DIY 10dB Pi-Attenuator

Submitted by gerry on Thu, 02/09/2017 - 09:26

Attenuators are expensive, especially the fancy calibrated ones, and it seems that they are the only ones available. SMD resistors are cheap, though, and I have SMA connectors, so I thought I would try to make my own. I used the calculator here, to come up with a design that used resistors I had, gave an impedance of 50 ohms and an attenuation that was useful (10dB). The schematic for the attenuator is as follows. 

pi attenuator

The formulas to calcuate R1, R2 and R3 are: 

R1=R3= Z ( K-1/K+1) 

R2= 2 Z ( K / K2 - 1) 

And in my case, the values I got (while selecting for resistors I already had) are R1 = R3 = 100ohm, and R2 = 75ohm.

I built it up on a pair of SMA connectors with a bit of PCB soldered to the bottom to give a bit of rigidity. I will eventually replace this with some heavy copper or tin sheet, which I will be able to fully enclose the device with. This is how it currently looks.

10dB Attenuator

I did some measurements on my scope. I took readings at 2MHz and 20MHz and found an attenuation of very close to 10dB in both cases.

At 2MHz, I read a peak-to-peak voltage of 10.5V when fed direct from my signal generator. With the attentuator inline, I measured a peak-to-peak voltage of 3.28V, which is very nearly 10dB of attenuation. 

Direct reading...

direct reading at 2MHz

Attenuated reading...

attenuated reading at 2MHz

At 20MHz, the figures were very similar... 9.92V p-p direct, and 3.2V attenuated. Slightly less attenuation, but close enough for my purposes.

Direct reading...

direct reading at 20MHz

Attenuated reading...

attenuated reading at 20MHz

This attenuator is not designed to handle power, but will be ideal for use with a reflection bridge and Red Pitaya for use as a Vector Network Analyser.






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