A simple SDR receiver

Software definied radio (SDR) prijímače sú veľmi populárne pre svoju jednoduchosť a tým aj nízke náklady. SDR receiver totiž obsahuje len nutné vysokofrekvenčné obvody a všetky ďalšie procesy – zosilnenie, filtrovanie a demodulácia sa deje digitálnej forme. V amatérskej realizácii sa ponúka využitie zvukovej karty ako A/D prevodníka. Software na počítači sa postará o všetky potrebné úkony.

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 prijímač tvoria štyri obvody: vstupný ladený obvod, zmiešavač, VF fázovač a oscilátor. Oproti TinySDR došlo ku zmene väzby antény. transformátor 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 seen the most changes compared to the original connection. Oscilátor s FET tranzistorom je oveľa lepší. FET menej zaťažuje LC obvod, čo prispieva k stabilite a takýto oscilátor je menej citlivý na nastavenie pracovného bodu, čo sa priaznivo prejaví na spektrálnej čistote. Cievka L1 má 20 závitov drôtom na kostričke s priemerom 10mm. Väzobná cievka L2 má 3 závity (na tej istej kostričke). Taktiež kondenzátory v oscilátore musia byť kvalitné typy.

Výstupy I a Q privedieme cable through a stereo jack 3.5" connector do vstupu zvukovej karty.

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. Odporúčam 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 technician will notice that toto zapojenie funguje aj opačne – ako vysielač. Ďalšou možnosťou je využitie ako úzkopásmového spektrálneho analyzátora, napr. pri dvojtónovej skúške vysielača. Taktiež je možné s SDR vytvoriť aj anténny analyzátor.