Friday, January 2, 2015

DSB Modulator,20 Mb

The Modulator,
               After perfecting my system with the 14318kc transmitters, cw. dsb and linear amplifier , push pull amplifier and wasting so much time and energy on making, learning,teaching things it is my time to be on air finally or forget the dream forever.
DSB Exciter

             So making it with my time proven modules. I have previously made the single ic modulator and mixers. Now giving it a little twist making on the copper plane. I call the copper board as a magic carpet. Better than pcb. It gives me the creative edge how to shape the project.

             For working on the copper board you need small hacksaw, big hacksaw, triangular file, round file, sharp knife, marker pen, steel ruler or straight bar, some crocodile clips, little vice. one sharp steel pin, de-soldering pump & good Magnifying lence,

As for 04jan2015 see the foto
I soldered the transistors. Used BC549c. Tried to use same components so that inventory can be good.

06jan2015:- soldered all the wires, jumpers, extra capacitors.

07jan2015:- At the modulator state fixed/measured the LC valued in the Test oscillator with the frequency counter.
Soldered it.
15jan2015:- finished up.

27jan2015:- Tested OK, slight careers leakage is there, but which need  to be reduced...
01feb2015:-uploaded the ckt diagram.

MY write-up for the contest.

14,194kc, 20 meter DSB MODULATOR by Indrajitsinh, VU3INJ,

For 20 meter only xtals widely available is 14318kc . Sometime we also get the 14166kc but not always. But this is far from the crowd. Most of the excitement is around 14200kc fone part. So this idea was born.

The DSB modulator using the two simple cheap easily available xtals and two TA7358AP, the 9pin SIL, FM mixer IC. Its widely available it contains one RF amp, Gilbert cell mixer and oscillator. It also cheaper than NE612 . Using two ta7358ap is a little bit over kill for a single frequency dsb exciter. Only advantage is simplicity, no critical tuning. When you are using the QRP the stability of signal is utmost important. Due to use of the DDS VFO and narrow xtal filters even 100hz deviation go dutch for some operators. This will give us a good solid stable signal and our friend will know where to find us.

            First IC is TA7358ap which modulates the signal. Internal coll-pitt oscillator between pin7& pin8 is used. Xtal here is 4.194kc. not pulled much. One stage bc549c used as MIC AF amp with 4k7 preset as crude af level control. Another af amplifier stage used is within ic. The DSB signal is taken out from pin6 by resistor to make it simple. So it does not effect the mixing/modulating much. But, a tuned amplifier is used to clean the signal, filter out harmonics & amplify. Common BC549c is used will give enough gain here. No need of RF transistor.

           After that the signal is given to a mixer second IC TA7358AP. Here 10.000 mhz xtal is little pulled up higher / to change the frequency few kc upwards, trimmer capacitor is used. But a small panel mount 50pf will be good. You can try also the varactor diode. Single stage tuned amplifier using BC549c follows the mixer ic to select the sum product i.e 14.194kc. Our goal is to arrive near 14200kc.

          Afterward this signal is filtered and given to the linear amplifier. Whole strip is driven from the 5volt regulator. Current consumption is about 45ma.... Signal is clean. There is not much tuning or testing. Except setting 4k7 trimmer for good sounding audio in receiver. Use whatever mic you have and accordingly modify the mic bias resistor. All the tanks are pre-tuned. i.e means the LC components were used as a tank in the collpitt ocillator. By adding standard capacitors values to arrive empirically at the given freq. The freq was measured by the counter. Now, this same components were soldered on the board. This is emperical and quick method of putting the fixed frequency coil for BPF work. To simplify 2.2 micro henry ready made rfc used as first tank. For second tank I could not find suitable candidate in commercially made RFC. Its 22swg wire,1/8 inch dia ( a metal dril bit used as a former) to wind it. 12 turns, tight wound. Then with the adhesive it was made rock solid.

         I had made two prototypes. First one a year ago on vero-board and another this time on copper board. This second prototype was mechanically more challenging than the veroboard. I had to cut groves on the copper plane , solder the copper pad with adhesive on copper board. But happy in the end that it worked. To solder/brew it took me three days But, testing required little more time as my receiver was not working....

        Components details, values and ckt diagrams follows. Please also see the PY2OHH & various TA7358 design on the web.

(x)indrajitsinh, vu3inj. Copy right. If you find this ckt useful please enjoy and pass it on with your refinement and above all enjoy the brewing.

Block diagram

Ckt diagram:-



Two prototypes:-

Bill of Material:-

Resistor Value Note Capacitors Value Note





C3,C6,C14,C21,C22, C23, C24,C250.1



R910K ---100KChoose as per MICC510 MFD


C70.1 / 10 MFD



C9,C10,C16,C17100 PF


C12,C19* TEXT




C26470 MFD, 25V

C271000 MFD, 16V

Transistor Value Note Diodes Value Note
Q1,Q2,Q3,Q4 BC549C NPN transistor D- LED

IC-1,IC-2 TA7358AP FM Front-end IC D- 1N4007 reverse polarity protection
IC-3 LM7805 Voltage regulator


Other items & hardware...

Mic & PTT switch, Relays.

Copper wire,20swg or similar for connecting the stages.

Copper islands cut for placing the components

Adhesive tube used in plumbing to join the water piples.10gm.

Fone pins, plugs,

Aluminium sheets to cover the entire units. Some paper for insulation.

Special note:-

Please make sure the Vcc is 5Volt dc .

How many contacts I have made with this ? None,

Is there any AM signal ? Yes there might some leakage of carrier due to the gain stages and absence of the xtal filters.

What development I can make ? well nothing much, in this one. if you don't like the tuned stage, please use the resistor as load but then there will be no harmonics filtering. This is just a simple DSB modulator. Take it as a basis for your own design.

Can I use the external VFO, DDS:- No need .its made for simplicity. If you want to use vfo/dds then it will be more rewarding to make a complete trx set of bitx.


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