Foxtech 2.4GHz Video Transmitter (a.k.a. SKY-TX321)
The unit under test is the one found in the following link:-
http://www.foxtechfpv.com/foxtech-24g-5 ... p-180.html
The module used in this transmitter appears to be the SKY-TX24500 (see attached datasheet) but the output power is 1W. It is possible that it is a custom modified version of the same module.
The nominal specifications are 2.4GHz, 500mW (+27dBm), 50ohms load. 8-channels are available. The particular board under test has seen some damage and repairs as evidenced in the following photo:-
Furthermore, the unit originally came with RP-SMA connectors which were not compatible with all my other equipment. As such, I had to re-fit the board with a regular SMA connector for the purpose of testing. Again, it should bear mentioning that the board that I received for testing has seen some re-working.
The specifications state that the operating voltage should be between 7V to 16V. The module comes with a switching power supply which efficiently steps the voltage down to the requisite level (the board itself appears to run on several different voltage levels for the different parts.) Tests show that the board draws approximately 3W of power across the specified voltage range (0.45A@7V, 0.25A@12V) with efficiency being slightly better at higher voltages. We will come back to this consideration in a little bit. Even so, the current draw is a little higher than what one might expect from a 500mW transmitter. The Lawmate 2.4GHz 500mW, for example, draws 0.45A@5V for a total power draw of 2.25W.
Testing reveals that the output power of the transmitter was actually +30dBm (1W) rather than +27dBm.
This comes as a bit of a surprise because the unit is being advertised as a 500mW unit. This would explain the higher than usual current draw during testing earlier. At 1W output power, the efficiency is in the order of 33%, which is quite good (compared the Lawmate's 22% efficiency). Output power dropped about 1dBm to +28.54dBm (714mW) when the supply went down to 6V (drawing 0.41A or 2.46W) and to +26dBm (398mW) at 5V (drawing 0.33A or 1.65W). At 5.3V you get 500mW of output power.
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Voltage Current Input Output Efficiency
12V 0.25A 3W 1W 33%
7V 0.44A 3W 1W 33%
6V 0.41A 2.46W 0.71W 29%
5V 0.33A 1.65W 0.4W 24%
Power stability is good with negligible degradation over temperature (output power crept up as the unit started to get hot.) Like the Lawmate modules, the heatsink was mounted on the wrong side and probably does little to help with heat dissipation. In fact, the heatsink actually covers up the ventilation holes and could actually be making for poorer heat dissipation. Starting from a room temperature of 29-degrees Celsius with normal circulation (but no forced convection), the temperature rose steadily until it stabilized around 61-degrees Celsius. With forced convection, the temperature should not reach this level. The output power remained stable up through to 59-degrees Celsius and then showed the slightest decrease above that (about 0.5dBm). Frequency drift was around +20Hz for the tested temperature range (positive coefficient.)
The board also sports a small electret microphone but it jut out a bit from the board and would be prone to damage. Furthermore, having a microphone on the transmitter itself (which is usually exposed to wind for cooling) may not be the best idea. It is there, nonetheless, for those who need it, and there is a little trimmer on the board that appears to be for adjusting the microphone audio level. There were no instructions for disabling this microphone but you might be able to use the trimmer to zero the microphone gain level. Additionally, there are two audio inputs (labelled 6.0MHz and 6.5MHz) in the instructions, presumably catering for different TV systems but no stereo audio input.
Measured channel frequencies are as follows:-
- CH1 - 2414 MHz
CH2 - 2432 MHz
CH3 - 2450 MHz
CH4 - 2468 MHz
CH5 - 2370 MHz
CH6 - 2390 MHz
CH7 - 2490 MHz
CH8 - 2510 MHz
* note that there is some confusion about the channel settings on the datasheet.
The input impedance was measured as 72ohms resistive, which should be close enough to standards for video terminating equipment.
On the whole, this looks like a decent system. The test was done with no video modulation so no comments can be made concerning the video deviation and quality. The first harmonic in the 4.74GHz was -35dBm or -60dB down from the fundamental so this is spectrally quite clean. The switching power supply board is not coupled sturdily to the transmitter module so some care should be taken when handling and mounting the unit to avoid stressing the joint. Special attention should be given to the fact that the CH1, CH2, and CH4 frequencies do not correspond exactly with that of Lawmate's, so if you happen to be using Lawmate receivers, you should avoid these channels as some performance penalty will be incurred due to the frequency mismatch. At USD62/watt, it's not a bad deal.
30 July 2010