Help me decrease an unknown noise in PCB

[aabs]

Hi
now i am working on a signal amplifier that should out signal from 20mV to 20V
but i have a high frequency (~50MHz) noise whit 20mV-pp amplitude in my PCB
i can not found noise source when i drive out all of the device from PCB i have that noise again in my circuit may be this noise is ground noise i test ground noise decrease rule in my PCB but noise remained.
is there any idea to decrease unknown noise in PCB.
thanks

 

[RF-OM]

noise pcb stage

What is the frequency band for your amplifier and what is your application? There may be a lot of reasons to have this noise. May be you have oscillation in amplifier? Check it first. You have 60dB voltage gain, so it is easily possible. If your circuit made with lamped element try to shot down one cascade at a time and check the noise. If it inside the chip try to turn off amplifier by any available method, may be just shorting the input (check DC path for safety). If noise is not from oscillation check what part of the circuit may produce it. SMPS may be a reason, or common IR voltage drop in return path. Sometimes noise can be originated outside of the board by powerful source like signal generator or transmitter in the same or adjacent room.

 

[rikie_rizza]

sources of noice in pcb

I think its an amplifier hum, try to tie output with 37pf to gnd, tie amplifier Vcc to gnd with 100nf, use proper grounding for your PS.

Try to touch one component (with out touching the conductor, eg IC body) and see if the signal frequecy or strength change. if it does, its possible a leakage occur.

 

[huzuhong525]

touch circuit noise

Place some filters in your board .
if your singal is audio ,Note your layout .

 

[aabs]

how to eliminate noise from pcb

thanks for your replys
first i use high frequency OPAMP in my design in a two stage form and i have power amplifier class AB in output. frequency range is wide (0.1Hz-5MHz)
i test the circuit when all IC was rejected from circuit so there was any amplification in circuit but a noise whit that frequency remain in ground and output.
so devices that i use does not made that noise.
is it possible to use a differential input/output stage to eliminate this noise any one test it before.
thanks.

 

[RF-OM]

Re: Noise in PCB

If your noise is common mode noise differential circuit will help. You need first to find the source. It is always better to find and eliminate the source that try to reduce its effect on the circuit. Try to touch other element and control the noise. May be you will find the aggressor. And check the possibility of external noise to be coupled.

 

[LE YAO]

Noise in PCB

professional ideas