PLL

[man...@digitalconfections.com]

I've heard complaints about how difficult it can be to hunt multiple transmitters when each one is on a slightly different frequencies hundreds of Hz apart. To ensure a good signal strength reading it is necessary to retune the receiver each time a different transmitter comes on the air. This would be an especially big problem when it is necessary to program in a different numeric frequency setting, and not simply turn the tuning dial on the receiver. Personally, I have not experienced competitions with such poor transmitters, but I certainly believe they are out there.

To address that issue, I am thinking about adding the following component to the Receiver board, and utilizing two of the currently unused ADC port inputs on our Digital Interface processor:

https://www.digikey.com/products/en?keywords=296-14109-1-ND

The approach would be something like this: 

o The VCO will be configured to range over a span of audio frequencies, say 200 Hz to 1200 Hz, nominally centered at 700 Hz. The PLL then, will be able to lo lock on any audio frequency in that range.

o The PLL input signal pin will be connected to the audio from the receiver.

o The processor will monitor the lock condition pin, and measure the VCO input voltage.

This way, the processor will be able to detect when the user has tuned to an audible CW signal such that its tone in the headphones is between 200 Hz and 1200 Hz. It will also be able to determine if the audio frequency is above or below 700 Hz. With that information the processor will be able to adjust the CW offset to bring the audio tone to the nominal center ~700 Hz. In "automatic frequency correction" mode, it will do this correction for any received signal automatically - as long as the signal's tone in the headphones is within +/-500 Hz of 700 Hz.

The PLL should also allow the receiver to do a better job of scanning for signals, since it will have both the RSSI indicator, and the PLL lock condition to indicate the presence of a signal on frequency.

Other thoughts, ideas, suggestions?

[Gerald Boyd]

Sounds like a good idea.

I just did some testing yesterday with a low power 2 meter CW transmitter (one transmitter out of the 5 cycle) and the prototype receiver. Did not notice any drift with either the receiver or transmitter during the outdoor test session. 

Given today’s technology stable oscillators can be used to net all the transmitters close enough so that there is not an issue.

The proposed solution should work. Would like the option to turn off so when tuning the receiver it won’t be trying to make corrections as I am changing the main VFO.

Also if there is some near frequency QRM may want to have the option to disable.

In some of my testing I have detected near frequency emissions from some consumer devices. 

On 80 I notice differences between transmitters and just keep going. They are small enough that I can still find the null with out messing with the receiver. Those transmitter have simple poor PPM oscillators.

The best solution is to have good transmitters. low PPM error compensated oscillators are cheep.

Sent from my iPad

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