Adding Multipath Outputs to a Tuner

These images show multipath-free and multipath-laden traces with the antenna pointed directly at a station and with it pointed at nearby hills. The broad curvature is due to the IF-filter response. The wiggles are due to multipath.

A multipath scope displays the envelope of the IF signal on the vertical axis versus the instantaneous signal frequency on the horizontal axis. When an FM signal propagates by more than one path, the time delay between paths disrupts the phase of the combined signal, distorting the detected audio. The path interference also causes peaks and dips in the IF-signal envelope; the signal contains AM as well as PM components. It is the AM component that you view on a multipath display.

I added multipath outputs to my Sony ST-S555ES tuner in a way that should be possible with many tuners. I used the buffered wideband detector output for the horizontal signal, and the signal-meter output from the IF chip for the vertical signal. The vertical signal should be AC coupled so that you don't need to recenter the scope trace when the absolute signal level changes. AC coupling is usually done in the scope.

This is a sample application circuit for the Sanyo LA1235 IF chip. The S-meter output on pin 13 follows the incoming signal strength. Its logarithmic response makes relative changes mostly independent of absolute signal level. True, the output will saturate on very strong signals, yielding a flat, useless multipath trace. And it may exhibit plateaus within the linear range, as shown in the curve below, where the recovered envelope modulation drops considerably. Still, a fair amount of circuitry would be needed to outdo this simple indicator signal, one available from all IF chips.

Proper filtering of the S-meter output is important. If too heavily filtered, the vertical signal will not respond quickly enough to track the horizontal signal, smearing the trace. If too lightly filtered, out-of-band power will fuzz the trace. Resistive loading matters, too. If too lightly loaded, the output will slew-rate limit into a capacitive load due to the current-source drive. If too heavily loaded, you may exceed the output current spec (2 mA for the LA1235). I used a 6.2kΩ pulldown resistor and paralleled it with a .01 uF ceramic disk capacitor that measured .008 uF. Response was noticeably worse with .015 uF, so it pays to experiment. I fed the output cable through a series 3.3kΩ resistor to add some protection.

I took the horizontal signal from the wideband op-amp that follows the ST-S555ES detector. Directly attaching a capacitive cable to a high-impedance detector can severely degrade stereo separation. Add an op-amp buffer if necessary. I used a 1kΩ resistor in series with its output to protect the op-amp against external hazards. I found that extracting the horizontal signal after the tuner's complex postdetection filter smeared the scope trace. The filter added way too much time delay.

I also tried a more linear filter, with no capacitance across the 6.2kΩ but with some to ground on the other side of the 3.3kΩ. This did not work as well, smearing the trace more for the same out-of-band rejection. Again, I think it might pay to experiment with your particular IF chip. In the ST-S555ES the S-meter output isn't used and the pin floats. When used it's likely to have a .022 uF or greater capacitor attached. You may need to reduce the capacitor value or isolate the part with a resistor and pick off the multipath output closer to the chip, where you can properly filter the signal for the scope trace.

More is here.

Updated May 11, 2008