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Adding two RF signals!!! - RF Cafe Forums

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Post subject: Adding two RF signals!!!
Unread postPosted: Wed Jun 01, 2005 6:03 am

Joined: Wed Jun 01, 2005 5:57 am
Posts: 2
Heey !
I have two antiphase Rf signals at 900MHz (not sinusoidal)...I wanna add them up. By doing this actually I intend to cacel out the odd harmonics and add up even ones.......
Any suggestions out there !!!
have fun bye!


Post subject:
Unread postPosted: Wed Jun 01, 2005 11:46 am

From the description of the method you suggest, you are not looking for a signal sumation but for some sort of harmonic cancelation.

Because would it be a summation the phase relationship of all harmonic would be the same.



Post subject:
Unread postPosted: Thu Jun 02, 2005 3:19 am

Thanks for the comments..
yeah that's right..........what I want is a second harmonic, If I can.....
one way is to do something like this. (with capacitors)...but this divides the amplitude as well....

In + In-
\ /
= =
\ /
\/______ Out



Post subject: Adding two RF signals
Unread postPosted: Wed Jun 08, 2005 11:39 am

Unfortunately, you can't use passive circuits to add voltages without loss.

You can only passively add power in watts: 1Watt + 2 Watts <= 3 Watts for any kind of passive combiner.

Otherwise, adding 1V to 2v would give 3V, which gives more power out than in: power for free - unless the signals aren't matched impedance to begin with. And you don't get power for free.

Good Luck!


Post subject:
Unread postPosted: Thu Jun 09, 2005 4:58 pm

I think you would want to actively add the two signals. If they were at a low frequency, you would use an op-amp, carefully adjust the input level of each signal to be the same, and adjust the phase of one signal so the fundamental cancels in out.

At RF, you might try to use a Gilbert cell active mixer such as the Intersil HFA3101 or some other one with sufficient frequency range.


Post subject: Why not use OP?
Unread postPosted: Mon Jul 18, 2005 12:40 pm

Why you cannot use an op. amp. with a high bandwith? for example AD800¿1? (I'm not sure) has 1GHz of bandwidth. Probably this a stupid question, but I'm new in the RF.


Post subject: Response to Cubeleg
Unread postPosted: Mon Jul 18, 2005 6:51 pm

When an opamp is rated at 1 GHz, the number is for the gain-bandwidth product. An amplifier with a gain of 1 isn't particularly useful - a 1 GHz GBW opamp is really only starting to be useful as you go below 100 MHz.

Good luck!

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