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oxxyfx

Max4311

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Hello,

I would like to use the Max4311 to switch betbeen 4 receivers. The circuit is on test board, with 2 receivers for testing. The brain of the circuit is a PIC 16F88. 2 pins of the pic is connected to the Max's address pins. The video output from the receivers are connected to In0 and In1 of the Max4311.

Based on the Max's datasheet, I did not wanted any gain on this signal, so I connected the FB signal directly to the Out. Also the datasheet says that for normal operation the Shutdown should be connected to Vcc - which it is.

Both incoming signals have a 75ohm resistor connected to the ground. The output has a 75Ohm receiver in series and then a 470uF capacitor before it connects to the TV.

The circuit is powered from a 5V Vreg, which has the stability caps in place.

However there is no signal coming out of the Max4311. I verified, the signals are there at the inputs, the A0 and A1 are low, and the output should give me the signal from the receiver connected to In0.

Do you guys have any idea why isn't this working?

Thanks,

Ox.

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Thank you, I will read through that thread. Regarding the Gain:

If I set it up for 2 - and later in the circuit I have another max4217 to bufer the video to 2 outputs - which is also adding to it a gain of 2 - wouldn't that be a problem? Like too strong signal?

Thanks, Ox.

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If I set it up for 2 - and later in the circuit I have another max4217 to buffer the video to 2 outputs - which is also adding to it a gain of 2 - wouldn't that be a problem? Like too strong signal?

The advise was based on your use of the 75 ohm terminations. They divide the amplitude in two. The gain must be set to account for them. If you will be following the MAX4311 mux with a 2X gain buffer, then just change the circuit so that unity gain can be used on the mux.

BTW, the MAX4311 has plenty of drive current for two video-out signals. In my opinion there is no need for a buffer. Just use two pairs of output caps and 75 ohm series resistors, along with a gain of 2X on the mux. KISS.

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Thank you, this is good advice, this will simplify my circuit. Now, I read through the thread you suggested. that mainly talks about the LMH 6574 chip - I am not sure if Terry ever changed that to the Max. One thing I noticed is that I will be using the same Airvawe receivers, DC coupled to the Max4311 - with a 75Ohm resistor to the ground - from a single supply of 5V - I built this design based on the reccomandations and discussions from the above thread. However I just noticed, that you mentioned that the gain of x2 could very well put my signal over the Vcc.

If I use two outputs as you suggested directly off the mux, will this still be the case?

The reason I am affraid of this because om my plane tracking receiver the video is off the airwave and goes into the Max4217 - (See this thread for the schematic: Max 4217) but on the tv screen I could see a little bit of ghosting on the right side of the white objetcs all the way through the recording. Can this be caused by the issue above?

Thanks, Ox.

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I will be using the same Airwave receivers, DC coupled to the Max4311 - with a 75Ohm resistor to the ground - from a single supply of 5V

In that case, omit the terminating resistor to gnd on the output of the Airwave. DC couple it to the MAX4311 and use unity gain (better still, configure it for slight variable gain so that you can adjust the signal level). On the output of the MAX4311, install two pairs of coupling caps and series terminating resistors. These will be your two video outputs.

...on the tv screen I could see a little bit of ghosting on the right side of the white objects all the way through the recording. Can this be caused by the issue above?

Ghosting is normally due to "ringing" on the video signal. This is often a sign of incorrectly implemented terminations. Long cabling can invite it too, especially if the terminations are not perfect. Just treat video like you would treat 10Mhz RF and you will do fine.

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Ghosting is normally due to "ringing" on the video signal. This is often a sign of incorrectly implemented terminations. Long cabling can invite it too, especially if the terminations are not perfect. Just treat video like you would treat 10Mhz RF and you will do fine.

I would like to understand this a little better if you don't mind - the termination part. The signal comes out from the Max4217, it is connected to a 75 Ohm resistor in series and a 470uF cap in series - from where it goes to the rca video connector.

something like this:

Sig -----> Resistor ------> Capacitor -----> RCA connector.

In my mind termination would mean taht somewhere on this line I should have a 75 Ohm resistor connected to the ground. Is this right?

In this case the schematic on the thread I quoted previously is nto correct, there is a resistor missing?

I may be off with the PCB from the 10Mhz RF standard, but I tried to avoid 90 degree or sharp turns, I made the video line as wide a possible and as straight as it was possible - this however is not an assurance that is 100% correct. Like yourself I aim for perfection - and ghosting like this does bother my eye...

