Discussion in 'Wireless & RF Design' started by nqchanh194, Jun 20, 2014.

1. ### nqchanh194 Thread Starter New Member

Jul 5, 2013
27
1
I'm reading some datasheets of UHF receiver, and the "image rejection" parameter makes me confused. For example: if an UHF receiver get the image rejection value is 50 dB in 434 MHz, it means that the ratio of signal at carrier frequency and the one at image frequency in the actual running must be higher than 50 dB in order to get the desired result ?

2. ### vk6zgo Active Member

Jul 21, 2012
677
85
No!
For a wanted signal & an unwanted image signal of the same level at the input of the receiver,the unwanted signal appears to be 50dB lower than the wanted signal.

Or to put it another way,for the interfering signal to be detected at the same level as the wanted signal,it would have to be increased in power by 50dB.

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3. ### MrChips Moderator

Oct 2, 2009
12,658
3,461

A superhet radio mixes the signal from a local oscillator with the incoming RF signal to produce an intermediate frequency (IF).

What is image frequency?

The image frequency is an undesired input frequency that is capable of producing the same IF as the desired input frequency.

Here is an example. Suppose your IF is 50MHz and the signal you are trying to receive is 450MHz. You set the local oscillator to 500MHz.

Signals at both 450MHz and 550MHz will produce 50MHz IF. The signal at 550MHz is called the image frequency. Both signals will get through to the IF amplifier.

50bB image rejection means that the RF filter preceding the mixer has to suppress the image RF (550MHz) by 50dB compared with the desired signal at 450MHz.

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