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 Post subject: 74HC04 Franklin Osc based AM Transmitter. (Cool circuit!)
PostPosted: Sep Sun 26, 2021 8:40 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 734
Two 74HC04 inverter IC used.
IC1 is in Franklin configuration. LC tank circuit is heavily isolated to prevent drift. Two gates in series provide additional isolation. Then two more gates drive IC2 where all six inverters have been paralleled to increase current output capacity to 25ma or so as we do not want a shortage of drive power when TX output is touching 100% modulation peak. An emitter follower wired around TIP31 powers the PA section, a 2n2222A transistor. It's driven quite hard into saturation by IC2. Notice how oscillator IC1 is heavily isolated from the power supply with 100uH RFC and few capacitors to ensure an interference free and stiff 5V power supply for stability purposes. At 12V DC, output of LM386 swings 10Vpp. R2 of TIP31 can be lowered a bit so the point V is exactly at 5V. This would ensure 100% voltage swing at point V from
5 to 0V and 5 to 10V! The 2n2222A is purposely driven hard into saturation or class D mode. It's better than class C and does not saturate the transistor to mere 80-85% modulation levels. The 2n2222A has a FT of 330MHz which is good for our class D amplifier for quick rise and fall times. The 100ohm series resistor in its collector prevents instability at class D operation and also limits final DC power input to <100mW. Output is 50ohm. A matching network to a 6 feet wire antenna is desperately required as I did not take the trouble to design one. Maybe we can use Vectronics 1290K output network. An audio compressor would surely make the audio sound 'loud and clear.' Why use crystals or complicated PLL when high frequency stability Franklin oscillator option is there. The coil L could be a permeability tuned. Its resonating capacitor in parallel should be a NP0 type.
Now you have got a perfect AM broadcast transmitter to transmit your favorite music around the house and yard!


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Last edited by Dare4444 on Sep Sun 26, 2021 9:45 pm, edited 12 times in total.
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 Post subject: Re: 74HC04 Franklin Osc based AM Transmitter (Cool circuit).
PostPosted: Sep Sun 26, 2021 8:45 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 734
Comments, thoughts, and modification ideas welcome! I feel it is one of my best circuits yet. A brass tuned oscillator would be so cool! The transistor is madly driven into saturation to boost the modulation index. I am eagerly waiting for comments, kinda proud of this design, lol. Well thought of design. Better than Vectronics!


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 Post subject: Re: 74HC04 Franklin Osc based AM Transmitter. (Cool circuit!
PostPosted: Sep Thu 30, 2021 2:35 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4724
Location: Lexington, KY USA
A couple of things come to mind.

The 74HC04 is a hex inverter. They do make hex non-inverting buffers as shown, but I doubt if this would be a good choice.

The 74HC outputs are moderately powerful; perhaps not so many need to be paralleled. Octal inverting buffers having greater current output are also a possibility.

Have you looked at the small signal phase shift through the 74HC04? How many stages are used in these? About three stages per buffer? And how hot do these get with the output centered? Note that spice sometimes gets it wrong when the part is used off-label. For linear use, they had to make special "UB" metal gate CMOS inverters when they went to three stages instead of one.

The LM386 should be able to source plenty of current for the modulator stage supply. You might ditch the TIP31 . This would operate the LM386 output class A, reducing crossover distortion. There are several ways a resistor can be used to move the LM386 DC output point up or down, if this is desired. If you are not happy with the LM386 used that way, there are plenty of other small audio power amp ICs.

And surely the TIP31 has at least one emitter?

Ted


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 Post subject: Re: 74HC04 Franklin Osc based AM Transmitter. (Cool circuit!
PostPosted: Oct Fri 01, 2021 7:08 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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Ckt was not tested on spice. Experience says it will work.
3 gates can now give 12ma peak current. TIP31's emitter is modulating the PA. Isn't it a class A emitter follower. Some Nonlinearity maybe introduced by the transistor but would it be audible? Extra chip omitted. A 82uH inductor on PVC pipe can be used for ant matching. Ant and tuning cap connected to bottom, top to V+ and collector to a tap nearby. I wanted 24max peak current, no chance of AM not sounding clean and natural then, comes from my past experience. This is okay I guess. The Franklin osc will work no doubt about it and class D can reach 95-100% modulation. Drive should be very stiff.

