Mw is another term used to state the power of the transmitter. To be legal on the 2.4 Ghz band in the Uk the maximum permitted power of the transmitter is 10mw.
50 ohm antenna impedance :
This means a wide variety of 50 ohm antennas can be connected to this unit. Nearly all Vtx units are sold with 3dbi Omni antennas but others can be used such as Dipole or Inverted Vee`s.
1Vpp Video Input level :
1 volt peak to peak video level. Really nothing for the user to get involved in much.
3V pp Audio Input Level :
3 volt peak to peak audio level. Again, no relevance to FPV
Input Voltage Range: 6 – 16V :
This is the operating range of the unit. Most if not all work the best on 12 volts of supply.
Vtx`s also have switching regulator so with the varied amount of voltage you can supply it with it will regulate the available power to keep its power output constant throughout
A 2.4 Ghz Vrx
This is a typical 2.4 Ghz receiver unit. Combined with a specific antenna it is the main factor of how far you can fly with your FPV setup.
The most common mistake the new user assumes is that the power of the Vtx is the deciding factor of overall performance.
It does in fact have some influence on over all range but doesn’t have any where near as much influence as the receiver antenna choice does.
As with the Vtx the Vrx unit has predefined channels you can choose to receive on.
Min Sensitivity: -91 dBm
Typ Sensitivity: -93 dBm
Max Sensitivity: -95dBm
The sensitivity of the receiver varies between bands. The higher the sensitivity the further out the receiver can see.
On average a receiver has a sensitivity of -85 dbm. So in this case the receiver is very sensitive and high spec.
Voltage: 9-12v :
The working voltage range of the receiver. Most receivers are run off 12 volt sealed lead batteries for long working life between charges. You can also power them off other batteries such as 3 cell lipos
A 3 cell lipo of 2200 – 5000 mah of capacity will run your receiver for hours of operation. The receivers themselves use only a small amount of current.
Variations between receivers :
Most standard receivers have a single 50 ohm input. This means one antenna of your choice can be connected via the Female SMA connector.
Other receivers have two inputs. This means two antennas can be simultaneously connected. There is a diversity circuit inside the receiver that picks the best signal to output.
Most standard receivers have two outputs. One video output and one audio output. Both are in the form of Av level voltage signals. They are fed to a female RCA connector to plug your goggles / screen or video splitter into.
Other receivers have two audio output connectors, one left and one right. These are less common. Some Vtx units transmit in stereo though so both channels can be produced. The reason for this is for example having an on board microphone.
One channel outputs the microphones audio, the other channel is used for the telemetry data sent down for data logging or antenna tracking purposes.