EMRFD Message Archive 7065

Message Date From Subject
7065 2011-12-20 13:57:00 ha5rxz A Problem Of Impedance
Dear Group

In the files section you will find a pdf diagram which generated considerable debate at our radio club this evening. Module One has a 50 ohm impedance balanced output port 'AB' and Module Two has a 50 ohm impedance balanced input 'CD'. Normally they are connected directly together.

If we wish to introduce a Low Pass Filter into each leg what impedance should the filter be to ensure a good 50 ohm match on both the input and output ports of the modules? Half of those present said 25 ohms and the other half said 100 ohms so obviously somebody is wrong here. The problem is complicated by the design of the Low Pass Filters as some circuits were found where the inductors were in series and the capacitors were wired between the inductors rather than to chassis giving a true balanced filter.

All thoughts welcome.

Peter HA5RXZ
7073 2011-12-20 19:56:08 kb1gmx Re: A Problem Of Impedance
7082 2011-12-21 08:20:34 Brooke Clarke A Problem Of Impedance
Hi Peter:

When you say that the source module has balanced 50 Ohm outputs do you mean that it expects to see a 50 Ohm load to
ground on each port?
If yes, then is it also the case that a 100 ohm load from center pin to center pin would also act as a matched load?

If so the filters, when tested by themselves should be matched to 50 Ohms. When installed the grounds of the filters,
source and load can be tied together.

There are additional parameters involved when working with balanced devices as compared to asymmetrical (50 Ohm)
devices. These involve how well they are balanced.

Balance is related to symmetry. In theory you can introduce a ground plane half way between a balanced device and use
the two new devices asymmetrically.

Have Fun,

Brooke Clarke