Image frequency

As a mirror frequency (even mirror receiving frequency ) is called the superheterodyne receiver the unwanted second reception frequency in the distance of twice the intermediate frequency f IF to the desired receive frequency.

Disturbing image frequency receiver can be suppressed

• By sufficiently low bandwidth or high selectivity of the filter between the antenna and the mixer or
• By compensating for the phase process.

Cause

The superheterodyne receiver, the reception frequency is mixed with the frequency of the local oscillator to a generally lower intermediate frequency for further processing:

The reason is that the lower intermediate frequency can be easily filtered and amplified.

However, the filter at the mixer output can distinguish no sign, so would a second combination

Generate the same intermediate frequency. Generating the oscillator frequency 2.0 MHz, can in an intermediate frequency of 455 kHz, and the lack of pre-selection, the two frequencies

And

Be the same well received. Both reception frequencies differ by itself.

This ambiguity is usually undesirable, so you have to select the desired frequency at least one sufficiently separated sharp filter ( tuned circuit ).

Image frequency spacing

In many cases, the intermediate frequency is below the frequency of the local oscillator. Then is the image frequency spacing

And corresponds to twice the intermediate frequency.

Is the intermediate frequency but above the beat frequency of the image frequency distance

Image Rejection

The ratio of the signal at the reception frequency and the image frequency is referred to as image rejection or image rejection and belongs to the far-off. In professional receivers values ​​between 60 and 70 dB are common in consumer electronics devices, the values ​​can also be substantially lower.

Phase process

Each mixer operation results in two sidebands and. The unwanted can with the IQ process without the use of filters, which are composed of resonant circuits, extinguish. Add to that the mixed signal yet another added, which is phase rotated to delete frequency range. This is added to the first mixing signal or subtracted - depending on which one wants to delete the two mixing frequencies.

As shown in the picture, you need an oscillator providing two 90 ° offset signals that control two separate mixer. In addition, it takes a delay element, which either rotates the input signal or one of the mixed signals by 90 °. Addition or subtraction of two mixed signals either deletes or ( constructive or destructive interference ).

The method is particularly suitable for digital signal processing, or at the highest frequencies as in the LNB. The disadvantage is the increased noise.