Accomodating Noncentered IF Filters

Digitally synthesized FM tuners seldom provide a way to accomodate ceramic IF filters whose passband is not centered at 10.7 MHz. Manufacturers normally use red-dot filters for synthesized tuners. These selected parts have center frequencies that range from 10.67 to 10.73 MHz. But filters with center frequencies at the extremities of this range may cause lopsided selectivity, with greater sensitivity to interference on one side of center than the other. Asymmetrical filter response also can increase distortion. You may encounter these issues with original filters or when upgrading to narrower filters to improve selectivity.

You can rehabilitate noncentered filters 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 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. This particular part did not have enough adjustment range so I added 68 pF across it on the back side of the PCB. I adjusted the trimmer for symmetrical clipping in narrow IF mode when overdeviating a 97.5-MHz stereo signal. This maximizes modulation acceptance for multipath-laden signals. Adjust a single-bandwidth tuner with a wide IF filter for minimum stereo distortion.

Changing the synthesizer reference frequency alters the tuning-step size. While filter alignment will be exact only at 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, the alignment error will be zero at 97.5 MHz and 3.2 kHz at the band edges, averaging just 1.6 kHz over the band.

You might even consider using this technique in a tuner such as the Yamaha T-80 that already provides IF offset in 10-kHz steps. Adding a reference-oscillator trimmer can lower the worst-case average tuning error from 5 kHz to less than 300 Hz.

More is here.

Updated February 1, 2008