Phase delay would matter highly in SSB. Here in the new modified circuit it shouldn't matter I think.


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 Post subject: Re: 74HC04 Franklin Osc based AM Transmitter. (Cool circuit!
PostPosted: Oct Fri 01, 2021 9:00 pm 
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In AM if you modulate a driver stage to, say, 25%, and then feed that into a Class-C power amplifier, it will have the effect of increasing the modulation depth. This will eliminate the distortion problems when you try to modulate a power amplifier to 100%.

Here T1 can easily be modulated to 50%. The colpitts sounds excellent if xtal controlled.

Not tested this on spice but experience comes into play here. Output primary winding should be optimized for max RF out with 100mW in. There are so many ways to build AM circuits. This latest circuit is traditional, but offers very good audio quality. I am thinking if a 1mhz Pushpull osc could be made with coil and two 2n3904s, modulated to 30% or so, and driving another modulated Pushpull 2n3904 PA stage, the sound would be exactly like the original audio plus 100% low distortion modulation index.


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 Post subject: Re: 74HC04 Franklin Osc based AM Transmitter. (Cool circuit!
PostPosted: Oct Sat 02, 2021 12:54 am 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 711
Location: Colorado Springs, CO 80917
Quote:
Experience says it will work.


You mean you've built one of these, right?


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 Post subject: Re: 74HC04 Franklin Osc based AM Transmitter. (Cool circuit!
PostPosted: Oct Sat 02, 2021 2:47 am 
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Joined: Mar Thu 01, 2018 1:30 am
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hwhall wrote:
Quote:
Experience says it will work.


You mean you've built one of these, right?


Yes, in parts, in full, in half, connected, checking sound quality, oscilloscope monitoring, etc


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 Post subject: Re: 74HC04 Franklin Osc based AM Transmitter. (Cool circuit!
PostPosted: Oct Sat 02, 2021 4:24 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 734
hwhall wrote:
Quote:
Experience says it will work.


You mean you've built one of these, right?


Take this circuit. Modulating a typical colpitts gives 50% max mod. Adding 43 material ferrite RFC in emitter and raising the oscillator current to max until it almost stops oscillating plus adding a buffer changes 50% to 100% mod but how?? Is the emitter RFC storing RF power and dumping it back? It was tested on 40m or 7.2 mhz but still with 1mhz crystal it should work as expected. The emitter RFC might need to be higher in value and two feedback 470pf caps maybe adjusted too for 100% loud clear modulated audio on receiver. Setting final power in is also easy, the emitter resistor of the buffer. I mean audio quality is really good. This is a new trick that I learned.


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 Post subject: Re: 74HC04 Franklin Osc based AM Transmitter. (Cool circuit!
PostPosted: Oct Sat 02, 2021 4:41 am 
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Joined: Mar Thu 01, 2018 1:30 am
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Take this for example. It only works with 2n2219A probably due to the low base resistor values. It ensures enough bias at modulation peaks. I didn't check it on an oscilloscope but peak voltage RF output nearly doubled on a diode power meter when I introduced 1khz tone near the mic. It means it was close to 100% modulation. RF power was 40mW into 50ohm as 7.2MHz is legal ham band. For Part15, increase emitter resistor for <100mW power in and use a 1MHz crystal


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 Post subject: Re: 74HC04 Franklin Osc based AM Transmitter. (Cool circuit!
PostPosted: Oct Sat 02, 2021 4:59 am 
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Joined: Mar Thu 01, 2018 1:30 am
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Again 100% mod by virtue of the emitter coil RFC.
2.2K coupling lightly loads the oscillator. Now just add 1MHz xtal and bias the PA for DC <100mW. The scope shows 100%
modulation at 7.2MHz

For 1MHz operation experiment by using a higher value for the inductor


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