Ham Radio General Class Practice Test

If a receiver mixes a 13.800 MHz VFO with a 14.255 MHz received signal to produce a 455 kHz intermediate frequency (IF) signal, what type of interference will a 13.345 MHz signal produce in the receiver?

1. Quadrature noise

2. Image response

3. Mixer interference

4. Intermediate interference

The correct answer is: Image response

The scenario describes the mixing process in a superheterodyne receiver where a 13.800 MHz variable frequency oscillator (VFO) is combined with a received signal at 14.255 MHz. This mixing generates an intermediate frequency (IF) signal of 455 kHz. To understand the interference caused by a 13.345 MHz signal, it is important to note the concept of image frequency. An image frequency is a second frequency that can create the same IF when mixed with the local oscillator frequency. In this case, the relationship is defined by the formula: \[ \text{Image Frequency} = \text{VFO Frequency} + \text{IF Frequency} \] or \[ \text{Image Frequency} = 13.800 MHz + 455 kHz = 14.255 MHz \] Now, if you consider a signal at the lower frequency of 13.345 MHz, when it's mixed with the 13.800 MHz VFO, it results in: \[ \text{IF Frequency} = 13.800 MHz - 13.345 MHz = 455 kHz \] The 13.345 MHz signal thus generates the same intermediate frequency of 455 kHz as the desired