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[ao16aprs] RE: Cheap Modem

So, can JAS-1 (FUJI20) also act as an DIGIpeater if they put it in JD
mode?  Just curious.  If this is so, then thats anotehr possible
digipeater for us...  But as a voice satellite, I doubt we will ever get
it...

bob



On Sat, 11 Sep 1999, Darryl Smith wrote:

> >>From the jas-1 modem discription:
> >The carrier recovery circuit used here is a digital "squaring loop"
> >
> >U4 (TL084 opamp) pins 2,3,1 is a limiter, which simply makes all subsequent
> >signal processing digital.
> 
> And here is the problem. I have not seen the design of this modem for about
> 3 years, so what I am about to say is based on my memory.
> 
> The incoming signal is basically put through a 1 or 2 pole filter and then
> limited... Or in other words rectified.
> 
> In english, the signal comes in from the radio, and goes through a low and
> high pass filter. This removes some of the out of band noice. Whenever the
> filtered signal is above 0V, it is high, whenever it is below 0V it is low.
> The problem with this is that it is highly succeptable to noise... minor
> variations in noise completely out of the normal frequency range will cause
> the sigital signal to be time shifted, or to be noisy.
> 
> The receiver will work well for perfect signals, but the performance for
> noisy signals is not good. To improve things the PLL needs to operate on the
> analogue signal rather than the digital signal, or at least the input filter
> needs to be tightened.
> 
> I suspect that a switched capacitor filter would provide far better
> results...
> 
> As for the transmitting side... It is just as bad.... It uses a low pass
> filter to shape the incoming digital stream after some digital processing.
> Thus rather than becoming real sine waves, the transmitted signals are more
> like capacitor charge and discharge curves...
> 
> >it looks to me that the pll is pulled to the audio and not the audio to the
> >pll but i may be mistaking
> 
> >From memory this circuit also has an early/late detector to move the radio
> up and down in frequency. Since the radio is SSB, this results in the audio
> frequency moving up/down
> 
> Darryl VK2TDS
> 
> 
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