Element 3 Topic 44: Mixers
3-44 F1 What is the image frequency if the normal channel is 151.000 MHz, the IF is operating at 11.000 MHz, and the LO is at 140.000 MHz?
129.000 MHz. Because mixing produces sum and difference frequencies of the received signals and the local oscillator signal, there is an opportunity for interfering signals that appear at the RF input to also produce intermediate frequency signals at the mixer output. In this problem, there is an image frequency that is greater than the station frequency by twice the intermediate frequency. The local oscillator is LOWER than the incoming frequency, so the image frequency equals the incoming frequency minus 2 times the intermediate frequency: 151 MHz - 11 MHz - 11 MHz = the correct answer at 129 MHz.
3-44 F5 If a receiver mixes a 13.8 MHz VFO with a 14.255 MHz receive signal to produce a 455 kHz intermediate frequency signal, what type of interference will a 13.345 MHz signal produce in the receiver?
An image response. When two signals are mixed, we get two primary products out: the sum of the two signals, and the difference between the two signals. In a superheterodyne receiver, where we use mixing to convert incoming signals to an I.F. (intermediate frequency) only one of these two products is desirable. the unwanted one is known as an image. In a superheterodyne receiver the image frequency is always twice the I.F. frequency away from the desired frequency. If our receiver has an I.F. frequency of 455 kHz, the image frequency is going to be 910 kHz away from our desired frequency. The image frequency for a radio signal at 14.255 MHz is going to be 910 kHz (0.910 MHz) away from 14.255. 14.255 minus 0.910 = 13.345 MHz.
3-44 F3 In what radio stage is the image frequency normally rejected?
RF. The radio frequency stage in any communications receiver has a double duty -- offer sensitivity to weak stations on the desired frequency, and provide plenty of selectivity to strong stations above and below the desired station frequency. The RF stage must operate linear, Class A, minimizing intermodulation interference problems. In the modern VHF or UHF communications receiver, the RF stage may also include helical resonators in order to minimize intermodulation and the problems of out-of-band image frequency response.
3-44 F6 What might occur in a receiver if excessive amounts of signal energy overdrive the mixer circuit?
Spurious mixer products are generated. Any time we overdrive the RF input, or the mixer circuit, the overdriven mixer will produce serious spurious products.
3-44 F2 What is the mixing process in a radio receiver?
The combination of two signals to produce sum and difference frequencies. Inside two-way radio equipment are several stages of mixers that combine two input signals to produce sum and difference frequencies. These two inputs are the desired RF signal and the local oscillator (LO) signal. One of the output signals becomes the intermediate frequency (IF) signal inside the receiver section.
3-44 F4 What are the principal frequencies that appear at the output of a mixer circuit?
The original frequencies and the sum and difference frequencies. The output of a mixer circuit is your oiginal two frequencies, and the sum and difference frequencies. The more elaborate the transceiver, the more mixing stages found in the sets. This helps filter out unwanted signals or phantom signals that could cause interference. The output of a mixer circuit is your original two frequencies, and the sum and difference frequencies. The more elaborate the transceiver, the more mixing stages found in the set. This helps filter out unwanted signals or phantom signals that could cause interference. There are also BALANCED and DOUBLE BALANCED mixers, which reject either one or both of the original frequencies, respectively. Double balanced mixers, DBMs, have become nearly standard in most modern receiver designs. However, a traditional mixer produces all four products, as well as many other minor products, of varying levels of importance. See Block diagram of a mixer circuit. p 139 GROL+RADAR Signal A - Usually the information signal, Example: modulated signal received by radio antenna. Signal B - Usually a single frequency signal from a circuit called a local oscillator. Usually larger in amplitude than signal A. Signal C - An output signal at a frequency that is the difference between the frequencies of Signal A and Signal B. If signal A is modulated with information, Signal C is modulated with information the same as Signal A.
