RADIO's

[Dr_Doggy]

k, so iv been reading about crystal radios, im just wondering based on my filter circuits couse, an inductor in paralel is a highpass filter, another way to do this is with a capacitor in series, so would I be able to drive a crystal radio with dielectrics only, no inductors, providing i used thevenin equivilant inductions???

 

[Nigel Goodwin]

No, it's NOT a filter as such, it's a tuned circuit (which is essentially a notch filter - either pass or reject).

 

[Dr_Doggy]

ya, a filter is esentially a pass or reject, where the inductor is a low pass filter, and the cap is the high pass filter, causeing a tuneing to a narrow band
so why cant we instead of useing an inductor in parallel use a thevenin equivilant capacitor in series. Where am I going wrong here?

 

[pc88]

well, can you post a picture of what your circuit would look like? then we can tell you why it should or should not work.

 

[Dr_Doggy]

note that I didnt bother matching impedance values, however these values can be detirmined with algebra

 

[pc88]

In the classic method, isn't there also a capacitor from ANT to ground? In your schematic, the detector diode will be given all the high frequency components of the incoming signal. By adding a cap in parallel with the inductor, the detector gets only a band of frequencies.

 

[ronsimpson]

where is the cap?

The “my method” is not going to work well.
The coil needs a capacitor! There must be an LC to resonate.

All coils have some capacitance just like all capacitors have some inductance. My first coil was wound on a toilet paper role. The coil self resonated in the AM band. There was no “capacitor” soldered across the coil. The coil was wound in such a way that it had a large amount of capacitance. Every time two wires get near each other but do not touch it forms a small capacitor. From every turn of wire to the next turn there are “capacitors”.

 

[pc88]

The "my method" schematic looks a lot like this design for an "all band" receiver:

http://www.techlib.com/electronics/allband.htm

 

[Dr_Doggy]

all im really sayin is that in a band-pass filter (parallel) you got your L and C in parallel, or for the same filter type you can stack L and C in series to get the same effects, however in doing this it switches the "roles" that L & C play(highpass-lowpass), so in theory couldn't we take that parallel filter, pull out the L(thats in parallel) and then replace it with a C that is in series to get the same filtering effects?

 

[Nigel Goodwin]

Are we still talking radios? - if so try reading the posts that have gone before, they don't use filters, they use tuned circuits.

 

[Dr_Doggy]

so whats the diffrence,? your feeding in the EM from the antenna then useing iimpedances to filter it down.

 

[Nigel Goodwin]

so whats the diffrence,? your feeding in the EM from the antenna then useing iimpedances to filter it down.

They are completely different, a tuned circuit is resonant, and has a magnification factor (Q). It's really fairly basic electronics?, I suggest you get yourself a copy of either the ARRL or RSGB Handbooks - an essential resource for anyone interested in electronics - doesn't have to be a new one, an old edition would be fine.

 

[audioguru]

An crystal radio has a tuned circuit, a detector diode and a DC load for the diode.
Yours is missing a couple of them.

You even had a battery, I don't know why.

 

[Leftyretro]

A simple L/C tuned filter gives a simple crystal radio a chance to receive a single specific frequency, it's selectivity is dependent on the Q factor of the L/C tank. If you make the L or C variable you have a chance to receive specific frequencies as you tune through the L/C's frequency range.

Feeding a simple crystal detector with a simple low pass (L) or high pass (C) will make the receiver respond to all frequencies above or below a certain frequency AT THE SAME TIME and makes it pretty useless as a receiver.

However as simple signal strength meter (detector diode driving a sensitive DC meter) normally uses a simple inductor or choke to allow a broadband response to relative near field signal strength it's seeing.

Lefty