cline@PROOF.ERGO.CS.CMU.EDU (Kenneth Cline) (02/05/91)
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 _
| | |-+ 3 |
| | Signal In ----|<+ 3 |
| | |-+ 3 |
---+ +----------- | +-------+
-----
---
-
The application is a transmitter for a locator beacon, where the
magnetic field of the inductor is picked up by the reciever's tuned
citcuit. This allows a micro-power transmitter to operate over a
distance of 50 meters for about 1000 hours using a nine volt alkaline
battery. I want to mazimize the magnetic field generated, and
minimize power input in a highly reliable circuit, while keeping size
and weight down (it should fit in a large shirt pocket).
Before I get to my real question, I will describe my circuit in case
anyone has any better suggestions. It starts with a crystal fet
oscillator, which is divided down to 2275 hz with a cmos divider.
This signal is shaped into a low duty cycle pulse (with adjustable on
time for tuning purposes) by a couple more cmos gates, a resistor and
capacitor, and the mosfet output transistor is switched in the class-C
configuration shown.
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. So far my best coil is 32 gauge
wire wrapped on a ferrite rod (from an AM antenna). This has
acceptable size and weight, along with reasonable values for
inductance (50mH) and resistance (10 Ohms). I am doing something
right, since the LC circuit develops several times the power supply
voltage, and the signal can be detected at a moderate distance given
reasonable low power input.
I am convinced that a tapped inductor is necessary, since otherwise,
the signal clips when it reaches ground (even with a bipolar
transistor). I have tried various configurations, and it appears that
tapping the center of the inductor yields the cleanest signal across
the circuit. It there a reason for this? It this necessary for
optimum efficiency? Theoretically, what is going on in this circuit?
Also, is an AM radio ferrite core acceptable here? I can cook my own
core out of low freq ferrite powder, but this is a pain and I would
prefer to avoid doing so unless really necessary.
Finally, the coil produces an audible buzz. I assume this is due to
vibrating wires and will go away when I pot the final version in
epoxy.
Thanks for any information.
Kenwhit@milton.u.washington.edu (John Whitmore) (02/09/91)
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 | > ---+ +----------- +--+ +-------+ > ----- > --- > - 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 both positive and negative (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
whit@milton.u.washington.edu (John Whitmore) (02/09/91)
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
commgrp@silver.ucs.indiana.edu (BACS Data Communications Group) (02/10/91)
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