Image Frequency Calculator

Calculator Inputs

Desired RF Frequency (MHz)

Intermediate Frequency (MHz)

Injection Side

Loaded Q of Each RF Stage

Number of RF Tuned Stages

Channel Bandwidth (kHz)

Target Image Rejection (dB)

Reset

Example Data Table

Case	Desired RF (MHz)	IF (MHz)	Injection	Loaded Q	Stages	Channel BW (kHz)
FM Broadcast Front-End	98.7	10.7	High-side	85	2	200
VHF Communication Receiver	145.5	10.7	High-side	110	3	25
HF Receiver Study	14.2	0.455	Low-side	70	2	6

Formula Used

1) Local oscillator frequency
High-side injection: f_LO = f_RF + f_IF
Low-side injection: f_LO = f_RF - f_IF

2) Image frequency
High-side injection: f_IMG = f_RF + 2f_IF
Low-side injection: f_IMG = f_RF - 2f_IF

3) Image separation
Δf = |f_IMG - f_RF| = 2f_IF

4) Approximate tuned front-end bandwidth
BW ≈ f_RF / Q

5) Single-stage image rejection ratio
ρ = (f_IMG / f_RF) - (f_RF / f_IMG)
IRR_single = √(1 + (Qρ)²)

6) Total image rejection for multiple identical tuned stages
IRR_total = (IRR_single)ⁿ
IRR(dB) = 20 log₁₀(IRR)

These equations are commonly used for superheterodyne front-end planning. They help estimate whether a chosen IF, tuned-stage Q, and injection strategy will reject the mirror response adequately.

How to Use This Calculator

Enter the desired received signal frequency in MHz.
Enter the selected intermediate frequency in MHz.
Choose high-side or low-side oscillator injection.
Provide the loaded Q for each tuned RF stage.
Enter the number of tuned RF stages in the receiver front-end.
Set the practical channel bandwidth in kHz.
Enter the target image rejection level in dB.
Press the calculate button to show results above the form.
Review image frequency, LO frequency, separation, rejection, and design assessment.
Use CSV or PDF export to save the computed engineering summary.

Frequently Asked Questions

1) What is image frequency in a receiver?

Image frequency is an unwanted signal that mixes with the same local oscillator and lands at the same intermediate frequency as the desired signal.

2) Why does image rejection matter?

Poor image rejection allows an unwanted signal to appear inside the IF chain. That can reduce sensitivity, distort audio or data, and create false tuning results.

3) What does high-side injection mean?

High-side injection means the local oscillator is above the desired RF signal. In that case, the image frequency appears above the local oscillator by one more IF.

4) What does low-side injection mean?

Low-side injection places the oscillator below the desired RF signal. The image then appears lower than the desired signal by twice the IF, if that result remains physically positive.

5) How does loaded Q affect image rejection?

Higher loaded Q narrows the tuned front-end bandwidth. That increases attenuation at the image frequency and usually improves predicted image rejection performance.

6) Why include multiple RF tuned stages?

Multiple tuned stages multiply the overall image rejection ratio. They can make a design meet strict rejection targets without forcing a single stage to use impractically high Q.

7) Why is channel bandwidth shown separately?

Channel bandwidth helps compare practical passband width against image separation. A wider passband may leave less frequency margin, especially in compact receiver designs.

8) When should I change the IF value?

Increase IF when you want wider image spacing. That often improves front-end rejection, though it may affect selectivity planning, filter choice, and conversion architecture.