Nuts and Volts - May 2015

built-in noise suppression to prevent computer noise from getting to the radio. To connect the audio, look for the audio-out jack on the radio.

Most radios have a provision for connecting to headphones or an external speaker. For the DE1103, the connection is easy as there is a line-out jack on the side of the radio. Finally, you need to know how to adjust the line-input level in the sound card recording section of your PC.

Getting to Know Your Receiver

Spend some time getting acquainted with your receiver’s operating controls. There will generally be controls to select the audio bandwidth, adjust the frequency, select the SSB mode, and adjust the BFO frequency. For the DE1130, the narrow-wide switch selects either 4 kHz or 6 kHz wide audio. In most cases, use the wide setting as powerful audio filters in the software will filter the noise. To select the desired radio frequency in kHz, enter the desired digits, press BAND+, and select SSB.

With this radio — as with many radios — you can only select the frequency to the nearest kHz since only the five most significant digits can be entered. That’s okay for receiving AM stations, but for receiving SSB voice signals smaller frequency adjustments need to be made. This is done via the BFO knob on the side of the unit.

Understanding the BFO

The BFO effectively provides variable adjustment of about 1.5 kHz above and below the dial frequency as seen in Figure 4. It is usually un-calibrated, however. In other words, the last — or lesser — three digits of the true dial frequency setting are unknown. More frequency precision needed for data transmissions can be achieved by calibrating the BFO.

Typically, seven digits of frequency are needed to locate specific radio bands and for accurate signal reporting. Thus, we need to determine the true RF receiving frequency of the radio when using SSB and the BFO together.

For my DE1103, the BFO knob allows frequency adjustment of plus 1,600 Hz to minus 1,300 Hz from the displayed dial frequency. The maximum BFO frequency (1,600 Hz) is attained with the BFO knob set fully counter-clockwise (CCW). Turning the knob CW from the fully CCW position causes a decrease in the BFO frequency, gradually reducing it to 0 Hz and then further to -1,300 Hz.

Here is an example of how the BFO affects the true radio dial. Suppose the displayed radio dial is

14,020 kHz and the BFO is set to +755 Hz. Adding the two, the true dial setting would be 14020.755 kHz ( 14.020755 MHz). In this example, we now have eight digits of frequency. In practice, the last digit will not be known to high accuracy. However, it will be good enough to get us to the right shortwave band segments — usually within 10 Hz.

If the BFO knob was calibrated in frequency, this would be the end of the story. Most BFOs — particularly in low cost radios like the DE1103 — are not. Another problem is that the BFO frequency will change a small amount with dial frequency. The good news is that we can easily overcome these problems using an audio spectrum analyzer on our PC and two standard candle radio stations separated widely in frequency.

These standard candle radio stations can be an AM radio station near 1,000 kHz and a shortwave station like WWV at 10 MHz. WWV has a very accurate frequency, and AM stations are required to be within 20 Hz of their assigned frequency. This allows us to perform a calibration for our radio’s BFO. It is easy and fun. Here’s how to do it.