PBL3852 查看數據表（PDF） - Ericsson

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PBL3852

Universal Transmission Circuit

Ericsson  

PBL 3852

Short about Radio Frequency

Interference RFI.

HF suppression at the microphone

input

The HF-signal at the microphone input

can be seen composed as of two

components. One component being the

differential (between pins 12and 13) and

the second related to ground at pin 14. Of

these two, the first is the most serious,

entering the amplifier directly being

amplified and detected. The second

component is less serious because it

affecting both inputs alike and most of it

will be balanced out of the amplifier.

There might be the case where the HF-

signal will have such an amplitude that

the amplifier can not balance it out. Then

components must be filtered with

capacitors and maybe resistors. It is

extremely important that everything that is

done at the input is in balance, otherways

the problem might get worse instead of

better. The extreme balance requirement

goes all the way to the PCB-layout. Small

unbalance signals can be corrected with

capacitors marked with *) this requiring

high precision components. See fig 28.

The solution shown is rather expensive

but with precision components it renders

good filtering at the input. If the main

problem is the signal between the inputs,

try to increase the 1nF capacitor but

make the others procentually smaller in

order to maintain the frequency re-

sponse. A more simple solution, that is

sufficient in most of the cases is also

shown in fig. 27.

10n 10n

100Ω

Mic.

100Ω

*

1n

*

10n

10n

11

13

M

12 +

PBL3852

14

Line

<20n

Mic.

1n

<20n

11

13

M

12 +

PBL3852

14

Line

Dynamic microphone

Dynamic microphone (simplified)

Figure 27. RFI elimination at microphone amplifier input .

+

11-15k

10n

11

200-

470Ω

+

13

1n

1µ

Mic.

M

+ 12 +

10n

200-

470Ω

1µ

PBL3852

14

Line

Electret microphone

HF-suppression at the receiver output

The problem here is of the same kind as

at the microphone amplifier input but will

be easier to solve because of the much

lower impedance and level of gain. The

solution is shown in the fig. 28. No

capacitors should be connected directly

from pins17 or 18 to ground because of

the low outputimpedance, series

resistance of at least 10Ω must be used if

there is a tendency to self oscillation.

Other paths for the HF-signal to enter

the audible system

To find out if the problem originates in the

DTMF-generator disconnect the generator

and short the mute input to -line, pin 14. If

the problem is small try to connect a

capacitor from mute input to -line pin 14.

Modern CMOS circuits are more sensitive

to RFI because of their high impedance at

the input pins, especially the keyboard

inputs to the DTMF-generator. These

inputs are not possible to filter with large

capacitors because of the keyboard

scanning pulses (1µs) that will be loaded

down. To shield the keyboard will some

times help. The polarity guard bridge can

also act as a rectifier and demodulator, of

the HF-signals. Connect 1nF capacitors

across each diode in the bridge. There is

a capacitor across the line C10, this is for

RFI suppression but also to stabilise the

whole system.

The cappacitor C10 shoud be connected

like in figure 30. The frequencies at which

the RFI comes through are in the region

of 10-1000MHz. The resistance of the

C10 will be somewhere 0.01-10Ω hence

even the shortest lenght of connector on

the PCB board or wire wil be in the same

region of resistance and thus of greatest

of importance. These actions described

above should, when applied correctly,

take care of the RFI coming in from the

telephone line. The second way for the

RFI to enter the system is to penetrate the

PCB board capacitively. The test methode

is to place a metal sheet under the

telephone set to be tested and inject the

sheet with RF signal. The most

used and effective counter measure to

this kind of RFI penetration is to shield

the telephone set, at least the bottom

of it, that is closest to the main PCB

board by metal foil or by spraying the

plastic casing with metallic matter. See

figure 29. This methode does not

necessarily count out the RFI

components that are recommended

earlier.

17 10-100Ω

-

<47n

Rx

16

10-100Ω

<47n

PBL3852

15

14

- Line

Figure 28. RFI elimination at receiver

amplifier output.

18

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