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Simple AM transmitter




 
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K1WIZ
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« on: November 06, 2019, 02:08:38 PM »

Hi folks, I'm looking to build a simple AM transmitter for broadcasting my old time radio collection to my radios.  I have drafted the attached circuit in Fritzing and my goal is to keep this as simple as possible with a minimum of components.

Desired frequency is 1MHz.   Can someone assist me with LC filter value selection?

* AMTX_schem.pdf (484.61 KB - downloaded 142 times.)
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« Reply #1 on: November 06, 2019, 11:26:34 PM »

Do I take it you want a completely SS unit with a Class A, source modulated final?

Do we also assume you want this to operate under Part 15 Rules?


Phil - AC0OB
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K1WIZ
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« Reply #2 on: November 07, 2019, 07:39:26 AM »

Phil,

Thank you for your reply.  Yes letís assume the following:

1) completely SS
2) keeping the crystal oscillator (I have one id like to use
2) more power output than part 15, but the ability to turn down to part 15 levels (Iím planning for carrier current)

Best,

John, K1WIZ
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K1WIZ
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« Reply #3 on: November 07, 2019, 07:44:26 AM »

if there is a better way to  setup the final (with a minimum of parts) and still achieve 100% modulation and good fidelity, Iím open to ideas/suggestions.   Smiley
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K1WIZ
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« Reply #4 on: November 07, 2019, 09:50:05 AM »

Phil, I wouldn't be totally opposed to a "glowing" final.  I'm looking for developed power of about 5 watts (for carrier current), with the ability to lower it to 100mW if an intentional radiator is used (for Part 15 compliance).   I'm hoping to leave all the options open but keep the design as simple as possible.
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M0VRF
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« Reply #5 on: November 07, 2019, 11:42:43 AM »

Simplest and best design to date is single ended class E.

Using and IRF510 driven by an NCP1074A driven by a Cardianal CPP osc from DK.

19 components total, including decoupling and V reg etc.

Easily acheive a 10W carrier with 12V.

Modulation wise use the class D amps from CH using the TPA3116D2, run in mono (PBTL) and use either O/P to PWM modulate the RF stage.

As it's series mod you'll get 1/2 supply so with just 6V at resting carrier you get 2W5.

You can, of course, use 24V and get the full 10W.

If you want part 15 compliance just run it thru' an attenuator!

You can use your own osc if you want as long as it stuffs out 3V p-p but the CPP's are $5 and ANY freq you want.

Simples...

J.




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K1WIZ
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« Reply #6 on: November 07, 2019, 12:28:25 PM »

J,

Do you have an alternate part number for NCP1074A?   Google responded: Your search - NCP1074A - did not match any documents.

Excellent suggestion on your other ideas!  Can you attach a rough schematic?   I think my osc puts out 5v  (it's TTL).

Very interested to try building this.

Best,

John, K1WIZ
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K1WIZ
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« Reply #7 on: November 07, 2019, 12:34:35 PM »

Would the IRF540 be a good substitute for the IRF510?   I have 540s on hand.  Looks like they are 100W devices where the IRF510 is rated for 50W
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K1WIZ
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« Reply #8 on: November 07, 2019, 12:39:00 PM »

Found this class D amp: https://www.amazon.com/LM-YN-TPA3116-Channel-Amplifier/dp/B01M0VZ4SA
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« Reply #9 on: November 07, 2019, 09:23:31 PM »

Phil, I wouldn't be totally opposed to a "glowing" final.  I'm looking for developed power of about 5 watts (for carrier current), with the ability to lower it to 100mW if an intentional radiator is used (for Part 15 compliance).   I'm hoping to leave all the options open but keep the design as simple as possible.


For a vacuum tube design see:

http://amfone.net/Amforum/index.php?topic=42107.0


Phil
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« Reply #10 on: November 07, 2019, 09:36:33 PM »

Another alternative is to use the Modulator and final from this 5W ClassE transmitter portion of this kit and simply change the component values in the final.

http://www.4sqrp.com/n-75.php


Phil
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K1WIZ
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« Reply #11 on: November 07, 2019, 10:27:08 PM »

Simplest and best design to date is single ended class E.

