Dear Expert

May I know why in the design of the synthesizer or other circuit for the RF Communication there was always the frequency mixer in there?
Why we need to mix the input frequency with the other frequency? What is the purpose ?

Please advice.

Thank you very much.

 

A frequency mixer is another way to say multiplier. When two frequencies, are mixed or multiplied, the result is the sum and difference of the frequencies. This all goes back to the trig identity:

sin(x) sin(y) = (cos(x-y) - cos(x+y)) / 2

which says that when you multiply two frequencies together, the resulting output is the sum and difference of the two frequencies. This is important for demodulation of AM signals. One of the two input signals is the modulated signal, whilst the other is the carrier. Therefore, the output signal from the identity is the carrier minus the modulated signal, which is the intelligence signal (what was modulated at the transmitter, i.e. what you want to listen to), and a much higher representation, which is filtered out. If this sounds a bit cryptic, research the demodulation of AM.

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Mixers are used for frequency conversion and are critical components in modern radio frequency (RF) systems. A mixer converts RF power at one frequency into power at another frequency to make signal processing easier and also inexpensive. A fundamental reason for frequency conversion is to allow amplification of the received signal at a frequency other than the RF, or the audio, frequency.

The ideal mixer is a device which multiplies two input signals. If the inputs are sinusoids, the ideal mixer output is the sum and difference frequencies.
Typically, either the sum, or the difference, frequency is removed with a filter.

Attached Images

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Hi Dragonball 6,

It is difficult to construct a tunable filter with a Q of 10000.
But it is easy to build a intermediate amplifier with a very narrow
bandwith and also a very high gain because we can make use of
ceramic or crystal filters. The reason why we use mixers in receivers
is selectivity.

on1aag.

 

A friend of mine keeps telling that there were adjustable bandwidth filters at IF range, appears while one section of gang cap adjusts upper frequency to lower side, the other one opposite trying to increase the lower side , the net result being reduction of bandwidth. I don't exactly remember whether RACAL has used this technique in their communication receivers.

__________________
Regards,
Sarma.

 

Dear All

Thank all the valuable info. It helps me to learn more.
As raviram87's attached picture, there are RF signal and LO signal mixing and output the IF signal.
RF signal = Modulated signal from the transmitter.
(i) May I know how about the LO signal?
(ii) Is it the LO signal is generated by our self with specific center frequency that we want?
(iii) Is it IF signal is the frequency that the output signal with same power level but in lower frequency?

Please advise.

Thank you.

Peter

 

Hi Dragonball_6,

The frequency of the LO will determine which (single) frequency
will be receive, not a entire frequency span. If and how much
of the sidebands (the modulation) will reach the demodulator
depends on the bandwidth of the IF-amplifier.

The output power of the mixer depends on the type of mixer
selected, it can be -6 dB if the mixer is "passive" like a diode
mixer (DBM) or it can be up to +20 dB when the mixer is active.
Some mixers have even a internal buffer/amplifier for the LO signal.
It will all depend on the type of mixer used.

The frequency of the signal from the mixer doesn't have to be
lower than the frequency of the received signal, it can be higher
as well. Most shortwave receivers (0 to 30 MHz) use upconversion
which means that the frequency of the first IF is well above the
frequency of the received signal. They usually choose 70 or 90 MHz
for the first IF, 21,4 MHz or 10,7 MHz for the second IF and 455 kHz
for the third IF.

on1aag.

 

There is a book on radio receivers which could be of general help, free on line at www.mikroe.com

__________________
Regards,
Sarma.