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Subject: Re: Adding a DC bias to a differential clock signal

Date: 04/15/02 at 10:23 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk
Message Posted:

In Reply to: Re: Adding a DC bias to a differential clock signal posted by Mario Santos on 04/15/02 at 9:32 AM:

Hi Mario,

To try to answer your questions in turn:

1) All I am saying in my first paragraph is your signal can be coupled via a transformer or capacitor because the Manchester encoding scheme always gives an a.c. signal regardless of the binary data you are transmitting - even if it is a string of zeroes or a string of ones.

But that implies nothing about what is needed to establish d.c. biasing at the inputs of the receiver - this is a different aspect of the problem. Most receiver chips have high impedance inputs, and would need a biasing source to define the d.c. level on which your Manchester encoded signal rides - and the simplest way to achieve this is to use a potential divider (2 resistors) to derive the desired bias voltage from the d.c. supply.

However, it may be the receiver device is self-biasing, in which case no biasing components would be needed; I would have to know more about the receiver device to be able to comment further on this.

2) Yes. You couple the signal to your receiver via a high frequency signal transformer. You would use the 120V to 7.8V d.c. supply as a source from which to derive the d.c. biasing of the receiver inputs.

3) You do need to be concerned with proper impedance matching. If the receiver already has 130ohms input impedance, the easiest way to ensure that the correct impedance is seen by the signal source is to use a 1:1 transformer - any other ratio will alter the impedance level presented to the line. The d.c. biasing network, if needed, will then have to have an impedance much larger than 130ohms (perhaps 10 times greater or more) to minimise it's effect on impedance matching.

Regards,

David Ashby


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