Author Topic: Ways to sum RF+baseband signals  (Read 1213 times)

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Offline PartialDischargeTopic starter

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Ways to sum RF+baseband signals
« on: February 20, 2020, 12:01:14 pm »
How would you sum 2 signals, lets say that come from low impedance sources, one DC-10MHz, the other 10MHz to 500MHz, about 1Vpk-pk each
Output must be 50Ohms, the DC is important and cannot be lost.  A passive way would be preferred but I guess op-amps could be an option.

Maybe some commercial Tektronix or HP use circuits to do this but I can't recall any..
 

Offline mikerj

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Re: Ways to sum RF+baseband signals
« Reply #1 on: February 20, 2020, 12:25:50 pm »
A simple 50 ohm power combiner (a passive splitter connected backwards) will sum the signals and preserve DC, but the sum will be 3dB down e.g. a 1volt DC level on each input would only yield 1v on the output rather than 2v.
 

Offline PartialDischargeTopic starter

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Re: Ways to sum RF+baseband signals
« Reply #2 on: February 20, 2020, 01:07:38 pm »
Are you sure you want to do that? Because I see no reason to do so.
Did you mean to mix them?
To sum them. Let's say some channels are optimized to transmitting RF and not DC, so you put the load of bandwidth on these. Other channels, with other technologies, may be slower but able to transmit DC.
So in the end you need to sum them
 

Offline PartialDischargeTopic starter

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Re: Ways to sum RF+baseband signals
« Reply #3 on: February 20, 2020, 01:36:12 pm »
To do this in a lossless fashion, use an RF transformer/balun to get the RF riding on top of your presumably zero impedance DC. You will get a complex output impedance, though, inductive at DC and 50R at RF.

The problem is that the RF LF corner is not close to DC, but in the few MHz, 10MHz the better. To restore just DC on RF is easy.
 

Offline David Hess

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Re: Ways to sum RF+baseband signals
« Reply #4 on: February 20, 2020, 11:39:30 pm »
If you have a fast and powerful OPAMP that can maintain low output impedance at a few MHz, why not?

Some operational amplifiers will be fast enough to use in the inverting configuration and current feedback operational amplifier can be used this way.

The higher frequency extension of this is to replace the operational amplifier with a transistor using shunt feedback.  A DC correction loop can be used to correct the DC offset if necessary.

The broadband method used by oscilloscopes to add two signals is suitable to 100s of MHz.  Convert each signal to a current using a transconductance amplifier, and then combine the two current outputs in parallel.  No conversion back to a voltage is necessary in this case because the  combined current can directly drive a transmission line with the source terminated by a shunt resistance.  There are some very fast integrated transconductance amplifiers which would be ideal for this and some integrated parts with dual inputs which might be suitable by themselves.
 
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Offline Alex Eisenhut

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Hoarder of 8-bit Commodore relics and 1960s Tektronix 500-series stuff. Unconventional interior decorator.
 


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