[GreenKeys] Might be a bad weekend...

[Bob Camp]

Hi

One way to look at a very standard 88 mhy toroid based "discriminator"  
is as a pair of AM detectors. The output is summed to make the  
decision about which one is "on" easy.

The harmonic of the "on" tone shows up inside the passband of the  
"off" tone. That limits your detection capability. Synchronous  
detection of the so called AM signals would help reject the bad  
sideband, but that's not practical.

Bob


On Dec 1, 2007, at 3:12 PM, Ralph Mowery wrote:

>
> ----- Original Message ----- From: "Bob Camp" <ham at cq.nu>
> To: "Daryl Cline" <darylcline at ntelos.net>
> Cc: <greenkeys at mailman.qth.net>
> Sent: Saturday, December 01, 2007 1:34 PM
> Subject: Re: [GreenKeys] Might be a bad weekend...
>
>
>> Hi
>>
>> One way to look at an FSK signal is as two ASK (amplitude keyed)   
>> signals. Since we don't run phase coherent switching that's  
>> usually  what we have anyway.
>>
>> With 170 shift and 100 baud your first "sideband" on the signal is  
>> 50  Hz from carrier, the third is at 150 Hz. Since it's a square  
>> wave the  odd harmonics are going to be there. 150 Hz is dumping  
>> energy very  close to the "carrier" of the second signal 170 Hz away.
>>
>> With 425 shift you get away from the problem. You can also  
>> tolerate  more drift on the link. My guess is that drift /  
>> frequency tolerance  is the bigger issue for the military.
>>
>> Bob
>
> Bob as there is only one tone on at a time it does not make any  
> differance in the harmonic content of one tone  as far as one tone  
> to the other tone. They will not both be on at the same time to  
> affect each other.
>
> I am not sure where the origional ammount of shift came from.  I  
> think for the ham bands it was in the FCC rules the tones could only  
> be 900 hz wide. So the hams used 850.     Mabye the military or  
> comercial was using that differance in the tone pairs.  Then 450 was  
> half of that.
>
> The receivers and transmitters did have a bunch of drift in the  
> eairly days. The seperation needed to be large between the tones  
> partly for this reason. As the equipment improved the tone  
> seperation could become less.
>
> YOu usually get a beter signal to noise ratio with narrow shifts.   
> The signals could be spaced closer to other signals also.   One  
> thing wide tones will do if you have the proper equipment is beter  
> selective fade resistance. I have seen it to some extent on the  
> narrow shift by watching the oscilliscope on 170 shift.  This is  
> where one tone fades and then the other tone fades as the first one  
> comes back.  Some demodulators will copy on either or both tones  
> where some need both tones for good copy.
>
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