TDA18219HN signal distortion

10.12.2013 19:34

I've been writing previously about some AGC problems I noticed with the NXP TDA18219HN tuner chip. Here is how they look like in practice.

Below is the TDA18219HN intermediate frequency output signal captured with an oscilloscope. There is a -60 dBm, 783.1 MHz constant wave on the tuner's antenna input. Channel filter is set to 8.0 MHz bandwidth. Tuner has a balanced IF output, so yellow trace shows non-inverted pin IFP, blue trace shows inverted pin IFN and orange trace shows IFP - IFN. The signal is as expected at this point, with no visible non-linear distortions. The integrated automatic gain control keeps the signal peak-to-peak amplitude at around 400 mV regardless of the input level.

TDA18219HN output with 8 MHz filter, -60 dBm input signal.

The following capture is from the same setup, except channel filter is set to 1.7 MHz. The input frequency was also lowered to 780 MHz to keep the frequency at the IF stage approximately the same. There's a severe signal distortion at this point. Note that the signal level is also higher now (this screenshot was made at 500 mV/div compared to 200 mV/div in the other two pictures). The triangular shapes make me think that the slew rate limit of some amplifier stage has been reached.

TDA18219HN output with 1700 kHz filter, -60 dBm input signal.

When input level is lowered to -75 dBm the distortion dissappears:

TDA18219HN output with 1700 kHz filter, -75 dBm input signal.

The new TV band receiver for VESNA will have an external channel filter anyway, so this isn't such a problem. I'll simply run the tuner using the 8 MHz setting and rely on the external filter to reduce the IF bandwidth further.

If it's really a bug in the hardware and not some artifact of my own circuit, I'm guessing that this setting wasn't well tested in production since it's not commonly used for DVB-T transmissions. A note in the datasheet about it would be nice though.

Posted by Tomaž | Categories: Analog

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