[ao16aprs] RE: Cheap Modem

["Darryl Smith" <vk2tds@ozemail.com.au>]

>>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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