Dear LF group,
Rik's method of producing an accurate audio tone for calibrating
the soundcard is very similar to what I am doing, and I can confirm
this gives an accurate and unambiguous result. I did have some
problems, however - the logic level square wave (in my case 1kHz)
contains many harmonics, and I found that my soundcard did not
have a very good anti-aliasing filter. The result of this was that
many spurious frequency components were present in the output
data, which could confuse the calibration program. I overcame this
by adding a simple filter - 10k in series with the divider logic output,
connected to a parallel - tuned circuit consisting of a 100mH
inductance in parallel with 220n + 22n capacitors. This made the
output a fairly clean sine wave. You may have to fiddle with the
capacitors to peak up the output, or for different audio frequencies.
If you don't want to build a calibrator, there are a couple of ways of
using a receiver as a calibrated audio source. If the reference
oscillator output in the RX is reasonably accessible, it can be
coupled with a capacitive probe wire to the RX input. If the RX is
tuned to the reference frequency, the audio output will then be the
nominal value within the tolerance of the reference; errors in RX
tuning due to the reference will cancel out. This should work well
with an RX like the RA1792, where it is easy to get at the
reference output.
If the reference is not available, the frequency offset can be
eliminated by a differential method; Tune the RX to a stable carrier,
so as to produce a fairly high frequency audio output - I used
1.3kHz. Record several minutes at 8k samples/sec, and measure
the exact frequency using the Wolf -m option. Re-tune the RX on
the same carrier to produce a low frequency audio output - I used
300Hz. Repeat the frequency calibration using Wolf -m. Calculate
the difference between the two RX frequency settings, and the
difference between the frequencies as measured by Wolf. The
soundcard sample rate is then:
8000 x (difference in RX tuning setting) / (difference in measured
audio frequency)
This gave me almost the same results as calibrating with an
accurate audio frequency. Both methods assume the receiver
reference oscillator is accurate, but it's accuracy will normally be
pretty good compared to the soundcard. Note, however, that both
methods may be defeated by receivers using DDS synthesisers,
which tune in odd-frequency steps, but display the output to the
nearest Hz.
Note also that the soundcard sampling rate error depends on the
actual rate selected - for example, measuring 800Hz using
Spectrum Lab with 8k sample rate gave me a frequency error of
about +4.5Hz, but with the 11k rate, the error was about -1.6Hz.
The errors are different with different types of soundcard. So using
other software to measure frequency offsets will not be helpful,
unless it also uses an 8k sample rate. I think Argo uses the 11k
rate. I don't know about the PSK31 software. Oh joy.....
Hope this is helpful,
Cheers, Jim Moritz
73 de M0BMU
|
