However when the radio receiver has a fixed frequency first local oscillator this is not easy to ensure as it will sweep over a band of frequencies. Normally intermediate frequencies are chosen so that there are likely to be no very large signals present which might cause problems. IF BreakthroughĪnother problem which can occur with a superhet occurs when signals from the antenna break through the RF sections of the set and directly enter the IF stages of the radio receiver. Typically it falls with increasing frequency because the percentage frequency difference between the wanted and image signals is smaller. This is because the level of rejection will vary according to the frequency in use. The frequency at which the measurement is made also has to be included. it would be 1/1000 lower in terms of voltage or 1/1000000 lower in terms of power. This means that if signals of the same strength were present on the wanted frequency and the image frequency, then the image signal would be 60 dB lower than the wanted one, i.e. The image rejection of a receiver will be specified as the ratio between the wanted and image signals expressed in decibels (dB)at a certain operating frequency. The specification compares the levels of signals of equal strength on the wanted and image frequencies, quoting the level of rejection of the unwanted signal. It is clearly important to specify the level of image rejection. In view of this it is very important to reduce the image response to acceptable levels, particularly for exacting radio communications applications. When they interfere heterodyne notes will be heard and as the receiver is tuned, the pitch of the signals will change. In addition to this the image signals will "tune" in the opposite direction to the wanted ones. This means that unwanted signals are received along with the wanted ones and this means that the levels of interference will be higher than those with a high level of image rejection. When a radio receiver has a poor level image rejection signals which should not be received as they are on the image will pass through the IF stages along with the required ones. In view of this, radio receivers to be used in high performance radio communications applications need to have a good image rejection performance. Superheterodyne receiver image is twice the IF away from the wanted signal Effect of poor image rejectionĪ receiver with a poor level of image rejection will suffer from much higher levels of interference than one with a high level of image rejection. In other words with an IF at 1 MHz, the image will be 2 MHz away from the wanted frequency. These will be separated from the wanted channel by a frequency equal to twice the IF. It does not need to reject signals on adjacent channels, but instead it needs to reject signals on the image frequency. It is clearly unacceptable to receive signals on two frequencies at the same time and it is possible to remove the unwanted one by the addition of a tuned circuit prior to the mixer RF tuned circuit removes image in a superheterodyne receiverįortunately this tuned circuit does not need to be excessively sharp. However if a signal at 4 MHz is also able to produce an output at 1 MHz. For example with the local oscillator set to 5 MHz and with an IF of 1 MHz it can be seen that a signal at 6 MHz mixes with the local oscillator to produce a signal at 1 MHz that will pass through the IF filter. The basic concept of the superhet radio means that it is possible for two signals to enter the intermediate frequency, IF, amplifier. Read more about the Superheterodyne Radio. By using a variable frequency local oscillator, a fixed frequency intermediate frequency filter can be used. In this way the signals can be converted in frequency to an intermediate frequency where they can be filtered. The superheterodyne radio uses a technique where incoming signals are mixed or multiplied with the signal from an internally generated local oscillator.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |