Preamplifier and Bandpass filter for SDR

Posted on Feb 5, 2014

The SDR hardware in its simplest form consists of a wideband switched balanced mixer and low noise LF amplifier. This simple hardware has an amazing sensitivity and linearity which is sufficient for test and even regular use. Here are some of the problems in this simple hardware: 1. The mixer is converting also the input spectrum around odd harmon

Preamplifier and Bandpass filter for SDR
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ics of the oscillator. For example the theoretical attenuation of the third harmonic mixing versus first harmonic is only 10 dB. 2. The sensitivity of this simple SDR is somewhere around -124dBm to -128 dBm at 500 Hz bandwidth (with good low noise preamplifier). But sometimes there are conditions, on higher frequencies, when the atmospheric noise is very low and the sensitivity should be above -130dBm. 3. The second order distortions are the weak point of this type of radio. To avoid these effects, an input filter is needed for out of band strong stations and also a step attenuator. 4. The switched mixer has excellent parameters when the signal source has low and resistive internal resistance. The antenna as a signal source is not the best choice even if the antenna is tuned, not to mention the random wires. The image rejection and dynamic range are drastically reduced when the signal source has reactive component. 5. There is strong leakage signal of the heterodyne oscillator at the mixer input. This signal is heard as a strong carrier if a second receiver uses the same antenna. A parallel L1, C1 circuit with high Q-factor is tuned with variable capacitor in the range from 1. 8 to 18 MHz. The signal from the low Z antenna is fed with inductive coupling with transformation ratio 1:12. The FET follower transforms the high resonance impedance into low resistive impedance which is fed to the switch mixer. The voltage gain...

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