Video connections from Airwave module to Video IC

[AXI Motor Models]

I am working out how to connect the video output from an Airwave receiver module (AWM625) to a Maxim video switching IC.

(below) I am also wondering what termination method I would use to connect two MAXIM video switches in a daisy chain.

Between pin 8 and pin 4 below, do I not need the resistors?

Do I need a 0.1uF capacitor for AC-coupling?

Do I need a voltage divider to shift the video signal upwards so that it does not go below the rails?

(below right) Also, is that how I would connect the final video output to an RCA connector? (Just a 75Ohm resistor to the pin)

Here is the circuit diagram for the Airwave module:

I don't know if the 75Ohm termination resistor should go before or after the 470uF cap that Airwave reccomends.

Do I need a 0.1uF cap just before the input pin 4 of the MAXIM?

Do I not need the 75Ohm termination resistor to ground?

Do I need a voltage divider to shift the Module's video up to levels within the Maxim's rails?

Thanks,

Matt

Edited January 1, 2010 by AXI Motor Models

[Mr.RC-Cam]

I am also wondering what termination method I would use to connect two MAXIM video switches in a daisy chain.

I think you would be much better off with the MAX4311 (1x4) mux. It would be a cleaner design and eliminate all the related questions about the cascading.

I don't know if the 75Ohm termination resistor should go before or after the 470uF cap that Airwave recommends.

In your implementation it would go after.

Do I need a 0.1uF cap just before the input pin 4 of the MAXIM?

To support your existing design you would need a cap on the input. The size will depend on the bias resistors (not seen on your drawing) you use on the MAX's input. The cap value should ensure that your frequency response is good down below 10Hz.

Do I not need the 75Ohm termination resistor to ground?

It is typical and would be required in your implementation.

Do I need a voltage divider to shift the Module's video up to levels within the Maxim's rails

Yes, you will need to include resistor bias on pin 4 & 5 to ensure that all possible video output levels stay withing the MAX's rails.

However, there are other solutions. For example, you should be able to eliminate the 470uF cap and 75 ohm resistor on the output of the Airwave. With some experimentation, I think you will find that a simple DC connection using a 3dB level attenuator would work out. I can't say what would be optimum, but I would try a pair of 1K resistors in a L-Pad arrangement and tweek as needed to achieve standard 1Vpk-pk video.

Also, your MAX's monitor output needs a 470uF cap. It is not shown in the MAX data sheet, but is highly recommended to ensure compatibility with common A/V devices that you may connect to your receiver design.

You didn't say what you are building. Perhaps 1 x 3 diversity? Is it for personal use or possible commercial design?

[AXI Motor Models]

Hi, thankyou for the help. Video terminations can be confusing! I am just experimenting with building a diversity receiver. Airwave modules are the only ones that I can find (apart from Comtech) which have RSSI out.

I have revised the output drawing and was wondering if the capacitor should go before or after the output resistor?

And also is the polarity of the capacitor the correct way around? (with positive towards the chip)

I changed the chip to one with a fixed gain of +2V/V because to my knowledge the 75Ohm terminations halve the amplitude of the video signal.

I wonder if switching the video at random times (not in-between sync pulses) would cause any noticable interference on the video?

Thanks

[Terry]

Looks fine to me, the switching is so fast and as the video from all receivers is the same you should not see any problems.

Terry

[AXI Motor Models]

That is good that the switching between receivers should be seamless.

Now the last obstacle for me is the bias circuit between the Airwave module and the Maxim video switch. I have no idea whether the circuit below is correct.

Will the video termination circuit below work if I select the correct value of pullup resistor? (Rbias) Or have I done it wrong?

Matt

Edited January 2, 2010 by AXI Motor Models

[Terry]

try the 0.1 the other side of the resistors and use 560r for the bias if I remember right

Terry

Edited January 2, 2010 by Terry

[AXI Motor Models]

Is it true that every time you have a standard 75Ohm video termination (75Ohm resistor to ground + 75Ohm resistor in line) that the video level is divided by 2? Is that why some of the MAXIM video switches have a fixed gain of 2V/V?

For the 470uF capacitor, I might try a Tantalum with an ESR of 100mOhm. This should work well shouldn't it?

[Kilrah]

Exact.