[Laser] Ultrasonic Modulator?

[TWOSIG at aol.com]

Kurt
 
First, let me suggest that you use a sharp cutoff low pass filter  filter on 
the voice component.  The cut-off frequency should be  less than one half, and 
I would recommend less than one third of the carrier  frequency.  I do not 
know what microphone you are planning to use, but I  would guess that it will 
pick up lots of energy into the frequency spectrum that  you are trying to use 
for a carrier.  ( I am betting that it will  oscillate. )
 
Second, I suggest that you use the highest frequency for your  carrier.  The 
power levels that you are suggesting would be very painful  for those of us 
that can still hear.  Even if you limit the voice component  to 3KHz with a 
brick wall filter, the mixer products of that with a 20KHz  carrier would include 
sounds down to 17KHz.  I still have (after more than  five decades) at least 
80 percent of normal hearing in that range.  If you  can push the carrier above 
30KHz, it would be better for your neighbors.
 
Third, reconsider your modulation scheme.  A simple 555 timer can be  used to 
frequency modulate, or even pulse width modulate in the 30 to 50 KHz  range.  
There are also phase lock loop circuits that will work there.   If this seems 
acceptable, then search for the circuits for an FM wireless  intercom.  The 
idea with those was to use a 30 plus KHz carrier which was  coupled to the AC 
house wiring.  You would just couple to the audio  amp.   ( You might even find 
a commercial unit at a garage sale.  )
 
 
If you still want an AM modulator, I have a suggestion for a circuit, but  it 
will only work for you if you do not care about generating spurious signals  
at the odd harmonics of the carrier frequency.  The reason is that the  
simplest way to generate the carrier is with a 555 timer circuit, and it will  
produce square waves that are rich in harmonic content.  The modulator will  
produce modulated waveforms in that part of the frequency spectrum as  well.  This 
circuit is based on what I use for AM modulation of an LED for  my light 
communication demonstration.
 
In my circuit I use 324 op amps to produce an audio signal biased at 6  
volts, which is fed to through two 10K resistors to ground.  The common  point of 
the two resistors provide half the voltage of the signal to the  non-inverting 
input of one op amp.  The output of the amp is fed through a  curent limiting 
resistor of 1K to the base of a 2N2222.  The emitter of the  transistor has a 
10K resistor to ground and a 100 ohm resistor and a diode to  the output of 
the 555.  There is also a line from the emitter to the  inverting input of the 
op amp for feedback.
 
I use an LED from the positive supply line to the collector of the 2N2222,  
but you would use a transformer to couple to the audio amp.
 
The circuit works this way.  With no signal, the 6 volt bias goes  throught 
the divider to put 3 volts on the input of the op amp, which then  drives the 
transistor to conduct.  If the 555 output is in the high state  the diode 
blocks any current from it.  The transistor then only sees the  10K path to ground 
and will provide only 0.3 milliamp, which is all that is  needed to develope 3 
volts across the resistor.  The 3 volts are fed back  to the op amp.  If the 
555 output goes low, the 100 ohm resistor is now in  parallel, and needs an 
additional 30 milliamp to satisfy the op amp  feedback.  With no audio input, 
the circuit provides a steady 300 microamp  of current and 30 milliamp pulses 
through the primary of the transformer.
 
Now imagine what happens if you have an audio signal comming in on the 6  
volt bias.  The op am will try to match the input voltage ( and please note  that 
the op amp never sees the pulse rate of the 555, it could be an RF signal  as 
long as the 2N2222 can respond to the pulses ).  A 4 volt peak sine wave  in 
would swing from 2 volts to 10 volts, resulting in the current through the  
transformer going from 100 microamps to 10 milliamps on some pulses, then up to  
500 microamps to 50 milliamps.
 
You could use an opto-isolator instead of the transformer, or some  other way 
to convert the current pulses to the input you need for your audio  amp.
 
Oh.  You will also get the base band audio signal as well as the  carrier 
frequency and all the odd harmonics.   This would be a really  lousey circuit if 
used for RF, but it works for my light communication  demo.  If you can use 
it, I am glad to share.
 
 
Best Wishes.
 
James 
N5GUI
 
 
 
 
In a message dated 4/25/2006 6:50:53 PM Central Standard Time,  
kd7jyk at earthlink.net writes:

I am  looking for a circuit where I can mix a VFO from around 10 - 30 KHz
with a  microphone.  Nothing fancy, AM OK.  This group is as close as I  can
think of for this project, I want to super-impose my voice on an  ultrasonic
carrier and send it a distance using some high power tweeters I  have.  I
have tracked down numerous sources of amplifiers that will  cover this audio
range up to several tens of Watts, looking for a  modulator.

Any ideas?

Kurt