|DSB 14318KC QRP TRANSMITTER|
I am brewing it. Parts are all standard.
For AF amp LM741 is used. TA7358AP ic does the work of osc and dsb modulator.
Then these stages are followed by three stage amp. One tuned amp. broad band driver amp based on 2n2218 and last single ended RF PA based on IRF510.
I am testing it and if my ham / friends from (100km my nearest hams) tells me critical report 54 then i will be cofident to announce that it is a boy !
I am not sure whether this little tx can span more than that... but who knows this might put me on air ! If it works i will consider it a xmus miracle.... vu3inj
29 dec 2013. My ham friends were looking for me as a sked was announced but they could not find me due to mismatch and low power at my end. On dummy load it gives a good audio and good power but problem is with antenna working on it. Please friends do visit me... i plan to work on air with this tx in new year and daily after 10pm in the night. all are welcome....
NB:_ As of 2jan 2014 working on resovling the power issue. Solved it maybe it is comfortably providing about 700ma at 12 on 4 watt dummy load , 50 ohm. It becomes warm on just calling cq cq cq de vu3inj....... so i need some sort of tuner..... brewing the atu..... without atu antenna is not loading.
As of 11 Jan 2014, power increased but ATU needs to be brewed and confirmed SWL report from my nearest hams 100km far is awaited then will finally say pass or fail.and publish all write-ups.
As of 14 Jan 2014 a self oscillations in tuned stage was detected. This stage was looking innocent but there was no emmiter degenerations ( 0.1 mfd + 100 ohm resistor ! ) and on 24 volts power supply heating up of the final RF amp coil was noticed. However , there was no self oscillations. So, these things needs to be cured.....
As of 16 jan 2014.
cured by changing the tuned amp stage. but power went down. i also did put 100k plus 0.1mfd in gate to drain feedback, which reduced the distortion and also power. so, by feedback you can reduce the distortion, increase linearity with the penalty of the reduced power .
checked the field stregth with newly brewed rf detector. it explains why nobody answered my call pa need to give more power or external pa is needed. that it falks, dsb projects rd ends here.
Modules as used in the transmitter.
AF amplifier and DSB modulator
Here, ic TA7358AP used as balanced modulator. This 9 pin SIL ic is originally used in the front end of FM radio, so widely available and can do almost all the work of hard to find IC NE602, NE612 etc. internal oscillator is used between pin 6 and pin 7. a xtal of 14318kc is put with 33pf so it gives me the frequency of 14313kc. output from pin 6 is followed by a emitter follower. from this stage modulated rf is given to the buffer amplifier. in cw tx this buffer is also used. after emitter follower stage one 1k resistor is put to reduce the rf going to buffer stage.
buffer amplifier stage
5volt dc regulated.
I had one old carbon mic from russian origin. so put it in a small box a small press to on switch was also installed. and that became PTT switch not shown in the ckt but it is there and connected between point 2 and point three. r1-1k, r2-4k7,q1- any pnp transitor here BC557 is used. relay is of 12 volt use whatever you find. diode accross is a 1n4007 please do put it correctly. Now a days i dont find the relay with more than two switched connections. So, you can paralled for switching pa stage also. i have left the pa stage vcc always connected.
out of three stage of rf amp chain . first is the tuned amplifier. all the selectivity is here. this is the only tuned stage. for spectral purity there should be more tuned stage but as it increases the complexity and this stage give input to the driver stage.
Driver and Final PA stageThen a driver stage based on 2N2218 and final amplifier stage based on the IRF510 was brewed main constraint was that i did not have the suitable ferrite material for the power amp, only toroid available was the balun core one piece and few toroidal core from CFL tube-this dont work beyound the 10 mhz, so this was a very serious problem, then i saw the farhans first bitx using the air core coils on plastic washers. i could not find the suitable washers but 2cm plastic waterpipe was available and a very standard thing which can be available for some time.
As, this was experimental transmitter all the play with the settings, bias, components were done, it was abused so much that each day change was made and seen how it performed.
This is the same thing given in farhans bitx 1 version and also found in many other designs. there is not so much scope of any innovations or problem. Use it.
After the power amplifier one stage LPF was put and then dummy load of 50 ohm (or dipole antenna).
One point in time it tuned up very nicely. So, i published for sked, but aftersome time there was some problem in curing that power went to down, at first i did not realize that thing,then tested again and still problem needs to be rectified.**********************************************************************
LETTER OF THANKS
Lots of friends, VU3HMM, VU3HML, OM S Kamble, VU3YFD, VU2KD, OM gurudatta and others tried to listen and help in improving, even OM VU2ESE also suggested one or two things. OM Sandeep Lohias actively supported with his expertize & experience. From delhi VU2ATN, VU2YK,VU2RAK also gave positive feedbacks, but, anyway this ckt did not stand to the point but gave a very valuable lessons and stirred lots of interest. VU2TOO also gave me moral support and VU2UPX was also closely watching and lots.
At this point in time this phase is over, will try it some other time with success.I will try to write-up fully within short time, as busy in new mqth,
Thanks you friends.... vu3inj, indrajit 17 jan 2014
thats it folk wheel will be re-invented soon !
Jan 2015 problem found. Power is low about 2watt also antenna was not on high building. With input from VU2KD & VU2DEV antenna was raised high, linear was loaded and it worked very well......