[Lf] [Fwd: LF: Frequency Calibration Argo etc.]

[Andre' Kesteloot]

Talbot Andrew wrote:

> The recent talk about soundcard accuracies set me thinking.   Tuning
> error and soundcard accuracy do not have exactly the same effect on the
> apparent observed frequency line in Argo or any DSP system where the
> sampling rate is not locked to the same frequency as used for tuning.
>
> Tuning error is reflected as a frequency shift which applies equally to
> all tones within an audio bandwidth.
> Sampling rate error appears to change the tones by an amount
> proportional to frequency.
>
> eg at 8000 Hz sampling rate + 10ppm  = 8000.08 Hz
> a 1kHz tone will appear at to be 999.99Hz (0.01 Hz low),    1500Hz will
> appear to be 1499.985Hz (0.015Hz low)   etc
> If the receiver tuning is 0.1 Hz in error these would be 999.9 ands
> 1499.9 respectively.
>
> So to calibrate out both uncertainties a minimum of two measurements are
> needed.  Here is a procedure to do this :
>
> Tune into a transmission whose frequency is known EXACTLY - such as MSF
> at 60kHz or the centre of Loran at 100kHz - using your favourite piece
> of narrow band software and receiver as used normally.   Adjust tuning
> for an audio tone of 500Hz and measure the exact tone frequency
> resulting.     Then alter the receiver tuning to get a tone of 2500Hz
> and measure this figure exactly.
>
> The difference between the two measured frequencies  expressed as a
> fraction of the wanted separation (here 2kHz) is the soundcard sampling
> error rate.   Any error that applies equally to both tones is due to
> receiver tuning alone.  Obviously the widest tone separation as possible
> is desired to minimise measurement error.
> This test does pre-suppose that the error in the receiver is determined
> only by its internal oscillators and not by errors in transferring the
> desired frequency to the display.  Most modern PLL and DDS controlled
> receivers do in fact generate exactly what the dial says, subject only
> to their oscillator calibration.
>
> In most cases the receiver frequency setting error should swamp that of
> the soundcard but this may always not necessarily be the case.   For
> example an IC746 with TCXO option and trimmed to 0.2ppm accuracy (it can
> be done) used to receive directly on 137kHz will exhibit an error of
> 0.027 Hz as all frequencies within the reciver are locked to the master
> source.   A laptop integral soundcard could easily be 30ppm out even if
> the oscillator is 'meant' to be exact ie. not allowing for poor
> implementation which can give errors up to 0.2 percent !!!   30ppm would
> give 0.03Hz at 1kHz tone and 0.06Hz at 2kHz tone frequency, error
> exceeding that from the receiver.
>
> Andy  G4JNT
>
> >
> > Assuming an overly optimistic stability of 10 ppm over the
> > whole temperature excursion range
> > for that crystal, this means a deviation of 0.01  Hertz for
> > an audio signal of 1 kHz.
> > At this frequencies, even cheap crystals can prove adequate.
> >
> > Thanks for your tests Alan,
> >
> > 73   Alberto    I2PHD
> >
> >
> >
>
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