A simple SDR receiver

Software defined radio (SDR) receivers are very popular for their simplicity and thus low cost. The SDR receiver contains only the necessary high-frequency circuits and all other processes - amplification, filtering and demodulation - are done digitally. In amateur implementations, it is possible to use a sound card as an A/D converter. The Software on the computer takes care of all the necessary operations.

TinySDR SDR receiver

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

Schematic of the SDR receiver

The principle of connection of the TinySDR SDR receiver

SDR receiver tvoria štyri obvody: vstupný ladený obvod, zmiešavač, VF Fázovač a oscilátor . Oproti TinySDR došlo ku zmene väzby antény. transformer it is wound on a two-hole core from an old balancing member (larger size). The main winding has 6 turns, the connection to the antenna 1.5 turns. The LC circuit needs to be tuned to the required frequency, for example 3.75MHz. The capacitor capacity of 470pF is only indicative.

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

The RF phasor consists of a 100pF capacitor and a 1k trimmer. Both parts must be quality types. A 90° phase shift on the mixer is set using the trimmer.

The oscillator has undergone the most changes compared to the original connection. The oscillator with a FET transistor is much better. The FET loads the LC circuit less, which contributes to stability and such an oscillator is less sensitive to setting the operating point, which will have a positive effect on spectral purity. The coil L1 has 20 turns of wire on a ground with a diameter of 10mm. The coupling coil L2 has 3 turns (on the same ground). Also, the capacitors in the oscillator must be of high-quality types.

We will connect the I and Q outputs via a 3.5” stereo jack connector to the sound card input.

Setting up the SDR receiver

I did the setup with a GDO, a two-channel oscilloscope and an auxiliary receiver. First, you need to tune the oscillator to the desired frequency. We check it with an auxiliary receiver or counter. The input circuit can be adjusted using GDO or by increasing the signal from the band to the maximum.

Setting the 90° phase shift of the RF phasor (the situation occurs when Xc = R at the operating frequency) is only possible with a two-channel oscilloscope. Software, e.g. Rocky can calculate the shift from strong signals on the band and make a correction. The second trimmer 1k sets the symmetry of the mixer. The wiring works even with bad Settings, but the 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 puts little strain on the computer. Attention should be paid to the sound card. Especially on laptops, the input is only monophonic! In that case, it is necessary to reach for an external sound card.

I implemented the described SDR receiver with small modifications on the printed circuit board from a direct mixing receiver. A nice treat was that the SDR was working on the first engagement. The sensitivity is relatively low, but I was pleasantly pleased with the selectivity and durability of the receiver. You can listen to a sample of the reception:

SDR options and benefits

SDR offers many possibilities. The attentive eye of an experienced Projects / Homebrew will notice that this connection also works the other way around – as a transmitter. Another possibility is to use it as a narrowband spectrum analyzer, e.g. for two-tone transmitter testing. It is also possible to create an antenna analyzer with SDR.