RF attenuation circuit

[Yuri1737]

Hi,
Im trying to make a digitally selectable RF attenuator.
First question do I need a RF balun to convert differential RF signal to single ended or a capacitor will be enough. Can i use as a balun RF transformer such as http://www.coilcraft.com/ja4220.cfm?
Second question i found a simple circuit shown on a picture bellow - signal path containing an attenuator which can be bypass using a relay will this circuit work for low signals or will it disturb the waveform? I think i should terminate the relay connection with 50 Ohm when it is not used.

[rs20]

Please say what bandwidth you're aiming for.

That aside, the shield of the cable is typically connected to ground; in that case, a simple capacitor is sufficient. I only see baluns in use in connection with mixers and so on, where it's sorta kinda part of the mixing process maybe. But I'm clearly not an expert.

[aveekbh]

If you're using a balanced RF line (I can only think of twin-lead as an example) instead of the more common coax, then you will need a balun on either side of the attenuator. Of course, balanced attenuator topologies exist as well - http://www.rfcafe.com/references/electrical/attenuators.htm.

In the circuit that you have shown, I presume the "resistors" are actually attenuators. In general, you cannot use relays of this nature - the open end of the relays are stubs, and they will affect the impedance. Instead, you need to use SPDT switches (one on each end of the attenuators) to send the signal through the attenuator or the bypass line.

Hope this makes sense. 

[Yuri1737]

About the attenuators circuit will the circuit from picture below work? Im not quite sure if the 50 Ohm terminator near 20dB attenuator wont affect the circuit.

[aveekbh]

How are the junctions realised (near the 10 and 20 dB attenuators)? You cannot connect them like normal wires, you need RF splitters of the correct impedance or SPDT switches which will either engage the attenuator or the bypass.

[Marco]

Please say what bandwidth you're aiming for.

And linearity and dynamic range.

[Marco]

About the attenuators circuit will the circuit from picture below work? Im not quite sure if the 50 Ohm terminator near 20dB attenuator wont affect the circuit.

Don't the input and output have to be impedance matched?

Can't you just use a digital VGA?

[Neganur]

I'm a bit puzzled why you start using AD's SPDT 1GHz switches instead of looking at digitally selectable attenuators (VGAs) from Analog, Macom, Perigrine, Mini Circuits etc. There's a bunch of them with differential inputs too. Unless it exceeds your power or bandwidth, in which case the switches might not be good either.