Using and IRF510 driven by an NCP1074A driven by a Cardianal CPP osc from DK.

19 components total, including decoupling and V reg etc.

Easily acheive a 10W carrier with 12V.

Modulation wise use the class D amps from CH using the TPA3116D2, run in mono (PBTL) and use either O/P to PWM modulate the RF stage.

As it's series mod you'll get 1/2 supply so with just 6V at resting carrier you get 2W5.

You can, of course, use 24V and get the full 10W.

If you want part 15 compliance just run it thru' an attenuator!

You can use your own osc if you want as long as it stuffs out 3V p-p but the CPP's are $5 and ANY freq you want.

Simples...

J.


I think I may want to go with this design, but I am unsure how to "PWM modulate the RF stage" (series mod)?

I'm assuming (maybe incorrectly) that the NCP1074A may be a bipolar transistor of some kind?  Google returned no hits on that part number.
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M0VRF
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« Reply #12 on: November 08, 2019, 08:44:44 AM »

Apologies, it's NCP81074.

Modulating the RF stage is achieved as described and Yes it's series mod. Very efficient as they are class D.

The 540 has a rather High Qg and hard to drive, mind you you're only up at 1MHz do maybe ok?


Regards.

J.

ps. Please PM me for more info.
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« Reply #13 on: November 08, 2019, 01:43:57 PM »


For a vacuum tube design see:

http://amfone.net/Amforum/index.php?topic=42107.0


Phil

On page 7 in the file of that topic, concerning the oscillator, should not the plate of the plate of V2A be connected to the junctiuon of C2 and R5?

(I asked this in there, before realizing we are here. spacetime coordinate error!)
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« Reply #14 on: November 08, 2019, 01:57:31 PM »

Here is a possible circuit for bread-boarding for your consideration using the KISS principle:

Heat Sinks should be placed on each transistor.

Added active pullup so final is off when in Stop.


Phil - AC0OB

* 10W 1 MHZ AM LPAM.pdf (115.13 KB - downloaded 56 times.)
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« Reply #15 on: November 08, 2019, 02:07:29 PM »


For a vacuum tube design see:

http://amfone.net/Amforum/index.php?topic=42107.0


Phil

On page 7 in the file of that topic, concerning the oscillator, should not the plate of the plate of V2A be connected to the junctiuon of C2 and R5?

(I asked this in there, before realizing we are here. spacetime coordinate error!)

Pin 3 IS connected to C2 and R5. There is an implied junction anytime there is a NOT a jumper symbol or a faint dashed line displayed.

Phil
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N1BCG
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« Reply #16 on: November 10, 2019, 06:17:43 AM »

You are likely to run into insurmountable him issues using the carrier current option. Imposing an RF signal onto power wires carrying 60Hz leaves the signal susceptible to mixing (the carrier-current hum effect). I battled this vigorously at my college station.

The Part 15 solution will result in a far cleaner signal as long as the antenna is not near your power wiring. Part 15 requires 100mW max input to the final PA, so using low loss L/C components in the output circuit is important.
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« Reply #17 on: November 10, 2019, 10:14:21 AM »

This a site  https://www.hobbybroadcaster.net/  deals almost exclusively with Part 15 Broadcasting.

Phil - AC0OB
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K1WIZ
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« Reply #18 on: November 11, 2019, 10:01:08 AM »

For the part 15 purists out there:

I may do as suggested (if not carrier current) use attenuation to drop the output to 80mW (ensuring an output that is part 15 compliant).

I also want to use the same circuit design for a 75 Meter QRP TX, so I will recycle the design with the appropriate LC components.   This way I can order most parts in quantity and build these for different intended uses.   As for the hum on carrier current, (which I still would like to try) I have heard AM CC stations that sounded just fine.

