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. A simple and very effective way to improve these radio stations is the implementation SDR technology.
I tried this method with minimal effort in the radio station 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).
With the use of SDR technology, it can be significantly better. How to do it? The MF signal needs to be taken out of the radio station. Probably the easiest way is to use NB amplifier circuits, specifically from the collector of transistor Q202. All you need is a decoupling capacitor of approx. 1nF and a connector on the rear panel. NB is a small printed circuit board with most 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.
The difference in receiving quality is significantThis 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!