I'm trying to transmit and receive voice to my GSM modem over a 10-meter cable from a handset connected board using TEA1062 line drivers. But when I connect a speaker to the handset, a sine wave noise rides on the line signal, so the speaker tinkles.

I tried using different speakers with different ohm values and realised that when I use a 130ohm speaker everything is okay. But if I use an 8-ohm or 16-ohm speaker, that noise signal noise happens. If I disconnect the speaker, the noise signal disappears.

There is not much information on the TEA1062 datasheet for the beginners since this IC is not used anymore. Are there any basic rules about audio based designs like impedance matching for speakers etc.? I just used the typical application schematic in the datasheet. I couldn't even understand the formulas for changing the line voltage level. This IC was mostly used for fixed telephones so they assume we are using 48V line level etc.

Edit: Sorry, I'm trying to add the schematics for half an hour but there is a failure about imgur. Below is the links for the schematics:

Schematic of the typical application in TEA1062 datasheet

The typical application only shows the telephone side of the 2 nodes. So, I needed to design the first schematic with a local supply.

  • 3
    Can you draw a schematic of your setup? – HandyHowie Aug 7 at 12:13
  • 4
    Sine wave noise means oscillation, oscillation means instability. A quick look at the TEA1062 shows that it is not designed to drive loads as low as 8 ohms, Rl = 600R. It's a line driver not audio amplifier. You could always add a dedicated amplifier after if you need to drive a speaker. – loudnoises Aug 7 at 12:19
  • I added the schematics. There is already an audio amplifier on my schematic, but there is not any audio amplifiers at the other side (typical app schematic which is used before my gsm modem input). – abdullah cinar Aug 7 at 12:50
  • You know imgur is blocked here in Turkey. Anyway, I designed a TEA1062-based intercom system for my previous company a few years ago. The load impedance was around 250 Ohms (A handset speaker). And there were no noise and any other problems. As @loudnoises pointed out, the IC is not capable of driving 8 Ohm speakers. By the way, TEA1062 (from NXP) is obsolete. UTC is now manufacturing the same IC with TEA1062N part number (You can buy from Ozdisan). – Rohat Kılıç Aug 7 at 13:20
  • @RohatKılıç Is there any reference links that I can use as a start of understanding the basics of this kind of circuits or ICs? There are not any application notes about TEA1062. So it is a bit hard to proceed without having analog design experience. I should better look for the app notes of TEA1062N. – abdullah cinar Aug 7 at 13:24
up vote 2 down vote accepted

First, please note that TEA1062A/AN is not capable of driving low-impedance loads (e.g. 4-Ohm- or 8-Ohm-speakers). This IC is used for telephone handsets or intercom units. So the load impedance should be higher (e.g. 250 Ohms).

Although the datasheet provides a lot of design-related useful information as well as typical application schematic, I'll try my best to enlighten some of the keypoints.

Below is the pinning table of the IC taken from the datasheet:

enter image description here

NOTE: I'll not describe all the pins here since the datasheet has done it for me. I'll only show some keypoints.

And here's the typical application circuit (I'll make the explanations over this circuit):

enter image description here

Brief explanation: It's obvious that the audio signal is carried on the LINE which has a DC component (probably greater than 4VDC) for powering the driver IC. To take advantage of this DC component a capacitor (C1) with a high capacitance is used across VCC and GND (VEE).

-Receiving: The DC component of the LINE is blocked by C5 and the incoming signal is applied to IR (the input of the receiver amplifier). Amplified signal is filtered by R4-C4 network and then transferred to the earpiece or speaker.

-Transmitting: The signal to be transmitted is received by a microphone (can be either a dynamic or a capacitive one). The signal is amplified and filtered by R7-C6 network. Then the signal is transmitted over LINE after side-tone suppression (Side-tone is a low level portion of the transmitted signal that comes to the earpiece during transmitting. And yes, it's like hearing what you're saying but with quite low level. Side-tone network formed by R2, R3, R8, R9 and Zbal is a crucial part of the circuit since it can can lead to acoustic feedback if not designed properly.).

