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cwd10

900mHz TX. Dissapointing output results with Tek-SA.

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OK. So I finally got around to testing the output of my 900mHz vidTX today, and the results were very dissapointing. Here's my setup:

I used a Tektronix 2711 SA. I attached the TX directly to the input with a 6" piece of RG-142 coax, using the internal 30dB attenuator.

Peak output was 22.5dBm, which equates to roughly 177.8mW nominal. Now I realize there will be a slight loss in the cable, but it shouldn't be more than half a dB, at most. This is very dissapointing, considering the TX is advertised at 500mW. I plan on opening this thing up and trying to fine-tune it for better performance. Has anybody already done this? Any pointers?

Edited by cwd10

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(1) Which 900MHz Tx is it?

(2) Did you disconnect the video input before you measured?

(3) What was the exact power supply voltage?

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Who is the manufacturer? If it’s Far East manufactured there’s a good chance it is a generic circuit/pcb and a close look at it could well show a number of vacant component places – components that are not “essential” (at least by their standards) and/or not in stock at the time the board is populated. A good example of components that get left out are the output filters.

So many 900Mhz (and to some extent 1200Mhz) video boards from the far east don’t have filters on the Tx output or the Rx input – just a pair of component holes, which are patch across with a wire where required. I don’t know why they do this, but they do, and while directly not a power issue, filters absent from either board do have an impact on range or performance (depending on how you make the measurement).

As far as power output goes – your measurement cwd10 is quite likely not the way the manufacturer measures power output. That’s no excuse of course, but if you contact the manufacturer you’ll likely get an explination for a setup conditions and circumstances that really don’t exist in real world circumstances. I’ll share with you common explination, one I have heard several times from manufacturers: the 500milliwatt quoted is firstly, a peak output figure, and secondly, it’s a peak output figure which only exists for a certain circuit voltage level at a certain frequency, outside of which the amp stages in the transmitter do not offer 500milliwatt output to the antenna. In other words, while those are figures that get produced in factory test conditions - and as such are "true", they are not real world figures you are likely to experience day to day using your hardware. I know what you are thinking .......

The other folk guilty of using output figures like this in their advertising literature, are so-called “ghetto blaster” manufacturers. They will quote something silly like “2400watts” – and it’s true: if you have a scope that’s quick enough and connected up to measure output, at a certain point in the trace you may well see a 2kilowatt plus peak, but it’ll be a peak that lasts a fraction of second (millisec’s at most) while the capacitors dump their energy! Not really representative of real world conditions, but that’s marketing for you. In my experience tranmitter power output figures are often only valid for the exact same conditions, circumstances and setup(s) that the manufacturer gives them for (much like car performance figures), which is information seldom published. Otherwise take them with a pinch of salt unless it can be demonstrated otherwise.

Theres lots that can often be done with many of the 900Mhz Tx and Rx's to realise real world usable performance improvements from these products e.g improvements to filtering and SNR are just 2, which along with more output power (or without) are areas which can often be reworked with worthwhile benefits. If you can get the lid off and a good qual digital pict posted up, I'll be happy to share some ideas with you (circuitry permitting).

Edited by Helix1

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Daniel recently tested a couple racewood 500 mW 900 MHZ video transmitters, and found that both only put out about 125 mW. It seems that these 900 MHZ video transmitters just do not put out the amount anywhere near of power that is promised.

Mike

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Very odd. I've tested several hundred Racewood 900MHz/500mW Tx's and all but a few were rejected due to RF power not meeting spec. Unlike the mainland China made offerings, the Taiwan made Racewood 500mW Tx has very consistent RF power. My experience has shown that they meet or exceed the advertised 500mW rating. The nice thing is that it provides this RF power level over a large power supply range (most other designs will have varied RF power, depending on battery voltage).

A low power level can be caused by a broken SMA solder connection due to excessive antenna flexing and other similar accidents.

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