#1 2014-12-02 11:07:22

DB1NTO
Member
Registered: 2014-12-01
Posts: 52

3.3V reference for ADC

Hi Mark,

you wrote in your Blog "The biggest change is that the demodulator ADC is now referenced from a regulated 3.3v supply instead of the 5V (rather noisy) bus."

Can i simply change the input reference voltage on R10 (old schematic, quickstart.pdf) from 5V to regulated 3.3V or do i change more to use this?

Thank you

Taner
DB1NTO

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#2 2014-12-02 16:54:49

markqvist
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Registered: 2014-12-01
Posts: 112

Re: 3.3V reference for ADC

Hi Taner!
There is three things you will need to do. As you say, you need to provide 3.3v to the R10 instead of 5V. You will also need to provide 3.3v to the AREF pin of the ATmega. Finally you need to change the code to use the external AREF pin instead of internal VCC reference. This is done by setting the ADMUX register to the correct value for external reference. By setting the two most significant bits of ADMUX to 0, the chip selects the voltage on the AREF pin as ADC reference. This could be done something like this:

ADMUX &= 0x3F;

The new MicroModem has these changes built in to the circuit, and the new firmware I'm writing will automatically detect the reference, and set the registers accordingly smile

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#3 2014-12-02 23:35:08

DB1NTO
Member
Registered: 2014-12-01
Posts: 52

Re: 3.3V reference for ADC

Thank you Mark!

i will wait four your new firmware which has some advantages for me smile
What do you think about a update of "Quickstart.pdf" about the 3.3V reference?
I think 5V reference ist too high for the most use cases. So some people needs a high output level on their receivers...
For the DRA 818V Module from DORJI and similar Modules it might be very helpful to reduce the voltage.
Maybe a R/C bandpass in the RX audio path might be also helpful to reduce noise and better decoding?

Taner
DB1NTO

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#4 2014-12-23 16:18:42

markqvist
Administrator
Registered: 2014-12-01
Posts: 112

Re: 3.3V reference for ADC

Ooops, I had totally forgotten to answer this post! I probably should mention it in the documentation, but I'm a little bit hestitant to make it a requirement to use it to have 3.3v for reference, since not all ciruicts will have a 3.3v source, for example if you make a basic MicroModem build without USB, you would need to add in a 3.3v regulator to get it working, and I really want to keep the "simple" version of the circuit as simple as possible smile

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#5 2014-12-23 16:21:41

markqvist
Administrator
Registered: 2014-12-01
Posts: 112

Re: 3.3V reference for ADC

But yeah, in general, lower voltage is a good idea. On the other hand, you do want a bit of "room to play", because there is actually very large variations in the received audio amplitude from station to station on the APRS network, some will be very loud, some softer. I did test with reference voltage as low as 1.1v, but I actually found I got better results with the 3.3v reference. This will of course vary from radio to radio, and I have only been testing on three different Baofengs, and a Yaesu FT897D.

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