 | Radio and Electronics (DED Philippinen, 66 p.) |
 |  | 6. RECEPTION OF RADIOSIGNALS (AM - TYPE) |
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6.8 INCIDENTAL REMARK ON MIXING FREQUENCIES
If two signals with different frequencies are mixed in a MIXERSTAGE there will appear several new signals at the output: beside the two original frequencies we will find two new frequencies
fa = foscillator + fradio
fb = foscillator - fradio
for example:
In case of a wanted RF-signal of 1.52 MHz and an oscillator frequency of 1.976 MHz the frequencies fa and fb would be:
fa = 1.976MHz + 1.52MHz = 3.496MHz
fb = 1.976MHz - 1.52MHz = 0.456MHz
If we make sure that the distance between the frequency of the tuned circuit and the frequency of the oscillator is kept constant, we will find for every radiofrequency the same frequency at the output.
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f1 (MHz) |
f2 (MHz) |
f3 (MHz) |
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2.0 |
2.452 |
8.452 |
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2.5 |
2.952 |
8.452 |
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2.75 |
3.202 | |
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3.01 |
3.462 | |
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5.28 | |
8.452 |
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6.682 | |
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9.852 |
8.452 |
Therefore we will have at the output of the mixerstage always a constant frequency which is called now the INTERMEDIATE FREQUENCY which is derived from the oscillatorfrequency and the radiosignal and which is therefore still modulated with the audiosignal carried by the radiosignal. Figure 44. shows how this always constant intermediate frequency is achieved.
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