Mike said:

Hi - I recently came across the term "3db bandwidth" used in a way

which I was not entirely familiar. I am most familiar with the term

when used with amplifiers. For example, say in a FET amplifier where

you know that the midband gain [in db] = 20 * log(midband gain [in

V/V]), and the 3db bandwidth is the frequency which gives you a gain of

3db less than the midband gain.

Where I'm seeing it is in the datasheet for a gyro - the Analog

ADXRS401:

http://www.analog.com/UploadedFiles/Data_Sheets/ADXRS401.pdf
Can somebody please explain what exactly it means in this context?

Thanks!

-Mike

Just the same as an amplifier.

The ideal gyro would give you an exact output (volts?) vs. rotation rate

input, no matter how fast the rotation rate was changing. Your gyro

will not do this. If you made a rate table that could impose a

sinusoidal rotation on the gyro at any frequency, you would find a point

where the output-to-input ratio was 0.707 the value that it is for a

constant rotation -- that's the 3dB point.

Firstly, is the output measurement as a power or simple voltage?

If it is power (V^2), certainly 0.707 is the figure.

A simple voltage will however be 0.5.

I am certainly no expert on gyros, but I suspect in this instance we may

be dealing with a simple voltage, feedback and all that......

As the test apparatus is probably quite involved mechanically it may be

a tricky thing to verify I suspect.

a dB is actually defined as 10 log(test/ref) and is dimensionless, so it

can apply to any sort of measurement you care when comparing against a

known reference.

20 log (test/ref) really is a convenient way to perform POWER

measurements from a voltage reading, the V^2 is done in the log domain

(20 instead of 10).

Perhaps if I am only half correct in my reply, I'm 3dB out!

By all means, happy to be corrected if need be

Cheers, Ray