Modification of the RF input filter in TEF6686 module

TEF6686 based radio tuners are very popular nowadays. There are two versions of the 10-pin modules.

• Current version, which is available on Aliexpress (left)
• Old design for TEA685X, which is pin compatible with TEF668X (right)

The main difference between them is a different RF input circuit. Both designs use balun FM antenna application, but only the old version implements RF filtering.

The balun FM antenna application schematic is presented below:

The first part of this circuit (with 270 nH inductor) is missing in the currently available PCB version. The second part (with 220 nH inductor and 12 pF capacitor) is used for 75 Ω → 300 Ω matching. Recently I have ordered modules with TEA6851, but instead of the old design, I received the new version with bad RF filtering.

Is the filter really needed? Unfortunately, TEF is not immune to strong out-of-band signals.

First screenshot presents the FM spectrum with TEF6687 installed in old PCB module with proper RF filtering:

Second screenshot presents the FM spectrum with TEF6687 installed in the currently widely-available module without RF filter:

The difference between them is a wide-band interference around 89.0 MHz (digital modulation) up to 29 dBf.

When an external low pass filter is applied, the interference is gone:

Luckily, there is some space left on the PCB where the missing components can be added. Use the NP0 (C0G) capacitors.

• Remove the original C9 and C10 capacitors.
• Cut the trace between C9 and C10 capacitor.
• Solder 1 nF capacitor.
• Prepare soldering spot at GND.

• Solder 4.7 pF capacitor to the GND.
• Cut the PCB trace near R7 and solder 12 pF capacitor.

• Cut the PCB trace between newly added capacitors to make space for the 270 nH inductor

• Solder the 270 nH inductor (I used NLV32T-R27J-PFD).

That’s all. No more nasty out-of-band signals. Of course the they can also affect the OIRT band, like this…

Before modification:After modification: