01 octombrie 2018

New QRP-uBITX in the shack - II - A few necessary measures

First of all, the board is "Rev 4" which means it came with discrete audio PA and "Audio pop mod".

We'll get back later to this diagram.

1. After unpacking, the parallel LCD was replaced with a I2C one as I want to free some hardware resources for further improvements.
I uploaded the new firmware, the one from KD8CEC. 

A quick test for the Raduino board. Everything is OK.

2. The crystal ladder filter must be grounded. A small 0.8mm hole was made and a ground wire soldered on all crystal's cases.
I used the occasion to implement the mod for the variable IF filter.
For that, I removed C217, C218, C219, C220 and C221 and replaced with 5 varactor diodes.
I found SVC236-TB-E to be suitable for this purpose as I need to have between 50-100 pF and a good stability over a large temperature range.

Here it is how it looks... You cannot see the varactor diodes here because of the 47k resistors but they are behind the line of resistors, right in the place of the removed capacitors.

Here is the diagram for the mod.

A few words about:
-The Rx and Tx are 12V from the K1 relay. I took the voltage from R48 and R38. 
-The 12V which came from the Rx/Tx relay are regulated by the two linear regulators. As we increase the voltage, the IF bandwith widens. For Rx, 0 to 8V measured at the common cathodes of D6 and D7 correspond to a variable bandwith from approx 250 Hz to 4 kHz. For Tx, a voltage around 2.9V gave a nice warm modulation of about 2.8 kHz.

I have on the panel only P1. You can use any linear regulator or even some Zenner diodes with 4k7 resistors.

The components are placed on a small universal PCB. The components are not critical.

3. Some mods are necessary also on the audio PA stage.
There are some crossover distorsions that made the audio bad. To solve them, I modified the polarisation of the final transistors. Remember I told you we will revisit the schematics of the audio PA? Good! Between D15 and D16 a resistor must be inserted. I tested various values and the best results was with  10 Ohm. The trace between the two diodes was cut and the SMD resistor was placed right between.

Here it is how IF variable bandwith sound (CW):

I also use this TRX to listen to MW broadcast:

........ more to come!

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