DC: Polyakov (II) Improvise 

Nov/19

The oscillator of the Polyakov DC receiver i had built need to be improved. of course, the Vackar. that's all ideas came about. 

How Vackar Oscillator Couple to Buffer

An interesting thing is the tank inductor is 10 uH, it's big. i don't know if this is a key factor to keep the oscillator stable. it's hard to get a 10 uH inductor based on my coil form, the 6 mm Nylon Hex Nut. I got 4.5 uH. I got to try 4.5 uH performance.  with experimental, i got the following configuration to ensure good waveform and the minimal bias for reliable starting oscillating.

Checking  the output from oscilloscope, the ARRL version's couple method had a problem. the collector waveform is too distortion and too low on my actually circuits. this because the BJT became conduction for a small period, and there is a very large bypass capacitor parallel with BJT's collector, the 1500 pF Silver Mica. 

the lower side of the tank  is the good point feeding the buffer.  I try several buffer, with tuned tank or not tuned. both of them stop the oscillator. i don't notice the actual reason lately discovered,  spending much time to change buffer's configuration. finally get a mixer version:

talking the problem confuse me:

Item 1. while i use a wide-band transformer, no output at all. the wind-band transformer inductor impendance is not enough, only 12 uH. i suspect the low impedance present to the first buffer collector take the responsibility.

Item 2.  while both buffer 1 and buffer 2 use tuned circuits:  unstable while  de-tune a little bit, very strong parasitic oscillation occur on very low frequency. why? still to heavy load for the oscillator? 

Item 3. final configuration i got is present to the above schematic.  100 pF couple capacitor present 500 ohm for 3.5 Mhz.

while all of these work finished. i recall some detail while i proceed the item 2.  at begin,  the oscillator make the oscillator stop oscillation too, until put the 330 pF capacitor parallel with the first buffer BJT's base. further recall, while i make the oscillator,  if oscillator's BJT not connect the it's base to ground via that 220 pF capacitor, the oscillator actually not work at all.  this is un-familiar to me, that's mean while the positive feedback is less, it's oscillate, but while feedback more, it's stop!!  

i know now,  why the first buffer i tried does not work, that's not because the buffer itself,  it's same as the latter buffer, the key is how the buffer couple to the tank. 

does this final configuration more stable than previous version?  yes, and NO.

yes. because, it does work better than early version.  NO, it's because the real problem is not the buffer itself (again!) i found the item 3's  circuits( the above schematic) have the same unstable problem!! and i finally find it's because the supply decouple is not enough!!  refer to the 2 0.47 uF capacitor with the *  with it.

ooh, god.  the first 2 buffer should work as the finally version does !!

obviously, i could not use the original mixer configuration,  solution is simple, as the following schematic, adding a 56 uH RFC, which force the LO driven the mixer, otherwise, LO energy is bypass to ground via that 0.47 uF capacitor.

Audio Stages is unchanged. Audio Gain block and LM386

Does I Cure Audio noise?

With new LO and front end, It made exceeding noise, barely hear anything. 

I then put the C1 to audio LPF output end.  C1 helps a lot to reduce audio noise. Later, It proof that noise is come from LO: the oscillator parasit  serious  oscillation.

R1 was added to mixer as RF terminator resistor. R1 make the mixer sound quite a little bit .