[Laser] OTT stable audio oscillator?

[laser@mailman.qth.net]

In a message dated 4/12/03 6:01:43 AM GMT Daylight Time, 
john.dalton@bigfoot.com writes:


> I would have thought that for audio frequencies, the best non-oven controlled
> crystals should be able to get stability in the range you mentioned.
> A glance at Hy-Q (http://www.hy-q.com.au) shows they do crystals down to 
> 3ppm.
> (20khz * 3ppm) is less than 100mHz.
> 
> 

QRSS is a great method of recovering very weak signals and it's been standard 
practice on the LF bands for the past few years. However, it's not so ideal 
for laser communications due to the blurring that occurs on transitions 
between on and off periods. Don't get me wrong here, I think it's a very good 
idea for extending the distances possible using lasers, but its limitations 
will be far more apparent because of the scintillation of the beam in  
disturbed air.
An example of the 'blurring' can be seen on a screen shot of the 60kHz 
transmission from Rugby in the UK.  www.g0mrf.freeserve.co.uk/qrs.htm   The 
effects from scintillation of a laser are much worse.

ARGO has several settings for dot period:  3  10  30  90 seconds  etc.   From 
tests here, the 3 second dot period looks very good via laser but the higher 
settings become increasingly difficult to use with the received signal 
resembling wide band noise.
Consequently you can predict the accuracy required from the audio source and 
for 3 or 10 second dots any 'old drifty' 100ppm xtal should be fine.

Components required.
1) Junk box crystal 
2) CMOS 4060
3) 2 x capacitors anywhere between 22pF to 68pF
4) 1 x resistor  between 1 and 10 Megs

Modulation can be keyed via pin 12.
The 4060 has about 9 or 10 outputs which can give a useful audio frequency 
from almost any crystal you have.

73

David G0MRF




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