LN: Phone line input is applied here (a low-pass filter can also be applied)

Typical phone line circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

And how this LINE is applied to the IC (The filter is optional, so the LINE can be applied directly to LN. Totally depends on the application):

schematic

simulate this circuit

GAS1 & GAS2 & SLPE: Gain adjustment and slope compensation pins for microphone input. A resistor (R7) with a resistance of a few tens of kOhms is connected between these pins (The higher the resistance, the more the microphone gain). Datasheet suggests a 68kOhm resistor for a dynamic microphone. Try different values for best results.

A capacitor (C6) placed across GAS1 and SLPE provides input filtering with a cut-off frequency of 1/[2 \$\cdot \pi \cdot \$ R7 \$\cdot\$ C6] (NOTE: If the circuit will be used for human speech then the cut-off frequency can be anything between 2.5kHz and 3.5kHz). So, select R7 first (68k ~ 150k), and then select cut-off frequency. Finally, calculate C6. For stabilization, a capacitor (C8 = 10 \$\cdot\$ C6) is required between GAS1 and VEE (GND).

MIC- & MIC+: Microphone inputs. If a dynamic microphone is used then the microphone can be connected directly to these inputs as shown in typical application circuit. If a capacitive microphone is used then application should be like the following:

schematic

simulate this circuit

RF-CF1-CF2 network provides a high-pass filter. The values above are chosen for human speech.

The following values for sidetone suppression may give good results: R2 = 130k, R3 = 3k9, R8 = 390R, R9 = 16R..20R and Zbal = 680R. If you are interested in how these values are calculated then please refer to the datasheet.

QR & GAR: Output and gain adjustment of the transmitting amplifier. The gain is determined by a resistor (R4) placed across QR and GAR. And the output is taken from QR. This pin is capable of driving piezoelectric earpieces and high-impedance (e.g. 250 Ohms) speakers. A capacitor (C4) placed parallel to R4 provides an output filtering with a cut-off frequency of 1/[2 \$\cdot \pi \cdot \$ R4 \$\cdot\$ C4]. And, of course, a capacitor (C7) having a value of 10 times C4 is required for stability.

The following arrangement is taken from my application which brings good results for a 250 Ohm speaker. You may increase R4 to 100k for higher gain but please note that you'll need to re-calculate C4 according to the desired cut-off frequency.

schematic

simulate this circuit

AGC: Automatic Gain Control pin. Detailed explanation is given in the datasheet, so I'll not say anything new. R6 = 100k can be used for this purpose.

DTMF: If you want to transmit a dial signal over the LINE you can use this pin. But you need to mute first the IC via MUTE pin (functions depending on the model number - TEA1062A or TEA1062. Refer to the datasheet).

That's all I can do. Hope this helps.

  • Sorry for the late response, I couldn't get into here for a couple of weeks. This answer is really very explanatory, I started to work through it. This will be very helpful for me, thank you so much! – abdullah cinar Sep 4 at 12:28
  • 1
    @abdullahcinar you're welcome. If you have any further questions through the design process please don't hesitate to ask. – Rohat Kılıç Sep 4 at 13:07
  • thanks, you are so kind. do you think I will need to create a new question or is there a way to send direct messages over stackexchange? I can have some simple questions about some basic stuff and I don't know if it would be appropriate to create a new question. – abdullah cinar Sep 4 at 13:52
  • 1
    @abdullahcinar depending on the content you can either post a new question or ask it under the main question as a comment. Editing your question according to the needs is also acceptable -since this will move the question to the top- unless its content is not beyond the main issue(s). – Rohat Kılıç Sep 5 at 5:21
  • 1
    @abdullahcinar Of course you can. But since its voltage level is about -48V (and will be about +48V after rectification) you may need to use zener diodes and some protective devices (You can see this in the typical application schematic in the datasheet). For mics and speakers, you can put the links for the datasheet into your question so we can check them for you and guide you through the design. Unfortunately I cannot say anything about GSM/GPRS usage. You may need to edit your question or post a new one for this issue. – Rohat Kılıç Sep 5 at 5:25

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