[Lf] [Fwd: LF: DSP56002EVM with Atomic Clock]

[Andre' Kesteloot]

Klaus von der Heide wrote:

> Hello LF-enthusiasts,
>
> some of you obviously use the DSP56002EVM.
> Therefore, let me again mention a simple method to
> synchronize an LF-receiver to an atomic clock.
> I do it the following way:
>
> The 24576 kHz of the CODEC oscillator on the EVM board
> is the mother oscillator of all frequency-relevant
> parts of the receiver. The signal is picked up by a
> small capacitor from the board and amplified by a
> gain block and then divided by digital CMOS circuitry
> as follows:
>              24576 / 64 / 3 = 128 kHz
> These 128 kHz (well LC-filtered) is used to downconvert
> the 136 kHz band to about 8 kHz, which is the input to
> the left channel of my EVM board.
> The digital 128 kHz are divided further:
>              128 / 2 = 64 kHz
> This signal (also LC-filtered) is used to downconvert
> DCF77 from 77.5 kHz to 13.5 kHz which is the right
> input to the DSP board. Only one receiver is written
> in assembly language. It is switched between left and
> right input. The DSP program generates cos/sin-pairs
> of 13.5 kHz and 7.7...9.8 kHz for downconversion into
> the baseband. After the DSP-broadband noise blanker
> (correlation of both inputs 136 and 77 kHz) a
> multirate filter follows and a quadrature demodulator.
> The rotation of the DCF77 signal in the baseband
> exactly says at what the 24576 kHz were in error.
> So the program can correct this error in software.
>
> In place of DCF77 also MSF and others (50 kHz)
> could be used by simple change of the LC-input
> filters of the receiver and the downconversion
> frequency generated by DDS in the program.
>
> The advantage of such a synchronization is to
> allow very high resolution over long periods.
> A spectrum of a transatlantic signal could be
> made over a whole night at a resolution of
> 0.01 Hz or even better. The transmitted signal
> must have the same quality, off course. The
> signal to noise ratio against a resolution of
> 1 Hz then is better by a factor of 10.
>
> My simple receiver is used therefore the other
> way also. At the moment an OPAmp produces only
> 1 Volt at 50 Ohm at any frequency within the
> 136 kHz band (no PA, no Antenna).
>
> It should be pointed out that only convertion
> frequencies (generated by DDS) and the sampling
> frequency are relevant to the receiving or
> transmitting frequency, and not the DSP clock
> and not the clock of the PC which does the FFT.
>
> Because of heavy QRL, this LF project is growing
> slowly.
>
> 73 de Klaus, DJ5HG