Darius UKW Tuner Schaltung und Bilder
Beschreibung des EC900 EF95 Eingangs- und Mischteils
A few words about the ratio detector
tube stereo decoder implementation with example
The VHF FM IF with EF89 2x 6BN6 and EF91
Low Phase Noise - push-pull input section with ECC91/6J6
A few words the ratio detector general. In tube receivers, the IF gain is usually not so high that even the noise of the precursor is limited. For weak signals it then leaves the ratio detector, the limit. In addition, the output of the last IF stage is not always the same. It depends on, inter alia, the operating voltage and the tube aging. This Influences balance is also an object of the ratio detector. 
The ratio detector is a dynamic limiter . Although the shares are AM suppressed within the audio range, but the average of the IF amplitude ratio determines the voltage on the capacitor and thus the average low-frequency amplitude. This means a weak signal which produces a smaller voltage ratio, is quieter than a total of a strong signal that generates high voltage ratio. 
Now it is for the stereo decoder already of importance that the Pilottonamplitude and in relation to the other signal amplitudes a certain value or exceed. Also, the position of the volume control the amplifier, depend to a certain loudness, not the strength of the received signal. Many radio receivers is therefore the ratio detector, a low frequency amplifier stage in a high μ triode Gittervorspannungserzeugung downstream from the starting current. (Eg 10nF EABC80 10Meg Ohm) It causes an adjustment of the loudness at the expense of dynamics. 
I have this problem with a Triodenstufe, the forward-regulated depending on the voltage ratio is thus controlled resolved. Only the ratio detector in conjunction with the controlled low frequency element of it is a complete FM demodulator.
Last updated March 4, 2008
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