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FSK modulator/demodulator

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rasmusms

Jan 1, 1970
0
Does anyone have a idea for a FSK modulator and demodulator witch is
frequency stable at a wide temperature range (-40 - 80) celsius
degrees. The carieer frequency should be around 1-2Mhz
 
A

Allan Herriman

Jan 1, 1970
0
Does anyone have a idea for a FSK modulator and demodulator witch is
frequency stable at a wide temperature range (-40 - 80) celsius
degrees. The carieer frequency should be around 1-2Mhz

What stability do you require? +/- a few percent , or +/- a few ppm?

If the latter, a DSP based modulator and demodulator clocked from a
TXCO could work.

Regards,
Allan
 
R

rasmusms

Jan 1, 1970
0
What stability do you require? +/- a few percent , or +/- a few ppm?

probably about +/- 1000ppm. It depends on how fare the 2 frequency are
apart.
If the latter, a DSP based modulator and demodulator clocked from a
TXCO could work.

Seems as a expensive solution

Regards,
Rasmus
 
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Rene Tschaggelar

Jan 1, 1970
0
rasmusms said:
Does anyone have a idea for a FSK modulator and demodulator witch is
frequency stable at a wide temperature range (-40 - 80) celsius
degrees. The carieer frequency should be around 1-2Mhz

A stability of 0.1% is not doable with pure analog.
Did you have a look at a DDS such as the AD9833 ?

Rene
 
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Rene Tschaggelar

Jan 1, 1970
0
rasmusms said:
probably about +/- 1000ppm. It depends on how fare the 2 frequency are
apart.

They need to be at least the baudrate apart. Better a bit more.
 
R

rasmusms

Jan 1, 1970
0
Did you have a look at a DDS such as the AD9833 ?
It is to expencive
They need to be at least the baudrate apart. Better a bit more.
I thought so. So they need to be a least about 300kHz apart (min.
200kbps)
A stability of 0.1% is not doable with pure analog
It doesn't need to be all analog
 
R

Rene Tschaggelar

Jan 1, 1970
0
rasmusms wrote
What stability do you require? +/- a few percent , or +/- a fe ppm


probably about +/- 1000ppm. It depends on how fare the 2 frequenc ar
apart
They need to be at least the baudrate apart. Better a bit more
 
A

Allan Herriman

Jan 1, 1970
0
It is to expencive

I thought so. So they need to be a least about 300kHz apart (min.
200kbps)

It doesn't need to be all analog

If there is a good balance of 0s and 1s in the bitstream you are
transmitting, then it's fairly easy to use a PLL to make the (analog)
modulator frequency stability as good as that of a reference crystal.

The PLL will need a fairly low loop bandwidth to avoid causing
excessive baseline wander.

The PLL will also need a fairly low comparison frequency to avoid the
problem that occurs when the phase modulation on the output (due to
the FSK) scaled by the feedback divider exceeds +/- 2 pi radians,
which is the linear range of the phase detector.

You might need to use a scrambler to ensure a good balance of 0 and 1.

Regards,
Allan
 
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