IEEE 185-1975, Standard Methods of Testing Frequency Modulated Broadcast Receivers, specifies an audio bandpass filter for several measurements. The filter should be no more than 3 dB down at 200 and 15,000 Hz, with slopes of a least 18 dB/octave. A third-order Butterworth filter can meet these requirements.
R7/C7 is the IEEE standard output load. Reduce the value of C7 by the input cable capacitance. For best accuracy, keep all component values within a couple percent of those indicated. Use a wideband, low-noise, quad op-amp, such as a TL074. Power supply bypassing is not shown. Only one filter is needed.
In addition to the passband response, the IEEE filter spec requires the stopband response to be at least 30 dB down at 19 kHz and above. No simple all-pole filter meets this requirement, which is intended to reject residual stereo pilot and subchannel signals. For distortion measurements, a spectrum analyzer eliminates the need for an output filter of any kind. But when measuring stereo S/N, a residual pilot signal may limit accuracy. Some tuners have ultrasonic output filters with a notch at 19 kHz. Other tuners use pilot cancellation. Often you can adjust these circuits to drop the residual pilot well into the noise. For tuners with inadequate pilot suppression, create a 19 kHz notch filter with the remaining op-amp in the quad package.
The bandpass filter output feeds the notch filter input. If necessary, adjust R1 and R2 to set the notch frequency, R3 to maximize flatness below 15 kHz, and R4 to maximize notch depth. These adjustments interact somewhat.
The high-Q notch filter has negligible loss at 15 kHz and below.
88–108 MHz