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« Reply #19 on: November 11, 2019, 12:30:57 PM »

Quote
Signal coverage may be extended to some extent by implementing "neutral loading" where the RF signal in injected into the neutral wire of the electrical service. Care must be made to ensure any radiated signal continues to comply with Part 15.221.

https://www.hobbybroadcaster.net/resources/carrier-current-AM-broadcasting.php

Isolation, transient voltage protection and Impedance Matching will be important here.

Phil - AC0OB
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« Reply #20 on: November 11, 2019, 08:23:03 PM »

This a site  https://www.hobbybroadcaster.net/  deals almost exclusively with Part 15 Broadcasting.

Phil - AC0OB

This is very interesting!

I notice one thing they discuss there is the so called range extender, a nice-looking product for sale.
http://talkinghouse.com/range-extender-2.5.htm

The range extender seems to be an antenna matcher and antenna combination that allows one to put the matcher with antenna in one place, say on the roof or 20FT mast, and put the transmitter many yards away on a desk inside, and run (presumably non-leaking) coax to the range extender.
But the fcc says:

"ß15.219   Operation in the band 510-1705 kHz.
(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts.
(b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters."


see:  
http://www.talkinghouse.com/faqs.htm
The last Q&A. - FCC approved the Talking House (i A.M. Radio) Transmitter for operation with interconnecting coaxial cable and its Range Extender, which is comprised of the 3-meter antenna/ground as a complete system under Part 15.219."

I could not find an installation manual for this piece of equipment that could explain it better. Check out this discussion about what seems to be going on, how this may work within the law:
https://part15.org/forums/topic/iss-range-extender/#post-111698

All this said, perhaps my oldies CDROM will play noiselessly throughout the house on a 9 FT antenna from a 100mW input transmitter.
And perhaps not in my QRM-plagued house. There is no big band and old-oldies station around here any more since 770 went back to the original religious format.

might be interesting:
US Patent Number US 7437130 B2
google:
https://patents.google.com/patent/US7437130
https://patents.google.com/patent/US6973294
https://patents.google.com/patent/US6295443
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M0VRF
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« Reply #21 on: November 12, 2019, 12:45:24 PM »

I'll post up a schematic of the simple class E asap.

But from the pcb design you can see how simple this really is.

19 components total

I use the C3M SiC's from CREE (TO247) and get around 30-40W carrier with 24V

For low power using 12V you get around 10W

So running PWM and half Vcc you get a 2W5 carrier and 10W peak, makes for a nice 'Around the house' TX.

Regards

JohnB.


* SIC PCB.png (85.73 KB, 1265x603 - viewed 68 times.)
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« Reply #22 on: November 13, 2019, 10:37:02 AM »

19 components! looks good, very compact.

For something FCC approved (100mW input) there are the 'talking house' units which have external audio input, frequently pretty cheap on the popular site but check the model number.

There are versions with 'music' fidelity and units with 'speech' audio fidelity and they look just alike. The reason for mentioning it is that a non-technical guest may come here looking for technical information, and the difference between the models is not obvious.
There is a comparison here and the big difference is audible:  https://www.youtube.com/watch?v=NQ2_ooYHcuY

high fidelity model: IAR-17
Enhanced Talking House AM Transmitter from I.A.M.

standard 'voice' fidelity model: Version 5.0 (and lower)
Talking House AM Radio Transmitter Version 5.0
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« Reply #23 on: November 13, 2019, 05:06:20 PM »

I'll post up a schematic of the simple class E asap.

But from the pcb design you can see how simple this really is.

19 components total

I use the C3M SiC's from CREE (TO247) and get around 30-40W carrier with 24V

For low power using 12V you get around 10W

So running PWM and half Vcc you get a 2W5 carrier and 10W peak, makes for a nice 'Around the house' TX.

Regards

JohnB.

Is there a way to operate this design at 100mW input, perhaps with a smaller output stage?
I ask because the easiest part 15 regulations with which to comply state the input power of 100mW and the length of antenna + ground lead to be 3 meters, and no field strength measurement is required.
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M0VRF
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« Reply #24 on: November 17, 2019, 02:55:12 PM »

Yes, quite simply...

Run it from a lower voltage!
Half volts = 1/4 power.

JB.
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