Hi,
I've got doubts with this circuit which is in Question 7 of the page..

http://www.allaboutcircuits.com/worksheets/filter.html

My Questions are.... (ALL QUESTIONS ARE EXPLAINED NEATLY IN ATTACHMENT)

1) Why is antenna coupled to the tank circuit and why not attached directly to the top of tank circuit?

2) How can the diode clip of the negative part of the voltage ? since the received voltage will be less than 0.6 volts and this voltage is not sufficient to turn on the diode... it might be greater than or equal to 0.6 volts if there are powerful transmitters nearby but I've got serious doubts about this...

3) What is the need for the potentiometer kinda stuff near base of transistor?


I am trying to build a receiver or something like that so I am asking all these questions.. I hav also got a thread in radio and communications forum.. thanks for all help in advance.. Waiting for replies...

 

Hi,
I've got doubts with this circuit which is in Question 7 of the page..

http://www.allaboutcircuits.com/worksheets/filter.html

My Questions are.... (ALL QUESTIONS ARE EXPLAINED NEATLY IN ATTACHMENT)

I am trying to build a receiver or something like that so I am asking all these questions.. I hav also got a thread in radio and communications forum.. thanks for all help in advance.. Waiting for replies...

1) Why is antenna coupled to the tank circuit and why not attached directly to the top of tank circuit?

Attaching the antenna directly to the top of the tank circuit would increase the damping of the tuned circuit, leading to poorer selectivity. The antenna capacitance would also fall directly in parallel with the tuning capacitance, limiting the tuning range.

2) How can the diode clip of the negative part of the voltage ? since the received voltage will be less than 0.6 volts and this voltage is not sufficient to turn on the diode... it might be greater than or equal to 0.6 volts if there are powerful transmitters nearby but I've got serious doubts about this...

This type of receiver (detector with AF amplifier) does have quite limited sensitivity. The diode should preferably be a Germanium or Schottky type, which would require less than 0.6V.

3) What is the need for the potentiometer kinda stuff near base of transistor?

The diode detector (demodulator) is essentially a rectifier and so must deliver a DC output current. The output to the amplifier is DC blocked to preserve the transistor bias, so a DC load is required. Whether or not a volume control potentiometer is really necessary will depend on the signal strength, and whether the user requires variable level.

 

Thanks adjuster for quick reply..

Attaching the antenna directly to the top of the tank circuit would increase the damping of the tuned circuit, leading to poorer selectivity. The antenna capacitance would also fall directly in parallel with the tuning capacitance, limiting the tuning range.

Can I know how is this antenna to tuner coupling is achieved or could u plz send a link to know the theory behind it.. How many turns should the windings in the antenna have??

Also plz see my thread started in radio and communications forum related to a similar topic.. Thanks for help

 

I just want to know how does the antenna attached directly above the tank circuit reduces the selectivity.. Also plz provide me with a link which discusses about the basic bandpass filter stuff... Thanks...