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From: commgrp@silver.ucs.indiana.edu (BACS Data Communications Group)
Newsgroups: sci.electronics
Subject: Re: Low frequency tuned circuit
Message-ID: <1991Feb9.163914.9596@bronze.ucs.indiana.edu>
Date: 9 Feb 91 16:39:14 GMT
Sender: daemon@bronze.ucs.indiana.edu (Mr Background)
Organization: Indiana University
Lines: 60

John Whitmore writes:
>In article <11780@pt.cs.cmu.edu> cline@PROOF.ERGO.CS.CMU.EDU (Kenneth 
>Cline) writes:

>>I am building a class-C amplifier for 2275 hz (yes - audio frequency),
>>as shown in the schematic below:

>>                                        V+
>>                                        |
>>                                    +---+---+
>>                                    3       |
>>                                    3       |
>>                         +--+--|<---3       -
>>    +-+                  |  |       3       _
>>    | |                |-+  _       3       |
>>    | |  Signal In ----|<+  ^       3       |
>>    | |                |-+  |       3       |
>> ---+ +-----------       +--+       +-------+
>>                         |
>>                       -----
>>                        ---
>>                         -

Sorry I missed the original posting.

25% duty cycle works well, and is easy to achieve with flipflops and 
gates.

Try a separate primary winding instead of a tap on the main coil; it's 
much easier to adjust the number of turns for optimum performance. 
BTW, coupling affects resonance.

An air-core coil will probably work better than ferrite unless there 
are serious size constraints.  The permeability gain of ferrite is 
only effective if the core is much longer than the coil.  

Ferrite will change the coil's inductance with temperature.  For 
reasonable values of Q at audio frequencies, bandwidth becomes _very_ 
narrow, so it's necessary to have stable components.  You may have to 
select the tuning capacitor form a whole box of like-valued 
capacitors. I have achieved best Q with high-voltage mica capacitors.

Signal strength of near-field-induction devices such as avalanche 
transceivers and cave radios is proportional to the inverse _cube_ of 
the range:  To double the range requires a 64-times power increase.  
Effort is better spent in a sophisticated receiver than in a powerful 
transmitter.

Range is proportional the _magnetic moment_ of the transmitting coil.  
Magnetic moment is the product of current, number of turns, and _area_ 
within the coil.  The best way to extend range is to use the largest 
practicable coil diameter.  The optimum "antenna" is all your wire in 
a single huge turn.  The inductance and Q of a single large turn are 
very low; such a coil needs no tuning at audio frequencies.

Reference:  _73_ magazine, February 1984, p. 42.

--

Frank Reid     reid@ucs.indiana.edu