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PBL38573

Speech Circuit for constant current feeding systems

Ericsson  

PBL 385 73

Short about Radio Frequency

Interference RFI.

HF suppression at the microphone in-

put.

The HF-signal at the microphone input can

be seen composed as of two components.

One component being the differential

(between pins 9 and 10) 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 22a. 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 response. A more

simple solution, that is sufficient in most of

the cases is also shown in fig. 22b.

a)

10n

10n

100Ω

Mic.

100Ω

*

1n

8

9

M

10 +

*

10n

10n

PBL385 73

11

Line

Dynamic microphone

b)

<20n

Mic.

1n

<20n

8

9

M

10 +

PBL385 73

11

Line

Dynamic microphone (simplified)

c)

+

8

200-

10n

470Ω

+

9

1n

1µ

Mic.

M

+ 10 +

10n

200-

470Ω

1µ

PBL385 73

11

Line

Electret microphone

Figure 22. RFI elimination at microphone amplifier input.

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. 23. No

capacitors should be connected directly

from pins 13 or 14 to ground because of

the low output impedance, 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 genera-

tor and disconnect the mute input at pin 5.

If the problem is small try to connect a

capacitor from mute input to -line pin 11.

DTMF circuits are sensitive to RFI because

of their high impedance at the input pins,

especially the keyboard inputs. These

inputs are not possible to filter with large

capacitors because of the keyboard scan-

ning 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 24. 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 24. This

methode does not necessarily count out

the RFI components that are

recommended earlier.

10-100Ω

13

Rx

14

-

PBL385 73

12

11

10-100Ω

- Line

<47n

<47n

Figure 23. RFI elimination at receiver

amplifier output

13

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