The only cable I use is the video cable from the RX unit to the camcorder. I don't think that brings in any ghosting.

I want to thank you for your help and patiente with me - I learned a lot, and I keep learning....

Ox.

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In my mind termination would mean that somewhere on this line I should have a 75 Ohm resistor connected to the ground. Is this right?

The chip's series terminator on its output is only half of the signal's treatment. The other half is the terminator inside the device you connect to. It will have the equivalent of a 75 ohm resistor to gnd.

FWIW, some commercially produced devices use designs that do not correctly terminate their video inputs. For example, the LCD test monitor on my workbench uses a 370 ohm factory installed termination. My handheld TFT LCD doesn't even have a terminator. The engineers to these consumer products screwed up. These design mistakes will often cause video problems.

If you get good video without your circuit, but it gets ghosts when yours is used, then that would probably mean that the problem is on your end (or can be improved by optimizing your design).

Lastly, I have yet to see any of the popular wireless system have properly adjusted video levels. Some are so far off it is shameful (video monitor AGC hides the problem). So, I recommend that you ensure the video level coming out of the Rx is correct before attacking video problems in your project or elsewhere. And yes, a o-scope is needed to check the levels.

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Thank you. I have a scope - I spent 450 (CAD) on a Welleman HPS-40 portable scope. It is most likely not a professional tool, but so far it suited my needs - however the video signal is so fast and activity reach that the small screen of my scope is not enough to display a cycle...

What level are the video signal coming out from the Airvawe 625 modules should be? The data sheet states: 1Vp-p +/- 0.2V.

The camcorder does not do ghosting with anything else - I previously recorded video from the airwave modules without the 75 ohm resistor and the 470 capactor in series and the quality was quite good. This was before I started reading this forum... :)

Only after I made the circuit with the Max4217 and the neccessary resistors and caps the ghosting showed up - I have to mention that all resistors and caps are through hole - I am having problems soldering SMT staff, and I avoid it if I can. The leads on the capacitors and resistors are as short as they can be...

Attached is the layout of the board's top

Edited by oxxyfx

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What level are the video signal coming out from the Airwave 625 modules should be? The data sheet states: 1Vp-p +/- 0.2V.

When driven into 75 ohms, you should adjust the module for 1V video. However, your Rx design is driving High-Z, into a 1X amp, so the level will need to be 2V. I'm not sure if the Airwave will be happy with a High-Z output (it could be fine or it could cause issues, hard to say without testing it out).

I previously recorded video from the airwave modules without the 75 ohm resistor and the 470 capacitor in series and the quality was quite good. This was before I started reading this forum...

When you use the video signal directly from the output of the Airwave you do not install the series terminator. It is built into the module. So, what you did sounds correct.

Only after I made the circuit with the Max4217 and the necessary resistors and caps the ghosting showed up.

I think you need to scope this out to see what your signal levels are doing.

BTW, I noticed a couple of issues with the layout:

It looks to me like you are missing the mfg's recommended bypass cap on the MAX4217.

Also, your analog and digital Vcc power rails are being shared in a long daisy chain arrangement. This is not recommended. I suggest you isolate the two and run them directly to the Vreg in a star arrangement. This can be done by cutting away the 5V trace to the RX and then connecting the Rx/MAX power line to the Vreg with a jumper wire. Add some bulk capacitance (>100uF) close to the Rx module's power pin, as well as install the bypass cap on the MAX chip.

Lastly, the video output caps look backward to me. Audio out caps seem correct.

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Thank you.

I will see if I can modify the layout without using a jumper wire. The 0.1uF bypass for the max is there, - true it is a little further away from the Max - it is C3.

About the video output caps - they are reversed on the design but installed correctly on the board. Sorry about that, I'll correct it on the schematic...

I will scope the outgoing signal if I have time tonight and maybe do a capture from the scope so we know what we are dealing with.

Also:

When driven into 75 ohms, you should adjust the module for 1V video. However, your Rx design is driving High-Z, into a 1X amp, so the level will need to be 2V. I'm not sure if the Airwave will be happy with a High-Z output (it could be fine or it could cause issues, hard to say without testing it out).

I thought - and I just looked again at the datasheet - the Max4217 is a Gain of 2... Does that kick me back to have 1V on the outputs?

Thanks again,

Ox.

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The 0.1uF bypass for the max is there, - true it is a little further away from the Max - it is C3.

