The vast majority of manufactured SSB and CW radio stations use crystal filters. These are a key factor determining characteristics such as selectivity and receiver resilience. In addition, age degrades the parameters of such filters. Jednoduchým a veľmi účinným spôsobom ako zlepšiť tieto rádiostanice je implementácia SDR technológie.
Vyskúšal som tento spôsob s minimálnym úsilím v rádiostanici President Lincoln. It is an extended radio station. From the circuitry, we learn that it uses a crystal filter at a frequency of 10.695MHz for SSB and CW. Although the filter is 8-pole, it does not exhibit excellent parameters, as every user of this radio station will confirm. The bandwidth is: 2.2kHz (-6dB); 4.9kHz (-15dB) and 8kHz (-60dB).
S využitím SDR technológie to môže byť výrazne lepšie. Ako na to? Z rádiostanice je potrebné vyviesť MF signál. Asi najľahší spôsob je využiť obvody zosilňovačov NB, konkrétne z kolektoru tranzistoru Q202. Stačí na to oddeľovací kondenzátor cca. 1nF a konektor na zadnom paneli. NB je malý plošný spoj s väčšinou SMD components mounted vertically on the main PCB. The connection point can be identified by the 1.5k resistor. NB will need to be turned on, but we will not be interested in it when listening via SDR.
The signal will be fed into the SDR receiver tuned to the intermediate frequency. I used SDR Ensemble, but any other type of receiver will do. Since it was removed from the device before the filter, we can see a wider spectrum from the band on the monitor and thus monitor the occurrence of signals.
Rozdiel v kvalite príjmu je markantnýThis effect on the shape of the 2.7kHz filter is the buffer setting in PowerSDR:
This description is not a complete guide, but it would like to indicate the benefits of SDR. The more skillful could, for example, integrate SDR receiver directly to the radio station or find another connection point. So don't be afraid to experiment with SDR, it's worth it!