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March 18th 14, 12:22 AM
posted to rec.radio.shortwave
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FM radio reception at ~24MHz?
On 03/17/2014 03:10 PM, matt weber wrote:
The 'front' end of many relatively low cost, wide range receivers has very poor selectivity. The output of the Mixer stage is the SUM and the DIFFERENCE between the local oscillator frequency and the incoming signal. The result is when you tune a frequency, you are really tuning to two different frequencies. The difference between one signal and the IF and the Sum of another signal and the IF, Usually there is some sort of pre-selector, bandpass filter or semi-tuned RF amplier that effectively blocks the unwanted frequency, so there is no energy in one of the two possibly tuned frequencies For example if you have a conventional FM receiver, traditionally the Intermediate frequency is 10.7 Mhz. So if you tuned the receiver Another strategy is to 'up convert' instead of down convert. This puts much more spectrum between the sum and difference frequency. All that your experience says is that SDR probably has poor front end selectivity. Thanks Matt. Jon |
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