Re: AM Modulation

On Tue, 21 Mar 2006 07:03:27 -0600, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Mon, 20 Mar 2006 08:06:47 -0800, Don Bowey <dbowey@xxxxxxxxxxx>
wrote:

On 3/19/06 5:48 PM, in article ORnTf.131$rg7.49@xxxxxxxx, "Tim Williams"
<tmoranwms@xxxxxxxxxxx> wrote:

"Tim Williams" <tmoranwms@xxxxxxxxxxx> wrote in message
news:jPnTf.130$rg7.93@xxxxxxxxxxx
Outside of saturation, constant
current devices such as pentodes and transistors will tend to have less
modulation (higher PSRR) than triodes or transistors wired for a resistive
characteristic.

In fact the perfect example is the quintessential analog voltage multiplier
circuit, a FET in the "linear" range (typically fractional volts Vdss). Id
is jointly proportional to Vds and Vg, not counting nonlinearities, which
can be smoothed out with a little help.

That's an example of a class A device (current always flowing), high-level
modulated (by varying Vds) and multiplies two signals, hence capable of
producing AM. Viola ;-)

By magic? Explain how a linear device operating linearly, will generate sum
and difference frequencies.

---
OK.

Let's say I have a nice, fast photoresistor in series with a plain
old resistor, in series with a signal generator, like this:


E1-----+-------+
| |
[POR] |
| |
E2-----+ [GEN]
| |
[LDR] |
| |
GND-----+-------+

Let's also say that the POR (Plain Old Resistor) comprises the
generator resistance plus some fixed resistance, and that the
generator is putting out a 1MHz sinusoid at a couple of volts or so.

Now let's take an LED and shine it on the LDR, like this:

+-------------+ E1-----+-------+
| +----+ | | |
| | ~|--P||S [POR] |
| |GEN1| R||E | |
| | ~|--I||C E2-----+ [GEN2]
| +----+ | | |
[DC SUPPLY] [LED] ------> [LDR] |
| | | |
+-------------+ GND----+-------+

GEN1 is another signal generator, but its output is set at 1000 Hz
and, at the moment, its output is cranked down to zero.

Now, we hook a scope from E2 to ground and adjust the DC supply so
that we get a conveniently viewable 1MHz waveform at E2, say a volt
or so.

Now, we crank up the output of GEN1 while watching the scope, and
what do we behold? A 1MHz signal actually being modulated by 1000Hz
through the miracle of linear modulation!!!


You're playing with words. Amplitude modulation is multiplication, and
whether you consider multiplication to be "linear" is up to you.

John

.

Relevant Pages

  • Re: AM Modulation
    ... a FET in the "linear" range. ... Let's also say that the POR (Plain Old Resistor) comprises the ... GEN1 is another signal generator, but its output is set at 1000 Hz ... The topic, however, is amplitude modulation, not how to make a waveform ...
    (sci.electronics.design)
  • Re: AM Modulation
    ... modulation than triodes or transistors wired for a resistive ... a FET in the "linear" range. ... Let's also say that the POR (Plain Old Resistor) comprises the ... GEN1 is another signal generator, but its output is set at 1000 Hz ...
    (sci.electronics.design)
  • Re: AM Modulation
    ... modulation than triodes or transistors wired for a resistive ... a FET in the "linear" range (typically fractional volts Vdss). ... Let's also say that the POR (Plain Old Resistor) comprises the ... GEN1 is another signal generator, but its output is set at 1000 Hz ...
    (sci.electronics.design)
  • Re: Advantages of Gray Coding for QAM
    ... restricted sense in which QAM could be considered a linear ... modulation, which is in opposition to what I've read, so naturally I'm ... Randy>> You seem to be putting this intermediate ... you consider the Gray code mapping ...
    (comp.dsp)
  • Re: Advantages of Gray Coding for QAM
    ... It does matter in the gadget world since a linear modulation requires ... modulation (e.g., PSK) does not. ... design attention and analog circuitry as QAM or even just AM. ...
    (comp.dsp)