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Welcome to the KnightLites KnightSMiTe Doc Page 6

 


 


                The Components of the KnightSMiTe
                  (Transmit/Receive Switching)


R5, C10, D1 and D2 are used to accomplish transmit/receive
switching.

** R5 **

R5 limits the amount of current wasted when the
KnightSMiTe is keyed.  Grounding the cathode of D1 reduces
the power supply of U1 to one diode drop above ground.
Without R5, the power supply of the KnightSMiTe would be reduced
to 0.7 volts and either rapidly deplete the battery or burn out
the diode D1.  If D1 is removed, R5 can be replaced with a jumper
to improve receive dynamic range slightly.  The improvement would
only be slight however while its presence along with C10 provide
a good degree of power supply noise suppression in the audio
amplifier.

1 k-ohm is an optimum value for R5 and I wouldn't recommend
changing it's value.  Making it larger improves noise immunity
but reduces the voltage supplied to the audio amplifier.  A
reduction in dynamic range results which will lead to distortion.
Making R5 smaller increases its dissipation during transmission
and increases the power wasted by virtue of shorting the cathode
of D1 to ground.

** C10 **

R5 and C10 provides form a low frequency bypass of the power
supplying the audio amplifier (U1) minimizing the effects of
noise on the power supply.

** D1 **

D1 isolates U1 from drawing current from the emitter bias of Q2
during receive.  Its function is to provide a short circuit path
to ground for U1's power supply during transmit thereby muting
the audio amplifier.  Eliminate D1 for a slight improvement in
power supply efficiency if listening to low level white noise
while transmitting isn't objectionable or U1 if is needed to
amplify a sidetone from a Tick keyer or other external tone
source.

** D2 **

D2 introduces a capacitance change between transmit and receive
to offset the oscillator frequency and enable detection of
signals that zero your transmit frequency.  D2 is reverse biased
in receive (minimum capacitance) and zero biased during transmit
(maximum capacitance).

D2 can be eliminated at the risk of not hearing stations that
respond on your transmit calling frequency or if an alternate
means of achieving transmit/receive offset control is introduced.

Any diode will work here, but feel free to experiment with
different diodes types to find one that produces the greatest
transmit offset.  One that exhibits a high change in capacitance
between zero and reverse bias will produce the best result.

 

If you are interested in the Knightlites and Amateur Radio contact Paul at

AA4XX@bellsouth.net