phase error measurement

[paul51]

Can anyone tell me a way to make a phase error measurement on a low noise amplifer.

thanks,

 

[buzzp]

Use a dual channel oscilloscope and measure the output and the input (assume both have a common ground-not isolated). These should be displayed on the same screen. Then you want to zoom in (time delay adjust on scope) to the point where the input crosses zero. Then measure the time til the output crosses zero (if you looked at zero cross as the signal fell, then you should look at the zero cross of the output as it falls as well). This time should be divided by the time to complete one cycle (frequency inverted) then multiplied by 360 degrees. This will give you the phase shift between input and output. If the input and output are isolated then doing this will not work since both do not share a common ground. Also, if your scope does not have isolated channels DO NOT attempt the above suggestion if the input and output are not isolated. This could damage your scope or the leads depending on the voltages your dealing with. This is because, internally, the ground of each scope lead are tied together and you will be trying to create a potential difference between two common points and this will definately create havoc. If the input and output share a commone reference (ground) then there will be no problem with the above. Chances are they are not isolated from one another since it is an amplifier. Hope this helps, Buzzp

 

[Lewish]

What buzzp described gives you phase delay, not error thru the amplifier. Phase error is how this delay changes with signal frequency. Interlated with something called phase margin.
If the amplifier in question is an op-amp, just look at a datasheet to get the information. Otherwise, make the measurement as buzzp described in increments over the band of frequencies you will be using the amplifier over. Make a plot of the change in phase delay versus frequency to get phase error. If you are only working at very low frequencies, the change in phase delay may be small enough to not matter. For example, if you are using an amplifier with a 1 megahertz bandwidth, just to amplify audio, don't worry about it.

Hope this helps.

Lewis

 

[lkm]

Hi Paul51,

The phase margin is not very meaningful in the real world. The problem we found about the phase margin is the phase linearity or phase margin Vs frequency.

If you want to measure the phase error or insertion phase, a vector network analyzer will help.

The other terms named "phase noise" may be used in the LO driver. We concern on the Lo' phase noise and the noise figure will add on it for the total noise.

I hope it help.