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I'm trying to make an AM reciever using a certain IC (Samsung KA22427C). More specifically, I want to understand roles of all bypass pins of this IC (these are 1, 7 and 16).
That's the IC

I looked over schemes of test circuits from datasheet and still destination of those three pins is not clear to me. Help will be appreciated!

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  • \$\begingroup\$ Those 3 pins are bypassed to pin#3, RF GROUND. \$\endgroup\$ Apr 25, 2017 at 6:54

2 Answers 2

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"Bypass" in radio circuits generally refers to a decoupling circuit, usually a low impedance capacitor to ground, to allow the high frequency signals to "bypass" the rest of the circuit. It could be a local power supply for an amplifier, or a reference point where you don't want stray RF signals. Or a measure of the signal strength, fed back into the RF stages, to control their gain (AGC) in which case you don't even want audio noise on it - hence the massive C17.

And in this circuit you can see pin 1 is connected to GND via C2, pin 7 via C7, and pin 16 via C16 and C17. The next page of the circuit lists all of these as "bypass" capacitors.

As Peter's answer suggests, some of them may have an auxiliary function : for example, with the waveband switch set to "FM", the "RF Bypass" pin is shorted to ground, which can reasonably be assumed to kill the RF amplifier.

As Pin 2 is the "FM IF input" we can assume the RF and mixer stages for FM are handled by a separate circuit, not this RF stage. (When these chips were made, IC processes were limited in frequency range : they couldn't handle 100MHz signals very well) - in fact the circuits on pages 6 and 8 confirm that.

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Looking at the Test Circuit, it appears to me that IF Bypass and RF Bypass are used to switch the receiver between AM and FM modes. AGC Bypass will turn the Automatic Gain Control off or on. Since Samsung doesn't provide an internal circuit diagram or theory of operation, we can't know what exactly these inputs do inside the IC.

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