Accommodating Offset IF Filters

Digitally synthesized FM tuners normally use ceramic IF filters centered at 10.7 MHz. But the characteristics of individual filters vary. Murata specifies filter center frequency as 10.67 to 10.73 MHz. Outlier filters may make a tuner more susceptible to interference on one side of channel center than the other. Filter offset also can increase distortion and modulation-induced noise. You may encounter these issues with original filters or when upgrading to narrower filters to improve selectivity.

You can accommodate a noncentered filter by offsetting the synthesized local oscillator to move the IF to the center of the filter passband. All that's necessary is to substitute a trimmer for a fixed capacitor in the reference-frequency crystal oscillator.

This shows the reference oscillator circuit of a Sony ST-S555ES tuner. The reference crystal is in the feedback loop of a CMOS amplifier within the synthesizer chip. I found the oscillator frequency to be most sensitive to C608, the shunt capacitor at the amplifier input on pin 3. I substituted a small trimmer capacitor for it.

This shows the trimmer installed in the tuner. To maximize modulation acceptance in narrow-IF mode, I adjusted it on a strong, overdeviated signal for symmetrical waveform clipping. An alternative is to adjust it on a weak signal for equal modulation-induced noise on positive and negative waveform tips. Perfectly symmetrical IF filters will simultaneously satisfy both conditions, but this seldom occurs. Adjust a single-bandwidth, wide-IF tuner for minimum stereo distortion.

Changing the synthesizer reference frequency alters the tuning-step size. While filter alignment will be exact at only one spot on the dial, the error will be no more than 10.7% of the filter offset if adjusted for zero at 97.5 MHz, the geometric center of the FM band. For example, if you correct a filter whose passband is offset 30 kHz, alignment error will be zero at 97.5 MHz, 3.2 kHz at 88 MHz, and 3.2 kHz in the other direction at 108 MHz.

May 5, 201488108 MHz