multiplier

How 2 signals can multiplies using transistor as active component, say the 2 signals to be multiplied are one with frequency=1KHz and other with freq=1MHz.

You need to understand a circuit called a doubly balanced mixer. It multiplies two signals together and produces components at the original two frequencies, (1 kHz, 1 MHz.). It also produces components that are the sum and difference of the two frequencies. These are called the upper and lower sidebands. They are at 1.001 MHz. and at 0.999 MHz.

Now, for the $64,000 question -- what happens when you run the upper sideband at 1.001 MHz. and local oscillator of 1.000 MHz. through the multiplier? What comes out?

electronspeed said:

How 2 signals can multiplies using transistor as active component, say the 2 signals to be multiplied are one with frequency=1KHz and other with freq=1MHz.

Click to expand...

For the production of sum and difference frequencies with no real multiplier accuracy, just operate the transistor bias in a non-linear part of the transconductance curve. That would be near saturation or near cutoff. Most mixers use them near cutoff, the more efficient (less power) option.

For accurate multiplier output to 10MHz, use a linear multiplier like the MPY634 IC:

https://www.electro-tech-online.com/custompdfs/2009/09/mpy634.pdf

When used as a 4 quadrant multiplier, it produces sum and difference frequencies and suppresses the original signals, like DSB transmissions. Give it a bit of DC bias on one input and it produces AM, complete with the original carrier.

Papabravo: Add a low pass filter to your $64,000 question and you get your demodulated output.

Bob

Give Mr. Bob Scott a kewpie doll, a long cigar, and...oh say $64,000.
Actually I know you know the answer, I was kinda hoping the OP might make the effort.

Papabravo said:

You need to understand a circuit called a doubly balanced mixer. It multiplies two signals together and produces components at the original two frequencies, (1 kHz, 1 MHz.). It also produces components that are the sum and difference of the two frequencies. These are called the upper and lower sidebands. They are at 1.001 MHz. and at 0.999 MHz.

Now, for the $64,000 question -- what happens when you run the upper sideband at 1.001 MHz. and local oscillator of 1.000 MHz. through the multiplier? What comes out?

Click to expand...

Like the pulsating of two sounds just barley out of frequency I'm guessing.

Njguy said:

Like the pulsating of two sounds just barley out of frequency I'm guessing.

Click to expand...

Hetrodyning (or Heterodyning) is the term you're looking for.