Tuesday, April 1, 2014

6meter tx and rx (ra3aae) experiments.

     Small transmitter

48mhz canned oscillator

local oscillator
pin details of can
Why this receiver

              There was a need to make  small RF oscillator for code practise and demo.  It was my deep desire to know far the small canned oscillator can work as some hams claims ! Is there any justificatins in the claims. Only way to know is test it yourself or beleive them.

             I had some 48Mhz canned xtal oscillator. Also i bought from metro, few 24Mhz fundamental xtals. This does not fall into the ham band but if you have the canned oscillator worth to try it.

     After initial successful testing 40 meter receiv er (PA2OHH design, see the march 2014 post) it was found that in the receiver, audio is bit noisy.

            In the KE3IJ (160 meter/40meter receiver design) there is one Common-Base amp stage. His callsign also contains 3IJ , my callsign is 3INJ ,what a remarkable co-incidence ! we both are brewer.
 
Transmitter
              Now, soldered the oscillator-transmitter and RA3AAE receiver. Will test it for range, antenna and all. 

Testing
          Powered up the tx. Switched on the rx and no tone. On increasing the volume it breaks into the oscillations. There is less noice unlike the common emmiter configurations but it breaks into the oscillations due to the  high gain of common base amplifier stage ! solutions- decouplings. or once again use noicy CE stage.  Ok. This receivers AF stage works. switched on off the QRPp tx (!). but there is no CW tone. RF probe shows tiny lights as it is very low power ( may be 10 miliwatt its not an issue).  Are they on right frequency ?

        Measured the freq. 

        48Mhz canned oscillator. reading shows stable 48.000.30 . thats OK.

        24mhz xtal oscillator shows some what like 24.020.5x

 OK. That means 24.020Mhz after the 24mhz it is up by 20k and on doubling it result into the 40k. No human being can hear it ! only BAT can hear hello cq. 

solutions, due to the VC (trimmer) freq is up,  Try to pull it down with L- a small inductor. But, this 24mhz osc is little wobbly in 100hz range. Put a little filter rc in the +vcc of osc.done not much improvement.

25march2014
         I am writing this on, day by day but will publish after all process is successfull. As one of my friend remarked is it working or not ?
28march2014

the thing which started out as the fun and simple is turning to be a major challenge. Reasons are two. Oscillations very-low freq motor boating due to high gain, and xtal oscillator (LO) is not coming down to even 24.001mhz. I need only 1k to 2k audio tone. Very strange. Tried lots of L and C combinations. It took my whole 3 hours. But, could not bring it down to the 24.005 Mhz. 

So, i will be leaving it and try to cure the audio motor boating first. Then test with the vfo 24mhz instead of the xtal. I think will make it in a bigger board with good audio stage utilizing the LF411, a pin compatible but better than LM741 op-amp ic.

(31march2014)

Deoupler circut :-

I saw this type of decupler in the ARRL dc radio R2 and Farhans(VU2ESE). Well now a days, i am seriously looking into the farhans fone stock website. What he has used and how its used. then look from my collections of the ckt compiled over the years (4 years) from the net. And then at-last the web. But, generally i find the answere from the compilations.

de-coupler
 This decoupler simply reduces the hums, noice going into the low noice pre-amp. It prevents noice coming from line to get into the input and amplified. Its design in not critcal, but use is higly recomended in the direct conversion receivers af pre-amplifiers. You will feel the difference.

VXO

vxo

I did not use the oscillator with two xtals as Low side shif was needed. An inductor was put to pull the frequency down toward 24.000Mhz but i can only manage to get the 24.005 Mhz with inductor -(10 turns,22swg, with ferrite core). So, it is not zerobeating the 48.000Mhz xtal QRPp tx. 

This will be ok with the sound cards-i.e. software radio but for humans ear s  up to 4k  tone is needed. 

So,it will be replaced with normal coil for 24Mhz and beat heard and the project will be declared success. in the front will put a cb rf amp with small whip.

Problem with this type of detectors are when a longish antenna is connected overloading is present and short antenna is not good enough. 

1april2014

It has become april, and project is taking time due to the morning QTH & other activity. I also have to do my share of chores....

Most probably, as shown in the G-QRP magazine SPRATs Mouse Trap receiver, will put a 1k pot in the input for the test.

8april2014 once again got the courage and put it on the table. heated up the soldering iron for an hour. changed few things.Lastly measured the voltages. Hmmm on the collector is less voltages then the base i.e first af stage is not amplifying. needs to change the bias net work. second discouragement was that 24mhz oscillator was not coming down to less than 24,005kc. so, still i will be not able to hear the audio note (10kc) for the test. Tried everything i knew on oscillator includin super oscllator style but no way.... Atlast removed the oscillator and will try it once again. If the compuer was used as receiver i might be able to see the 48kc wide spectrum. Hmm  from that wspr i had taken the ckt ! any way tx stage is okey. rx stage is also ok but they are not matching..... so, project ends here will try to put 20mhz can and with small antenna and batery will test the range. actually this was what i wanted to see.


(to be conitued........................)

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