Ratio Detector is Improved

WHERE HAS RATIO DETECTOR GONE?
The ratio detector has been very popular FM detection technique since vacuum tube days. It was used in the Panasonic radio cassette coder that my parents gave me 30 years ago. But recently other techniques took its place. One reason is the transformer, which is the key device of the ratio detector, cannot be integrated in an IC, but I think there is another reason. The transformer is difficult to understand, fabricate and adjust properly.

IMPROVED RATIO DETECTOR
This is a circuit diagram of the improved ratio detector, which is tuned to the center frequency of 10.7 MHz.
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The complex and troublesome transformer is replaced with a tapped toroidal coil. This detector has highly symmetric frequency vs. output voltage characteristic.
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PRINCIPLE OF CONVENTIONAL RATIO DETECTOR
Before explaining how the improved ratio dector works, I will start with the principle of the conventional one. This is a circuit diagram of the conventional ratio detector.
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The RF input V0 is applied to the coil L0. The capacitor C0 is not very essential for understanding the principle of the ratio detector. Its value is selected so that L0-C0 tank circuit resonates at the center frequency of FM modulation of the RF input, in order to cancel the inductance of the detector seen from the RF signal source.

Coils L1 and L2 inductively couples to L0. The resonance frequency of L2-C2 tank circuit is tuned to the center frequency. The voltage induced in L1 is in-phase with V0, while that in L2 lags 90 degrees at the center frequency. The condition necessary to make this phase difference will be discussed later.

The amplitudes of sum and difference of V1 and V2 are detected by D1 and D2, and the difference between the two amplitudes appears in the output.

As shown in the figure below, as the RF input frequency shifts from the center, the phase lag of V2 changes, making the amplitudes of the sum and the difference of V1 and V2 unequal. The difference appears as a voltage change in the detector output.
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When the frequency shift is much larger, the amplitude of V2 become small, and the output voltage goes close to the ground level again. As the result, the frequency vs. output voltage relationship shows "S curve". If the phase lag of V2 from V1 is smaller than 90 degrees, the symmetry of the S curve becomes poor.
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It is said that a large capacitance C4 reduces the affect of AM component in the RF input by adjusting load current of L1-C1 tank circuit, but it has nothing to do with the principle how the ratio detector detects FM modulation.

Although I won't show the theoretical calculation here, following conditions are needed to ensure the phase difference between V1 and V2 at the center frequency.

The inductive coupling between L0 and L2 must be very loose in order to make V2 lag 90 degrees from V0. On the other hand, V1 becomes in-phase to V0 only when the coupling between L0 and L1 is very tight and the resistive power taken from L1 is very small.

It is difficult to make a transformer that meets these conditions. Sometimes the transformer is separated in two pieces with loose and tight coupling to loosen the requirement.

HOW THE IMPROVED RATIO DETECTOR WORKS
But then why don't you connect the center tap of L2 directly to the RF input? Then you can omit the tight-coupled transformer. See the figure below.
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If you can prepare a floating voltage source that is in-phase to the RF input, the ratio detector can be modified as shown in (a). But, the newly introduced inductance LL must be very large and thus the amplitude of the floating voltage source must be very high.

The inductance and the voltage amplitude can be greatly reduced by connecting them to a pair of adjacent taps of L2 (b).

Though any pair of adjacent taps can be used, when the center tap and the one next to it is selected, the floating voltage source can be omitted (c).

By adding a capacitor on the opposite side, the inductance of the detector seen from the RF source can be cancelled and the symmetry of S curve is improved (d).

The remaining difference between the conventional and the improved ratio detector is that in the conventional one the midpoint of the load resistors of the amplitude detectors is connected to the ground and the output is taken from the midpoint of the smoothing capacitors, while it is reversed in the improved one. Since it is only because I feel it is easier to understand, you may of course use conventional connection.

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