Q.Write a note on the choice of local oscillator frequency.

In a superheterodyne receiver the local oscillator beats with the incoming signal to produce the fixed IF. It can be tuned either above (high-side) or below (low-side) the signal frequency, but high-side injection is normally chosen. …

[!TLDR]

The local oscillator is set to $f_{LO}=f_s+f_{IF}$ (high side) because this needs a much smaller frequency (tuning) ratio to cover the band, simplifying the ganged tuning capacitor and tracking.

In the superheterodyne receiver the mixer combines the incoming signal frequency $f_s$ with the local oscillator frequency $f_{LO}$ and the difference is made equal to the fixed intermediate frequency $f_{IF}$. This can be arranged in two ways:

• High-side injection: $f_{LO}=f_s+f_{IF}$
• Low-side injection: $f_{LO}=f_s-f_{IF}$

The local oscillator is tuned by the same ganged capacitor that tunes the RF stage, so what matters is the ratio of its highest to lowest frequency as the band is covered. Consider the AM band $f_s = 550$ kHz to $1650$ kHz with $f_{IF}=455$ kHz.

For high-side injection the oscillator runs from $550+455=1005$ kHz to $1650+455=2105$ kHz, a ratio of about $2105/1005 \approx 2.1$.

For low-side injection it would run from $550-455=95$ kHz to $1650-455=1195$ kHz, a ratio of about $1195/95 \approx 12.6$. …