Simple SDR receiver

Software definied radio (SDR) receivers are very popular for their simplicity and therefore low cost. The SDR receiver contains only the necessary high-frequency circuits and all other processes - amplification, filtering and demodulation takes place in digital form. In the amateur version, the use of a sound card as an A / D converter is offered. The software on your computer will take care of all the necessary tasks.

SDR receiver TinySDR

Very simple SDR receiver TinySDR described by Gintaras LY1GP. The described SDR receiver is based on this connection. It is suitable for the first acquaintance with SDR technology and the 80m band.

Schéma SDR prijímača
SDR receiver diagram

The principle of connection of the TinySDR SDR receiver

The SDR receiver consists of four circuits: input tuned circuit, mixer, HF phaser and oscillator. Compared to TinySDR, the antenna binding has changed. The transformer is wound on a two-hole core from an old balancing element (larger dimension). The main winding has 6 threads, antenna binding 1,5 thread. The LC circuit must be tuned to the required frequency, for example 3.75MHz. The capacity of the 470pF capacitor is for guidance only.

The mixer is unmodified. The diodes are schottky BAT-43.

The HF phaser consists of a 100pF capacitor and a 1k trimmer. Both components must be of good quality. Use the trimmer to set the 90 ° phase shift on the mixer.

The oscillator has undergone the most changes compared to the original circuit. An oscillator with a FET transistor is much better. FET puts less strain on the LC circuit, which contributes to stability and such an oscillator is less sensitive to operating point setting, which has a favorable effect on the spectral purity. Coil L1 has 20 threaded wire on a frame with a diameter of 10mm. The coupling coil L2 has 3 threads (on the same skeleton). Also the capacitors in the oscillator must be of good quality.

We bring the outputs I and Q cable via stereo jack 3.5 ”connector to the sound card input.

Setting up the SDR receiver

I made the settings with GDO, a two-channel oscilloscope and an auxiliary receiver. First, you need to tune the oscillator to the desired frequency. We check this with an auxiliary receiver or counter. The input circuit can be set by GDO or by setting the maximum signal from the band.

Set the phase shift of the 90 ° HF phaser (the situation occurs, when Xc = R at the operating frequency) it is only possible with a two-channel oscilloscope. Software, e.g.. Rocky can calculate the offset from the strong signals in the band and make a correction. The second trimmer 1k sets the symmetry of the mixer. The connection also works with the wrong settings, however, mirror frequency suppression is bad.

SDR software Rocky

After recovery, we can immediately listen to the band. I recommend the software Rocky, which is simple and lightly burdens the computer. Attention should be paid to the sound card. Especially on laptops, the input is only monaural! In this case, you need to reach for an external sound card.

I realized the described SDR receiver with small modifications on the printed circuit board from the direct mixing receiver. It was a pleasant pleasure, that the SDR worked on the first connection. Sensitivity is relatively low, but pleasantly pleased the selectivity and durability of the receiver. You can listen to a sample of the income .: YOUR :..

SDR capabilities and benefits

SDR offers many options. The attentive eye of an experienced technician will notice, that this connection also works in the opposite way - as a transmitter. Another option is use as a narrowband spectrum analyzer, e.g.. during a two-tone transmitter test. It is also possible to create with SDR as well antenna analyzer.

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