Question about Electronics textbook - diode switching circuits - Analog switch example

noweare

Joined Jun 30, 2017
115
The example talks about an AC source which is not shown. Would it be connected to the top of the circuit diagram, where the anodes of the diodes are? And capacitively coupled ?

WBahn

Joined Mar 31, 2012
30,035
And of course there's only one Electronics textbook and of course everyone has it and of course there's only one Analog switch example, so of course everyone knows exactly what circuit diagram you are referring to and so of course everyone knows what this AC source is that isn't shown.

Sheesh.

DickCappels

Joined Aug 21, 2008
10,168

noweare

Joined Jun 30, 2017
115
I am talking about the Electronics text book under the "Education" tab on this website
Volume III Semiconductors --> Chapter 3 Diodes & Rectifiers -->Diode switching circuit page
Analog switch example.

Question: There is no AC source in the example, but it is discussed. Would it be connected to the anode of the diodes?

Thank you

bertus

Joined Apr 5, 2008
22,276
Hello,

Do you mean this picture:

When the diode is in reverse mode, the resistance is high.
When the diode is in conducting mode, the resistance is low.

Bertus

noweare

Joined Jun 30, 2017
115
Yes, that is the circuit I am trying to talk about : )
Where would the AC source be connected ?
Thanks

bertus

Joined Apr 5, 2008
22,276
Hello.

There is no AC source used.
The diodes are switched using a DC voltage to have them isolating or conducting.
The anodes of the diodes are connected to the +5 Volts on the right of the picture.
When the "digital" voltage is also +5 Volts, the diode is isolating the capacitor from the circuit.
When the "digital" voltage is 0 Volts, the diode is conducting and placing the capacitor in the circuit.

Bertus

noweare

Joined Jun 30, 2017
115
Here is text from the example.

begin copied text
" The only problem is isolating the AC signal being switched from the DC control signal. The circuit is a parallel resonant network: resonant tuning inductor paralleled by one (or more) of the switched resonator capacitors. This parallel LC resonant circuit could be a preselector filter for a radio receiver. It could be the frequency determining network of an oscillator (not shown). The digital control lines may be driven by a microprocessor interface. "
end copied text

So the micro selects which capacitor forms the resonant circuit with the inductor but no AC source is shown. This circuit, I would
think, have an AC circuit coupled to it in order to work. Or am I confused?

bertus

Joined Apr 5, 2008
22,276
Hello,

The blue parts are the resonant circuit for the high frequency filter/oscilator.
The RFC's are isolating the high frequency AC from the DC.
The high frequency AC only sees the switched capacitor, wich is switched to the resonance circuit, changing the frequency of it.

Bertus

noweare

Joined Jun 30, 2017
115
Where is the high frequency AC coming from ? Is it just not shown in the schematic ?

bertus

Joined Apr 5, 2008
22,276
Hello,

The circuit shows only the way how to switch capacitors to a resonant circuit.
It does not show how the high frequency AC is made.

Bertus