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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Low frequency tuned circuit
Message-ID: <16133@milton.u.washington.edu>
Date: 9 Feb 91 06:40:49 GMT
References: <11780@pt.cs.cmu.edu> <16130@milton.u.washington.edu>
Organization: University of Washington, Seattle
Lines: 68


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

>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       |
> ---+ +-----------       +--+       +-------+
                          |
>                       -----
>                        ---
>                         -
        to which I would add the diodes shown; the one shunting the
MOSFET is built-in to the transistor, the other is added to prevent
the built-in diode from clamping the oscillator.

>The application is a transmitter for a locator beacon, where the
>magnetic field of the inductor is picked up by the reciever's tuned
>circuit.
        (9V battery power, low-loss oscillator design described)
>Now my question is how do I optimize the inductor in the output tuned
>circuit.  Clearly, I want the largest inductance possible while
>keeping resistance reasonably low.

        You want to maximize the Q of the circuit, while allowing
significant leakage of the magnetic field (so the detector can pick
it up).  First, the resistance of wire in the coil is the major loss
in many such circuits; minimize this by using core material of high
permeability (ferrite won't do; probably powdered iron or thin
laminated transformer steel is your best bet).  Commercial chokes
like the Miller 5500 series are a good bet (but you'd be applying
a different winding to the core).
        Yes, a tapped winding is a good idea; it's a LOT easier to
get a good high-voltage capacitor than to get a high-value capacitor
with low resistance.  Aim for a few hundred volts on that capacitor!
And, because a stick of high-permeability material can saturate,
consider putting TWO taps on, like

     +WWWWWWWW+WWWWWWWW+WWWWWWWWWWWWWWWWW+
     |        |        |                 |
   FET1      +9V     FET2                C

and driving pulses 180 degrees out-of-phase in the FETs (so the ferrite is
not driven with DC).  Yeah, it means another FET/diode and another
drive tap on the divider, but it could easily pay off in efficiency.

        Last suggestion: the inductance of the coil will change with
temperature.  In order to keep the operating frequency fixed, you
may find it necessary to either (1) degrade the Q (which loses
efficiency), or (2) choose a capacitor with a compensating
characteristic change with temperature.  The latter is highly
recommended (and many makers of inductors and magnetic materials
have specified their materials to make this easy).  Temperature
compensating capacitors (usually high-voltage ceramic types)
are commonly available.

        John Whitmore