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I wanted to know the primary difference between an Inphase/Quadrature (I/Q) Demodulator and an I/Q Mixer.

My requirements are as follows: My input signal is a 500 Mhz bandwidth signal centred around 4.25 Ghz. I need to obtain the I and Q components of the baseband signal with respect to the Local Oscillator( of 4.25 Ghz ).

Previously a Hittite chip [HMC620](http://www.hittite.com/products/view.html/view/HMC620I/Q Mixer) which has been described as a "mixer" in the datasheet. But I am unable to find an I/Q "mixer" from big companies like Texas Instruments or Analog Devices (the companies that are easily available). If my understanding is correct, an I/Q demodulator (TI and AD ics available) should also do the job I require.Is this correct?

UPDATE 1 :

The TI I/Q demodulator chip i am talking about is TRF371135 . while the Analog Devices one is ADL5380. They do not exactly satisfy my 500Mhz bandwidth requirement, but they are the closest I could find on Digikey.

Are all I/Q demodulators I/Q mixers ? Or am I going fundamentally wrong somewhere ?

Thanks

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    \$\begingroup\$ Datasheets for the parts you're proposing to use would be helpful. TI and Analog aren't known for serving markets that use that high a frequency --- but I don't know their complete product lines... \$\endgroup\$
    – The Photon
    Commented Jan 23, 2012 at 17:19
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    \$\begingroup\$ I wonder where you can legal use 500 MHz of bandwidth :-) \$\endgroup\$
    – Kellenjb
    Commented Jan 23, 2012 at 17:21
  • \$\begingroup\$ Sorry I don't know much about this kind of RF myself, but I did find this information about what is an I/Q (de)modulator: minicircuits.com/app/MOD11-8.pdf \$\endgroup\$
    – The Photon
    Commented Jan 23, 2012 at 17:30
  • \$\begingroup\$ @Kellenjb : Yes it is, between 3.1 to 10.6 GHz as long as your signal in space satisfies power spectral density constraints ( -41.3 dBm/MHz) set by FCC. This is the basic idea behind Ultra-Wideband Communication. \$\endgroup\$
    – gururaj
    Commented Jan 23, 2012 at 17:30
  • \$\begingroup\$ Key point: "Q: Is there a difference between a modulator and demodulator at the same frequency? A: Not much. Mini-Circuits' modulators and demodulators are tested for the specified function to confirm the specs. The higher the frequency, the greater the chance of a modulator not performing as well as a demodulator." \$\endgroup\$
    – The Photon
    Commented Jan 23, 2012 at 17:31

1 Answer 1

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A mixer is just a circuit that multiplies two signals.

The purpose of a mixer is to shift a signal in the frequency domain.

Normally a mixer multiplies a signal carrying information (some range of frequencies, e.q. received signal or signal that is going to be tranmsitted) with a signal without information, consisting of exact one frequency (from the local oscillator).

An I/Q-Mixer consists of two ordinary mixers that each multiply the signal carring the information with the local oscillator signal, but one being 90° phase shifted.

A demodulator in general is a circuit that extracts the actual information of an RF signal.

There are many different demodulators (as there are many ways of modulation) and even for one particular modulation (e.g. FM, SSB) sometimes more than one type of demodulator is possible.

An I/Q demodulator is just some kind of demodulator using (among other subcircuits) an I/Q mixer as mentioned above.

Summary:
An I/Q mixer is a part of an I/Q demodulator.
An I/Q demodulator always contains an I/Q mixer, but I/Q mixers may also be used in circuits that are no demodulators (e.g. modulators).

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