Switching fixed attenuator circuit

[ericz]

atenuator +circuit resistor cascade

Hello all,

In the tutorial of Variable attenuators in:
**broken link removed**

in the last page there is a mention of switching fixed attenuators.

My question is, can I cascade such configuration in order to achieve a variable attenuator (with fixed gain steps , i.e., -5dB ,-10dB,-15dB) by switching the spdt's?

another question I have is, when you cascade Pi attenuators, the Input and output impedance is still 50 Ohm (consider cascading 3 attenuators).

Thanks

 

[flatulent]

1. You can switch attenuators in and out with switches. The problem comes in when the switch losses are different in the on and off states. You will have reduced accuracy and also a fixed minimum attenuation.

2. The port impedances stay the same when you cascade them.

 

[frannk]

There are many commercial variable attenuator designs where fixed attenuators are switched in and out.

If you have the following attenuators:
1dB, 2dB, 4dB, 8dB, 16dB, and 32dB.
Then you can realize a variable attenuator in 1dB steps from 0 to 63dB.

Check out the following data sheet.
**broken link removed**

 

[ericz]

Is there a major problem in the layout of such attenuator?

the pads of the resistors which are layed act as capacitors and may creat an impedance missmatch