Common mode noise suppression

[Dave]

I have three cables to a radio receiver.
One is power, one ground and one is digital data from the receiver.
I have interference to the receiver from a grounded casing which contains
high speed processors and other noisy devices.
I believe the noise is transferred to the receiver via common mode (but
cannot be sure)
If I wrap all three wires through and round a toroid ferrite it helps.
If I remove the grounded metal casing the problem disappears.
I have to have the metal casing, the receiver has to go in the same casing
as the other components, the position of the receiving antenna makes no
difference, the receiver is itself in a tinned can...help!
Anyone?
Dave

 

[Martin Griffith]

I have three cables to a radio receiver.
One is power, one ground and one is digital data from the receiver.
I have interference to the receiver from a grounded casing which contains
high speed processors and other noisy devices.
I believe the noise is transferred to the receiver via common mode (but
cannot be sure)
If I wrap all three wires through and round a toroid ferrite it helps.
If I remove the grounded metal casing the problem disappears.
I have to have the metal casing, the receiver has to go in the same casing
as the other components, the position of the receiving antenna makes no
difference, the receiver is itself in a tinned can...help!
Anyone?
Dave

You probably need something like this
http://www.neutrik.com/content/products/detail.aspx?id=210_1576769505&catId=CatMSDE_audio

or this
http://www.maplin.co.uk/Search.aspx?criteria=digital transformer&source=15&SD=Y


martin

 

[Paul Mathews]

I have three cables to a radio receiver.
One is power, one ground and one is digital data from the receiver.
I have interference to the receiver from a grounded casing which contains
high speed processors and other noisy devices.
I believe the noise is transferred to the receiver via common mode (but
cannot be sure)
If I wrap all three wires through and round a toroid ferrite it helps.
If I remove the grounded metal casing the problem disappears.
I have to have the metal casing, the receiver has to go in the same casing
as the other components, the position of the receiving antenna makes no
difference, the receiver is itself in a tinned can...help!
Anyone?
Dave

The cables probably provide a return path for parasitic currents that
follow the basic capacitive current relation:

I = C dv/dt

Your common mode ferrite probably forces the parasitic currents to
take some other less harmful path. More effective remedies usually
involve reducing dv/dt, reducing capacitance, or shunting parasitic
currents through low impedance structures, i.e., shields.

There is lots of dv/dt associated with your clocked circuitry and/or
any switchmode power supplies. You'd like to have such currents flow
in as short a path as possible, well away from any sensitive 'victim'
circuits. One way to accomplish this is to add an additional shield
around the radiating circuitry, connecting that shield to the local
ground. Another remedy is to reduce dv/dt by slowing down rise and
fall times and eliminating ringing.

Paul Mathews

 

[Jamie]

I have three cables to a radio receiver.
One is power, one ground and one is digital data from the receiver.
I have interference to the receiver from a grounded casing which contains
high speed processors and other noisy devices.
I believe the noise is transferred to the receiver via common mode (but
cannot be sure)
If I wrap all three wires through and round a toroid ferrite it helps.
If I remove the grounded metal casing the problem disappears.
I have to have the metal casing, the receiver has to go in the same casing
as the other components, the position of the receiving antenna makes no
difference, the receiver is itself in a tinned can...help!
Anyone?
Dave

Use a twisted pair balanced line driver system to start with..
also, use a shielded cable with grounded shield at one end
only..