C3 is too far away to effectively bypass the MAX4217 IC. You need the cap at the IC itself. Add another one. An SMD cap directly on the IC's power pin is best. Please refer to the data sheet for more information on IC bypass requirements.

I just looked again at the datasheet - the Max4217 is a Gain of 2... Does that kick me back to have 1V on the outputs?

If you are using it in the 2X gain mode then you need to permanently install a 75 ohm resistor {to gnd} at the input of the MAX IC. Then adjust the buffered video to 1Vpkpk .

Basic details to measuring video levels are found here: http://www.rc-cam.com/lawmate.htm

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Hello,

I am back again. So I did try to follow your instructions on the Lawmate page step by step, but the camera adjusted it's AGC to the light, and I was not able to produce a white picture. However being quite experienced with photography and white balance settings, I iluminated a white paper from the back and put this in front of the camera - in which case I've got quite a good white picture.

Here are the captures from my scope. The first is the first image I captured right off the Airwave module - this line has a 75 ohm resistor to the ground and from here it enters the Max4217.

post-6-1184804735_thumb.jpg

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The third is captured at teh exit of the receiver - after the signal passes through a 75 Ohm resistor in series and a 470uF capacitor in series.

post-6-1184804988_thumb.jpg

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The third is captured at the exit of the receiver - after the signal passes through a 75 Ohm resistor in series and a 470uF capacitor in series.

With the buffered video output terminated to gnd with a 75 ohm resistor, adjust your video level so that it is 1.0Vpkpk +/-5%. Then remove the test termination and try it out to see how it looks on your viewing monitor. While at it, verify that when it is plugged into the monitor, the buffer's amplitude is still 1Vpkpk (or nearly so).

If you have more than one transmitter then they should all be checked to make sure they are matched to the Rx.

EDIT: BTW, your scope's horz timebase looks like it is off by 25% or so (not a problem for this level test). It would be wise to double check the vertical scale to ensure that the voltage measurements are accurate.

Edited by Mr.RC-Cam

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Now I am confused again, the video level it shoudl be adjusted on the receiver or transmitter?

I popped the top of the receiver open and there is no pot to adjust...

pictures are here: http://www.rc-cam.com/forum/index.php?showtopic=1955

I see not pot. So it has to be on the transmitter...

Is the scale on the scope adjustable? The capture was done with the scope conencted to the PC and a software on the PC receiving ASCII strings from the scope - displaying the image in a window. Is it possible that scope is ok but the software is off?

Thx, Ox.

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Now I am confused again, the video level it should be adjusted on the receiver or transmitter?

In this case we have been talking about adjusting the Tx. However, in situations where there is a level adjustment in both the Tx and Rx, things can get very complicated.

Is the scale on the scope adjustable?

You'll have to speak to the scope mfg about that.

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You'll have to speak to the scope mfg about that.

Here is a capture directly from the scope - I need to make sure the scope is off before I call them. It is kind of small but that's how it comes off the scope. Is this off too?

post-6-1184812105_thumb.jpg

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one more image - this time I put the markers on it as well to measure the signal horizontally and vertically.

I think this is close enough... NTSC having the line duration at 63.55 ┬Ás - my scope measures 63... It must be the third party software...

post-6-1184814272_thumb.jpg

Edited by oxxyfx

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Now that the buffer's video level is 1V, how does the image look on your monitor?

Just to recap, did you check to see if the video levels are still correct when the monitor is installed?

In regards to the scope's horz timebase accuracy, it is all up the the mfg's published specs. For these level tests, any timebase error is really not important. Vertical accuracy is what counts here.

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I didn't get around in adjusting the level just yet. Perhaps tomorrow. The level on the previous post capture is still without the 75ohm termination. I will continue tomorrow. Thanks for your help again.

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The level on the previous post capture is still without the 75ohm termination.

That's unexpected to hear. From the progression of screenshots, it sure looks like the buffer's output has the termination (either from a terminating test resistor or via direct connection to a CRT/monitor).

Is the scope's probe input set to Hi-Z? I see you are using a X1 scope probe, so be sure to disable the scope's 50 ohm input terminator if it has one. Otherwise you will have incorrect terminations.

Just so there is no misunderstanding, the final buffer output, where it connects to the external video device, needs to be set to 1Vpkpk when terminated to a standard of 75 ohms. The intermediate levels are good to review (they look fine), but they are not the main concern.

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I just remembered. you are right. The TV was plugged into the RCA jack I took the image from. So it was terminated.

I will do some adjustments tonight make sure that the signal level is closer to 1V.

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