Topband: SDR Mythbusters - ADC Overload myths debunked...

[JC]

Hi guys

I would like to add my 2 cents on this matter. If you not  good, or a very
good, RF engineer it is hard to understand what DDU/DUC radio do for you.
The most important thing, DDC/DUC  radios are not the same just because of
the DDC. Number of bits, jitter on the clock works like phase noise from LO.
What's in front of the ADC sets the performance as well the software and the
ADC

ADC overload only can be the limit when everything before the DDC can handle
more power than the ADC itself.  For the band you are listen. Most SDR
radios are using preamps to cover  1 to 50 MHz and guess what? Poor
performance on 160m. Example a common used T50-2 toroid barely can handle
20db IP3!!! 

Most of DDC uses the Linear Technologies LTC2208 16 bit.  Or similar DDC
with 16 bit.

http://cds.linear.com/docs/en/datasheet/2208fc.pdf

These ADC devices overload near +10dBm , that's very high, its over +50db
IP3. However everything before that should be +50db better, and that is not
the case. I like the QS1R because does not have preamp and the LPF/BALUN is
quite good to handle strong signals. 

Another issue with DDC radios available nowadays is the lack of shield, they
looks like a computer board and that's it, performs like a computer board.
No good RF practice to protect the noise floor or common mode noise. Really
not ready for field use.

With all the new software development, construction and good RF design is
way behind. Lab tests on lab environment also does not tell the true value
of a radio. It gives you specific results for specific test conditions.

I believe DDC is the future, but we are not there yet.

Regards
JC
N4IS