Monthly Archives: January 2014

Back in the game

After few days of troubling results and re-designing the whole RF board I finally have some good news. A completely changed tx mixer, outputs a signal that more or less represent a sine wave. The screen is from actual tx mixer output, before any filtering or smoothing:

tx_mix_3_5

This is 3.5 Mhz + tuning offset pure carrier, no filters on the scope, also using lower frequency as the bandwidth of my scope is quite limited (15 years old machine).

Original schematics used 74LVC4066 in balanced mode, with 4 phase quadrature clock, but I was not able to tune it. I had to go for the most commonly used design for such mixer:

txMixSch

But it is now clear why this design is the most popular – fewer parts and stable operation in 1-30Mhz range. It is a winner!

SI570 as clock for Johnson counter – the problem

Sometimes you think you know something is wrong, but until you see what it is, you can’t really be sure.

digital_clockA quick glimpse over commonly available schematics for direct conversion radios that use SI570 and 7474 as Johnson counter (2bit quadrature generator) reveals the tip of how many things are wrong about this idea. But as those radios work in practice, I for one never bothered to look at the real life picture, which is this:

SCR0003And the resulting jerky clock is this:

si_clockAnd understandably, the final quadrature clock be as ugly as this:

quad_clockWhich is more or less, the fundamental modulated by its harmonics 🙁

Prototyping is tough

Quad pre-amp for the switching mixer is also done. Eventually even prototyping (for hours on end) gets annoying.

IMG_6601I have achieved almost 8 Vpp from each channel, looks good so far

IMG_6599Next step is the mixer, but that is for tomorrow…

Tx Exciter testing

It is time to test the TX chain. The Quad amp that will produce 4 different phases (0,90,180 and 270 deg) is soldered on a proto board and is ready to play with.

tx_ex_test

Using CW carrier, generated by the soft DDS process in the firmware, it seems four distinct signals are created and ready to be fed to the next stage.

IMG_2400

Hopefully some more soldering this afternoon…

Software TCXO for the SI570 LO

This was a good idea and after two days of coding and testing I have it all working. Locking is quite cool. The precision of course is as good as the look up table I use for the non linear relationship between SI570 temperature and output frequency – check it out in action