How do you calculate what to frequency to use in the Local oscillator for a 2.4Ghz Transciever

by Kriti Vaish   Last Updated May 15, 2019 22:25 PM

I am designing a 2.4Ghz transciever and there is a confusion regarding the frequency of the Local oscillator to be used. Should the value of it be somewhere close to 2.4Ghz itself or should be in the Mhz range? And secondly will this value of the local oscillator change for the receiving part( where down conversion is reuired) and the transmitting part ( where up conversion of frequency to 2.4Ghz is required) ?



Answers 3


The answer depends on what type of transceiver you're designing !

If it is a direct conversion ( also called zero-IF) transceiver then the LO (local oscillator) frequency will be the same as Frf, the frequency you want to receive or transmit.

If you're designing a superheterodyne transceiver then LO will not be equal to Frf, the LO will be running at Frf + Fif or Frf - Fif Where Fif is the intermediate frequency you're mixing to.

Both architectures have their pros and cons. Nowadays direct conversion transceivers are the most common I think as these used in almost all transceivers in smartphones for example.

Bimpelrekkie
Bimpelrekkie
September 10, 2015 07:49 AM

I'm not going to give an answer based on your requirement but instead explain a little about the 100MHz commercial FM broadcast band.

It generally ranges from 88 to 108 MHz (a 20 MHz span) and the Intermediate Frequency (IF) is chosen to be at 10.7 MHz (slightly higher than twice the range of 20 MHz). With an IF of 10.7 MHz the local oscillator (LO) can be 10.7MHz lower (or higher) than the signal to be received.

For instance, if you wish to tune a radio station at 88 MHz then the LO will need to be 77.3 MHz (or 98.7MHz) and here's the subtlety....

A LO frequency of 98.7MHz could also "pull-in" a radio transmission that is transmitted at 109.4MHz and that is why the FM broadcast band is limited to 20MHz with an IF of slightly larger than half of this. Clearly nothing is transmitted at 109.4MHz that has the power of commercial radio hence, having an IF of 10.7 MHz is just enough to ensure full coverage without double reception.

So, the LO frequency depends on IF and IF depends on the band of frequencies you want to receive.

Regarding up-conversion and, dependent on what your modulating signal is, you might not use a LO and directly convert to your transmission frequency. A lot of radio receivers at 2.4 GHz directly down-convert without using a LO.

Andy aka
Andy aka
September 10, 2015 07:58 AM

I am also trying to receive 802.11x wifi signals, in a bug detector context, purely for self education purposes.

I have a mixer, MAX2682EUT, rated for 2400MHz to 2500MHz.

The datasheets of various products aimed at a 2.4GHz WiFi indicate a 240MHz IF its commonly used. This is slightly more than double the 100MHz bandwidth, which seems ideal.

I am looking for a suitable VCO to provide the LO input at Frf - Fif, that is to say, from 2160MHz to 2260MHz.

This should down-convert the WiFi signal to 240MHz IF.

All good so far....

The 240MHz IF is within the range of my logarithmic RF detector module, so that's workable. I'm using that as the backbone of the detector.

I am having trouble finding a voltage-controlled oscillator to generate my 2160MGz to 2260MHz LO.

The VCO DATASHEETS I have found that indicate they're suitable for applications in the 2.4GHz ISM band, are way below my 2.16GHz LO frequency. They seem to go up to 600MHz.

Am I correct in assuming that these are for upconverting, and that I should continue searching for a matching VCO?

Should I be looking at 3rd order or higher products, as I am not going to demodulate the signal (at this time)?

Could I use a fixed LO signal to downconvert the entire band to a lower side band which is easier to down-convert? This might be possible as the RF band width is only 4%.(100MHz/2500MHz).

Is there anyone who can put me straight on this?

(My current RF knowledge is sources mostly from W2AEW videos on YouTube.)

Jimbo
Jimbo
May 15, 2019 22:17 PM

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