The President Lincoln is a popular 11/10-meter radio. From the circuit diagram, we can see that it uses a crystal filter at 10.695 MHz for SSB and CW. Although the filter is an 8-pole design, it does not have excellent performance, as any user of this radio will confirm. The bandwidth is: 2.2 kHz (-6 dB); 4.9 kHz (-15 dB), and 8 kHz (-60 dB). The solution is to connect an SDR to this radio, which not only provides better filters but also a panadapter that allows you to visually monitor activity on the band.
Dočítate sa:
Tapping the Intermediate Frequency from the Radio for SDR Connection
The IF signal must be tapped from the radio. Probably the easiest way is to use the NB amplifier circuits, specifically from the collector of transistor Q202. All you need is a decoupling capacitor of approximately 1 nF and a connector on the rear panel. The NB is a small printed circuit board with mostly SMD components mounted vertically on the main PCB. The connection point can be identified by the 1.5k resistor. The NB will need to be turned on during operation, but that won’t really concern us when listening via the SDR.
SDR connection point in the President Lincoln
Processing in the SDR Receiver
The signal is fed into an SDR receiver tuned to the intermediate frequency. I used the SDR Ensemble, but any other type of receiver will work as well. Since the signal was extracted from the device before the filter, we can see a broader spectrum from the band on the monitor and thus monitor the occurrence of signals.
The Benefits of SDR for Reception Quality
The difference in reception quality is striking. It becomes even more apparent when using a higher-quality sound card and optimal filter settings. This description is not a complete guide, but it aims to illustrate the benefits of SDR. Those who are more technically inclined could, for example, integrate an SDR receiver directly into a radio or find other ways to connect it. So don’t be afraid to experiment with SDR—it’s worth it!
