Typical large signal performance of direct conversion receiverWith the IQ mixer and the anti alias filters and a Delta44 audio board a bandwidth of well above 90kHz is obtained. The figure below shows at which frequencies a near saturating signal is received as spurs. The dsp radio was calibrated before these images were produced.A Tektronix TR503 was used to sweep 120kHz across the receiver passband which was centered at 7.050MHz. The receiver noise floor is near 0dB on the scale but the TR503 noise floor is about 30 dB higher, limiting the number of harmonics visible.
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This image shows that the second harmonic of the audio frequency is
the dominating spur.
For interference signals close to the center the second harmonic is
suppressed by about 70dB but when the audio signal approaches
45kHz the suppression is less good.
The second order distorsion may have contributions from the mixers,
the filters and the A/D board.
The spurs in the main part of the passband are 50dB above the noise floor when the signal is at 115dB. To not degrade S/N a preamplifier must be allowed to raise the noise floor by 15dB so in real life the spur is at 35dB when the signal is near saturation at 100dB. To lower the spur by 35dB the signal has to be lowered by 17.5dB so the spur free dynamic range will be slightly better than 80dB in a typical low noise VHF application.
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