Constant input/output impedance variable attenuator?

[neazoi]

I wonder if this small variable attenuator circuit (used for HF and below) can present a relatively constant input and output impedance, regardless of the attenuation level?

 

[volker@muehlhaus]

As a testcase, set your attenuator to minimum attenuation. You then have input impedance 50R (left) || 50R (right) || 50R (load)

 

[FvM]

Looking at matched T-attenuator resistor tables, you'll know that all three resistor elements have to be varied.

 

[D.A.(Tony)Stewart]

Look at the schematic and tell us if you see matched-T's ?

 

[crutschow]

I wonder if this small variable attenuator circuit (used for HF and below) can present a relatively constant input and output impedance, regardless of the attenuation level?

No.

 

[D.A.(Tony)Stewart]

No.

If No , why not?

 

[barry]

If No , why not?

If the pot is at the bottom the impedance is 50 ohms. If the pot is at the top, it approaches 25 ohms (depending on frequency).

 

[D.A.(Tony)Stewart]

{ above } > attenuator circuit (used for HF and below) can present a relatively constant input and output impedance,


If SWR is fairly tolerant, you could swap ends or oppose CW, CCW and probably get -10 to >-60 dB attenuation, then for that matter use 100 ohm pots.

But then you could any 50 ohm volume control and 50R in series out.

 

[crutschow]

If No , why not?

I answered his question.

Since the two pot wipers are tied together with an AC short, the AC impedance looking into or out of the attenuator will vary with pot position.

 

[biff44]

THIS would be a lot more "constant impedance" than your first circuit:

You might want potentiometers that are more like 75 to 110 ohms....
Optimise the impedance for the nominal attenuation value you want, such as perhaps 5 dB.