The mixer is used to downconvert the higher RF frequency to?

Question

Quote The mixer is used to downconvert the higher RF frequency to a common Intermediate Frequency (IF). Think of a mixer as a multiplier so when you inject two frequencies the signal (Fs) and the local oscillator (LO) the output contains the sum and difference frequencies. The LO frequency is selected such that the IF is generated. If the IF was 10.7 MHz and I wanted to listen to 101 MHz, my LO could be set to 90.3 such that FS-LO = 10.7 Quote

Ya but what is the Intermediate Frequency and why do they have to downconvert the higher RF ?

Why does the mixer have to inject two frequencies ?


And what does the oscillator do ?

Answer 1

1. The intermediate frequency is the frequency created by mixing the output of the local oscillator with the output of RF amplifier. The mixing occurs in the mixer transistor whose output contains the RF frequency, the local oscillator frequency and the sum and difference of the two frequencies. The IF frequency is the difference frequency (desired carrier frequency minus local oscillator frequency) . The reason to have intermediate frequency (IF) amplifiers is so that you can use the same filters at the input to the IF amplifier(s) for all the different RF channels received. The low signal strength of the RF signal requires several stages of amplification before it can be demodulated to audio, amplified more and fed to a speaker. Using the same fixed tuned IF amplifiers for all channels eliminates the need to switch in a different filter at the input of each amplifier stage for each carrier frequency (radio station) that you wish to receive. The reason you must down convert the higher RF frequencies is because the bandpass filters at the input of each IF amplifier will not pass the higher RF frequencies. There is radios available that use only cascaded RF amplifiers but It is more practical to use IF amplifiers with fixed input filters than to use cascaded RF amplifiers with different input filters for each different RF carrier received.



2. Rather than injecting two frequencies the mixer is injected by two frequencies, the local oscillator frequency and the desired RF carrier frequency. The nonlinear mixer-amplifier output contains the local oscillator frequency, the desired RF carrier frequency, and the sum and difference of the two frequencies. The sum and difference frequencies contain the same audio modulation as the desired carrier frequency does therefore the original audio information can be recovered from the difference frequency. The bandpass filter at the input of the IF amplifiers is fixed tuned to the difference frequency and rejects the other three frequencies.



3. The local oscillator when properly tuned with the user station selector (tuner) produces a local oscillator output frequency suitable for mixing with the desired input RF carrier frequency so that the difference in the two frequencies is the IF frequency. Without the local oscillator there is no IF frequency and of course no audio output.

Answer 2

The oscillator is not a tuner, it is a tuneable oscillator. When the radio is shifted to receive a fifferent frequiecy signal, the oscillator frequency is also shifted by its coupling to the signal tuner, to maintain the mixer output at the selected intermediate frequency.

The oscillator's output is a frequency that is always separated from the incoming signal's frequency by the IF frequency. The oscillator and tuner track together.

I think you have enough information about the superhetrodyne principal. AM uses just 1 intermediate frequency, usually 4555 kHz., but as FM is in the UHF band of frequencies, a double conversionis used.The 1st. IF for FM is usually 10.7 mHz.

The basic difference between FM and AM is, in AM the amplitude of the RF signal is varied with the input signal, and the RF signal frequency does not change.

In FM, the amplitude of the RF signal is constant, and the frequency is made to vary up and down by the modulating signal, i.e. the RF signal frequency changes, swings above and below the nominal RF frequency.

The mixer does not inject 2 frequencies, 2 frequencies are injected and mixed to produce sum and difference frequencies, one of which is selected as the output intermediate frequency.

Modulation is the impression of "intelligence" on a RF carrier signal, by AM or FM as above.

To produce a signal of 10.7 mHz for your radio station on 101 mHz., the local oscillator will be running at either 90.3 mHz for low side injection to the mixer, or 111.7 for high side injection.

Answer 3

As a kid, no, my mom had an electric mixer with the rotation plate, etc. And she'd let me lick the beaters if it was some- thing I liked. I used a hand mixer when I was a newly wed though. And it is a tough job to get that crank to keep going at a steady pace. So I learned to use one as an adult instead. Luckily I didn't have the need to do alot of whipping up of things. Or was it, that I avoided things that needed whipping? Anyway, it didn't take me long to get a portable mixer and have it to do the job when I needed it. Using the old hand mixer sure made me appreciate the electric one, when I got it. Cutting the task in a shorter amount of time, for sure. It made me think of the generations before me, who had no choice in using a hand held manual mixer. They were a hardy stock!

Answer 4

One of the two frequencies injected in the mixer is the signal. The other is the oscillator output. The IF amplifier has a narrow bandwidth to amplify the downconverted signal and reject other signals.

Answer 5

You are describing dual conversion or Superheterodyne, which was invented by U.S. Army major Edwin Armstrong. See http://en.wikipedia.org/wiki/Superheterodyne_receiver



"The advantage to this method is that most of the radio's signal path has to be sensitive to only a narrow range of frequencies. Only the front end (the part before the frequency converter stage) needs to be sensitive to a wide frequency range. For example, the front end might need to be sensitive to 1–30 MHz, while the rest of the radio might need to be sensitive only to 455 kHz, a typical IF. Only one or two tuned stages need to be adjusted to track over the tuning range of the receiver; all the intermediate-frequency stages operate at a fixed frequency which need not be adjusted."