60 May 2015
SpectrumView from WD6CNF. It will be assumed here
that your radio audio output is connected to the sound
card on your computer and SpectrumView is running.
Allow the radio to warm up to reduce drift. Remember
there are two things we want to find: the BFO frequency
and how it changes with frequency.
It’s easy to determine the BFO frequency. Tune in a
local AM station of known frequency and select SSB on
the receiver. There will be an audio tone and a peak in the
audio spectrum as shown in Figure 5. This peak is due to
the carrier of the AM station. Adjust the BFO knob back
and forth, and watch the tone peak move left or right. The
peak frequency is shown at the top of display. This tone
peak is the BFO frequency.
Okay, now we want to find out how the BFO changes
with RF dial frequency. We can do this by tuning in
another station far away in frequency from the AM station
and noting the change in BFO frequency. A good choice
is WWV at 10 MHz as previously mentioned. (In a pinch,
an RF signal generator can be used in place of WWV.)
Let’s say that we tuned to a local AM station at 1.130
MHz and set the BFO to 970 Hz by observing
SpectrumView. Then, say, we tuned to WWV at 10 MHz
and found the BFO peak had changed to 1,024 Hz. This
shows that the BFO increased 54 Hz over the 8.87 MHz
frequency difference between the two stations. This is a
rate of 6.08 Hz per MHz. It is a small change, but worth
correcting for in the interest of accuracy. (Your AM station
frequency and numbers, of course, will be different.)
Remember this rate value. It is all we need to set the BFO
to any value at any frequency.
Here is an example of how to do this.
Suppose we wish to set the BFO to 600 Hz at 14.095
MHz. (Note that this corresponds to the true dial of
14.095600 MHz in SSB mode.) We will use the standard
candle AM station at 1.130 MHz. Note the frequency
difference ( 14.095 – 1.130) = 12.965 MHz. Hence, the
BFO frequency at 1.13 MHz would correspond to 600 –
6.08 x 12.965 = 521.1 Hz.
At this point, tune in the AM station at 1.130 MHz in
SSB mode and adjust the BFO knob for 521 Hz. Now,
when you set 14.095 on the radio dial, it will correspond
to BFO of 600 Hz and the true dial frequency of
14.095600 MHz. In this case, the AM station is our
standard candle, and since it is a local station it is always
available for setting the BFO using SpectrumView. Station
WWV is no longer needed.
Note that the BFO setting can be a negative
frequency. Suppose you want a BFO setting of 0 Hz at
14.076 MHz which is a true dial frequency of 14.076000
MHz. Similar to above, we have 0 – 6.08 x 12.946 = - 78
Hz. For this case, just rotate the BFO knob past 0 Hz to
negative frequencies, as monitored by SpectrumView at
the standard candle of 1.13 MHz.
Once the BFO is set, do not adjust it except to
change to another effective dial setting. It may drift a little
with temperature, so check it periodically. Remember that
SSB mode must be selected to use it. BFO calibration is
very useful for selecting the starting radio frequency
segments for BPSK, WSPR, and JT65, which usually need
an accurate true dial setting.
Since many of the signals you will be receiving are
FIGURE 7. JT65 decoding windows.
FIGURE 6. SITOR
data received with