RE: [linrad] 10.7 MHz mixer question...

["Leif Åsbrink" <SMTP:LEIF.ASBRINK@xxxxxxxxxxxxxxxxxx>]

Hi Oleg and All,
 
> I have a little question regarding 10.7 MHz mixer (that is built 
> on three 74HC4053). 
>
> What do the capacitors (C101,C102 470pF from the mixer 
> arms to ground) do ?

These capacitors improve IP3. They also reduce higher order
spurious responses of the mixer.

The transformer (TR13) is an inductance at high frequencies 
and when the switch opens the current that was going through 
it has to continue. If there is no load, the voltage will 
rise to force current into the 4053, something which will 
lead to a voltage transcient at the 4053. This large
voltage will cause nonlinearity and ultimately clipping in
the 4053. The rather big capacitors will absorb the current
without much variation of the voltage.

73

Leif  /  SM5BSZ



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