Sunday, May 31, 2009

Attenuator, RF probe and some matematics

Made an rf attenuator for an radio front end. I decided on copying values from one of the radios that I like, the FT-1000 (no I don't own part of Vertex :P) so I decided on 6, 12 and 18db attenuation because if it's good to Yaesu it should be good to me :)

Went to an online calculator for attenuators at:
and got myself some resistors values... I chose the "Pi" attenuator because.. I like the "Pi" shape...

The values for the resistors are:
Att ----- R1---------R2---------R3
6 - 150.47 ---- 37.35 --- 150.47
12 - 83.54 ---- 93.24 ---- 83.54
18 - 64.40 ---- 195.43 --- 64.40

Schematic is on the pictures or in ascii art:

| |
R1 R3
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Then was time to test.... how do they do that ?....

Having brewed an rf probe I though... power on a 50 Ohm load gets smaller if we increase attenuation before gets there.... so applied power to an dummy load and measure the voltage across it in the different levels of attenuation...

In the paper schematic is missing the dummy load that is connected to the output of the attenuator pad's; rf probe measures voltage across the dummy load.

Labels in the various parts of the testing...

If my calculation is right, values are this ones:

For 6 Db I got 6.7, for 12 I got 14.17 and for 18 db I measured 22.29.... honestly for something measured with a frequency generator, and all the rest homebrewed, I am quiet happy with the results....I think the values are correct, anyhow I am open to sugestions.

Formula for relation in dB is => "20 * Log (V/V0)"
My V0 is my value without attenuation to the load.

p.s. There are lot's of online calculators here:
case you need further info.