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Let's say you have a 1 GHz signal that you want to receive and demodulate. I am familiar with the superheterodyne receiver, where an LO of 1 GHz is mixed with the RF signal, and the output is then used.

On the other hand, it seems that we can also use a frequency divider, e.g., a divide by 2 or 4 or even 8, where the 1 GHz can be reduced to a lower value.

What is the performance difference of each? And why are frequency dividers not as commonly used as LO mixers?

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  • \$\begingroup\$ Do you speak about "Subharmonic mixers" ? It is a kind of "direct conversion". \$\endgroup\$
    – Antonio51
    Jan 15 at 9:02

2 Answers 2

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it seems that we can also use a frequency divider, e.g. a divide by 2 or 4

The disadvantage of dividing the frequency (for systems that use frequency modulation) is that when you divide the carrier frequency, you also divide the frequency deviations and, eventually, you reach a point where the number carrier cycles per logic "1" bit becomes exactly the same as for a logic "0" bit i.e. it fails to work.

The following picture hopes to show that dividing by 2 reduces the detection window available in the receiver's demodulator (called "space filter" and "mark filter" in the picture below): -

enter image description here

Clearly, dividing by 4 reduces the space and mark filters by another factor of two.

Image taken and modified from this site and from this document specifically. As you can see, for each division by two the mark and space filters required by the demodulator gets tighter and tighter to implement. This inevitably requires more complication if things start to get tight. There are some chips that do this; I've used them when demodulating 400 MHz where the chip in question implemented a divide by 4 before using a mixer. Not ideal but, a divide by 4 didn't degrade what we needed to implement.

It was a high speed data link with an FSK/FM deviation of about 15 MHz for a very bespoke aerospace application.

Some example numbers

If the centre frequency is 1 GHz and the data rate is (say) 100 kbps, for a maximum FM deviation of 100 kHz (for example), you would naturally expect to see an average of about 10,000 cycles of carrier per bit. For a logic "1" data bit this might be 11,000 cycles and for logic "0" it might be 9,000 cycles.

So, if you divided the raw modulated carrier by (say) 1,000, you'd get 11 carrier cycles for a logic "1" and 9 carrier cycles for a logic "0". In other words, you haven't got a big count difference between the two logic levels. If you decided to divide by 10,000 you wouldn't be able to distinguish between logic "1" and logic "0" at all.

Clearly if you divided by 100 instead, you'd count 110 cycles for logic "1" and 90 cycles for logic "0" and that is a more reliable count difference.

The bottom line is that each division is degrading the signal compared to superheterodyning the original modulated carrier down to a lower frequency because, the spectrum width of the down-converted signal remains exactly as it was at 1 GHz.

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With a mixer, the IF output contains the same amplitude and frequency/phase information as the original RF.

If you divide a signal down, you preserve only the frequency/phase information, the amplitude has to be stripped off prior to division.

If you have complex modulation therefore, you must use mixer downconversion, not division. Very few signalling systems use pure FM.

Even for a frequency/phase signal, there is a difference in what the two processes do. Mixer downconversion preserves the absolute frequency modulation, while reducing the carrier frequency, effectively increasing the deviation/carrier ratio. Division divides both the carrier and the modulation signal by the same amount. This reduces the recoverable signal to noise ratio, and is rarely what you want.

There are rare circumstances where a frequency demodulator is preceded by a divider. Some 'Source Analysers' do this for wideband input frequency range, where noise performance can be sacrificed, but these same instruments also implement a mixer route for good noise performance across a narrow frequency range.

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