## Frequency Accuracy/Stability

**Frequency Accuracy:**

Frequency accuracy refers to the absolute frequency accuracy of the
equipment or process. The frequency accuracy of two stations at each end of a
communication link impacts on the difficulty of the initial tuning in of the signals from
the other station. Advanced microwave enthusiasts using CW for weak-signal work are
looking at better than 0.001ppm as they are looking for weak signals at 10GHz!!!
Some even use Caesium standards!!!

For LF narrowband at least a 2ppm TCXO is needed, although even the
standard 5ppm crystal in a rig like the FT-847 will do.

For Laser Comms DX the accuracy of a standard soundcard should be
sufficient to generate and capture the sub-carrier tones for some modes of narrowband
work. Tests I have done on several soundcards show that the initial accuracy of the
standard sample rates (11025Hz, 22050Hz and 44100Hz) should be within 100ppm of the
nominal frequency.

Here is the results of calibrating two different soundcards against a
local time/frequency standard station.

*I.
*__Creative Labs Soundblaster
in 200MHz Pentium.__

Above is the calibration results for a Creative Labs Soundblaster
soundcard for 11025Hz sample rate, resolution BW of 0.001314Hz. The actual
sample frequency is 11024.963Hz which is an accuracy of -3.320ppm. The two pips
either side are the sidebands were produced by the 60 second cycle of the pips which apart
from marking the seconds (those sidebands are way off the screen at either side) carry
information in codes formed by the duration of the pips. The pips are at 0.01667Hz
offset either side. The computer had been on for 4 hours before the calibration was done.

*II.
*__ESS Soundcard in a
Compaq Presario Pentium 266MHz Laptop__

Above here is the calibration results for a ESS soundcard in a Compact
Presario Pentium 266MHz laptop. This was taken straight after switch-on for its
11025Hz sample rate. Here the actual sample rate is 11025.558Hz which is an initial
sample rate error of +50.568ppm.

I make the assumption that the frequency of a soundcard would be within
+/- 100 ppm for the standard samples rates.

__Note:__ The non-standard rates (eg 5500Hz) are an
entirely different matter as the actual sample rates are more likely to be the closest
sample rate which can be produced from the same reference as the standard rates. For
example, my Creative Labs Soundblaster soundcard has an actual sample rate of 5512.5Hz
(11025 / 2) for a set sample rate of 5500Hz.

**Frequency Stability:**

Frequency stability refers to the drift in frequency over the period of a
record of the equipment or process. For narrowband work this impacts on the maximum
gain in S/N ratio that can be achieved by going to narrower BWs. If, for example,
you are working with an FFT bin BW of 0.001Hz and over the period of the record time the
signal drifts by 0.1Hz, the S/N gain by using 0.001Hz BW is lost.

Here is the results of re-calibrating the Presario soundcard after 5 hours
of warm-up.

*III.
*__Re-calibration of
Presario Soundcard After Warm-up__

Above here is the re-calibration results for a ESS soundcard in a Compact
Presario Pentium 266MHz laptop after a 5 hours warm-up. Now the actual sample rate
is 11025.601Hz which is a sample rate error of +54.511ppm.

This shows a drift of +3.943ppm from switch-on to warm-up stabilisation.

**Conclusion:**

Subsequent measurements 2 hours later on both soundcards showed that the
sample rate had drifted by less than one FFT bin (0.001314Hz). At 1000Hz this
translates to less than 1.3ppm drift.

Soundcards without software calibration should therefore have sufficient **stability**
to allow use of both AFK and FDK
modes using record lengths of one minute (BW about 0.02Hz) with character code spacings of
0.1Hz.

However, they do not have sufficient **accuracy**
without software calibration for the AFK mode. To this
mode it will be necessary to calibrate the soundcard against a standard of at least 10ppm.

E-mail me with comments,
suggestions and corrections.