# Exact meaning of 3db bandwidth?

• posted

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:

Can somebody please explain what exactly it means in this context? Thanks!

-Mike

• posted

Looking at the datasheet, it states:

------- SETTING BANDWIDTH

External capacitors CMID and COUT are used in combination with on-chip resistors to create two low-pass filters to limit the bandwidth of the ADXRS401?s rate response. The ?3 dB frequency set by ROUT and COUT is:

fOUT = 1/(2 x pi x Rout x Cout)

-----------

This is the standard -3dB point for a passive RC filter.

In this particular case, it refers to the rate of change OF the yaw rate (2nd order differential) output. The rate output is specified as v being proportional to rate (in degrees / sec) - i.e. a DC level for a constant yaw rate. (My take, anyway)

HTH

Cheers

PeteS

• posted

3dB point of the low pass filter on page 9? come on dude. I suggest reading some filter design books.
• posted

The best way to understand RC filters is use ltspice. try this, save as rc.asc or whatever.asc and run in ltspice.

Version 4 SHEET 1 1072 680 WIRE -48 192 0 192 WIRE 0 192 0 208 WIRE -128 192 -208 192 WIRE 0 272 0 320 WIRE 0 192 64 192 WIRE -416 208 -416 176 WIRE -416 288 -416 304 WIRE 416 192 464 192 WIRE 464 192 464 208 WIRE 336 192 288 192 WIRE 464 272 464 320 WIRE 464 192 512 192 WIRE 832 192 880 192 WIRE 880 192 880 208 WIRE 752 192 704 192 WIRE 880 272 880 320 WIRE 880 192 960 192 FLAG 0 320 0 FLAG 64 192 Output1 FLAG -416 304 0 FLAG 464 320 0 FLAG 512 192 Output2 FLAG 880 320 0 FLAG 960 192 Output3 FLAG -416 176 ACin FLAG -208 192 ACin FLAG 288 192 ACin FLAG 704 192 ACin SYMBOL res -144 208 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R1 SYMATTR Value 1600 SYMBOL cap -16 208 R0 SYMATTR InstName C1 SYMATTR Value 10pF SYMBOL voltage -416 192 R0 WINDOW 123 24 132 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value "" SYMATTR Value2 AC 1 SYMBOL res 320 208 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R2 SYMATTR Value 806 SYMBOL cap 448 208 R0 SYMATTR InstName C2 SYMATTR Value 10pF SYMBOL res 736 208 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R3 SYMATTR Value 1000 SYMBOL cap 864 208 R0 SYMATTR InstName C3 SYMATTR Value 10pF TEXT -296 40 Left 0 !.ac dec 100 1000 10000e6

• posted

--------8

• posted

Lord, I appologize for that...that ain't right...and be with the starvin' pygmies in New Guinea. Amen!"

• posted

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.

```--
Tim Wescott
Wescott Design Services```
• posted

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

• posted

The -3dB point is 0.5 Po/Pi, but 0.707 Vo/Vi for this sort of measurement.

0.5 Voltage (in dB) is -6dB

Cheers

PeteS

• posted

Hi PeteS - that clears things up quite a bit. Thanks!

-Mike

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