Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Oct Tue 20, 2020 3:25 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 24 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Part 15 AM transmitter
PostPosted: Aug Tue 11, 2020 1:53 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
I'm looking at possibly completely redoing my two tube 6GY6 transmitter and going solid state.

Would it be possible to use something like this

https://www.pongrance.com/DDS2018.html

and use it as part of a LM-386 transmitter with a 2N3904 in place of the TCXO?

That would give me the option of tuning the whole AM band.

I do have a receiver that receives between 6-9MHz which I want to send the signal to as well provided I can do it legally and this would fit the bill as I can save frequencies and easily recall them. The receiver is in the same room as the transmitter so whatever power is needed would be very small.

What I don't know is how the transmitter can be made to cover a wide range of frequencies while still making the most of the 100mW max output.

Would it be better to find an RF signal generator (analog or digital display) with an amplitude modulation input that has a modulation bandwidth of at least 30-5KHz which properly modulates the RF signal?

Just want something where I don't have to depend on tubes being good.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Wed 12, 2020 4:33 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
A 74hc4053 would easily make an AM Transmitter covering 1-10MHz and driven by square waves with DDS.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Wed 12, 2020 5:03 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
So how would I use that chip?

Also that VFO I linked to has a pin on the chip that can be used for amplitude modulation but its at a fixed DC level set by a resistor. I'd need a way to control that.

Clear transmission range needs to be no more than around 100' or so.

I would want to install the transmitter in the same cabinet as my two tube transmitter where the new circuit would replace the old.

Now if I can find a low cost RF generator that will do what I need I'll definitely go that route and if necessary add a small amp such as a 2N3904 on the output to get me the required range.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Wed 12, 2020 8:12 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
Here you go. The 74HC4053 functions as a mixer and audio frequency from LM386 is combined with RF and this results in amplitude modulated career. Connect a wire antenna directly to the output capacitor and it should cover your entire room. You'll need square waves to drive the mixer. It should work well from 1 to 30MHz.


Attachments:
IMG_20200813_004137.jpg
IMG_20200813_004137.jpg [ 687.73 KiB | Viewed 736 times ]


Last edited by Dare4444 on Aug Wed 12, 2020 8:39 pm, edited 1 time in total.
Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Wed 12, 2020 8:23 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
Ground the input pins of unused gates.


Attachments:
IMG_20200813_005208.jpg
IMG_20200813_005208.jpg [ 412.21 KiB | Viewed 735 times ]
Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Wed 12, 2020 9:13 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
Oh ok cool.

Now it provides a proper AM signal right?


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Thu 13, 2020 12:47 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
Yes


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Thu 13, 2020 5:17 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
One mistake in the schematic. Pin 9, 10, and 11 are connected together. The schematic shows only 10 and 11. Include pin 9. The square wave is fed to three pins. You should get a nice AM signal. Connect a LPF at the output if you want a clean signal with no harmonics.

Tbe 74HC4053 mixer contains three digital switches. All three are connected in parallel to lower the 'ON' resistance.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Thu 13, 2020 2:12 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
If your frequency of interest is 6 to 9MHz then use a 7 pole 10MHz LPF at the output to eliminate harmonics. It will drive a 50ohm load. I'm not sure of the output power but expect a 10mW career with peaks rising to 40mW with 100% modulation. Audio quality is high fidelity.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Thu 13, 2020 7:16 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
Cool.

Perhaps without a filter it will hit 8 or 9 MHz with a 1MHz TCXO on harmonics


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Thu 13, 2020 11:59 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
Yes.. try it. Order the components. Who knew building a wideband AM modulator was so easy!


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Fri 14, 2020 12:18 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
Yep it's easy for sure, but I gotta make sure that harmonics are not above the specified level for a part 15 AM broadcaster running in the AM band as my next door neighbor is a ham and I don't want to be a source of interference.

Depending on how the chip operates (if it is open when no signal is present on 9 10 and 11) maybe I could use two chips, two TCXO and two filters with a switch to select which TCXO gets power. That way I could have two frequencies in use.

Far as filters go I am thinking maybe I'll figure a filter that can be put for instance between the chip and a 2N3904 or other better transistor that can go to 9MHz or perhaps I could build an OP-AMP unity gain filter if there's readily available OP-AMPs that can handle up to 9MHz.

That way the filter is not dependent on the antenna impedance.

Or is it possible to have both chips operating at the same time while being fed by the LM-386 so I have a simultaneous output at 1MHz and whatever frequency that I can legally transmit on in the 6-9MHz band.

I know 6MHz and the 7MHz ham band is no go. 7.5 MHz is no go as well. I'm thinking maybe something between 8-9MHz.

Only reason I would want to transmit to the radio is because it will give the radio some use as there's nothing on the 6-9MHz frequencies I really care to listen to anymore.

I can hear preaching or foreign languages or some english language stuff I am not interested in.

Not much of a choice there.

Radio Havana Cuba used to be real good, but ever since they went to repeating just one hour english programming daily (instead of two like they used to do years ago) and updating their music I haven't much cared to listen to them.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Fri 14, 2020 1:01 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
Is a 2n3904 really needed? A 7 pole LPF will remove all Harmonics. Connect a few feet of wire antenna directly to the output of LPF.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Fri 14, 2020 1:09 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
I was thinking an active filter would be better and not dependent on the antenna.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Fri 14, 2020 2:45 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
You could try that too.
Or a filtering feeding a 50ohm common base amp and then to antenna. This way varying antenna impedance won't affect the filter.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Fri 14, 2020 2:45 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
You could try that too.
Or the filter feeding a 50ohm common base amp and then to antenna. This way varying antenna impedance won't affect the filter and sound quality.


Last edited by Dare4444 on Aug Fri 14, 2020 3:26 am, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Fri 14, 2020 3:04 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
I wonder if there's any prebuilt rf filter circuits like how there's prebuilt RF amp boards.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Fri 14, 2020 3:32 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
Use 1uH ready-made molded inductors. It's simple.


Attachments:
IMG_20200814_080022.jpg
IMG_20200814_080022.jpg [ 427.33 KiB | Viewed 627 times ]
Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Fri 14, 2020 4:02 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
How would I build a filter for 1MHz so that only 1MHz is sent out to the antenna? Guess what I'm wanting is a filter that makes the 1MHz square wave into a sinewave of 1MHz which will then be buffered by an amp stage before the antenna.

At one point I almost had the 1MHz TCXO in my 1MHz two tube transmitter producing a clean sinewave by using a variable inductor between the TCXO and control grid of the tube.


Top
 Profile  
 
 Post subject: Re: Part 15 AM transmitter
PostPosted: Aug Fri 14, 2020 5:17 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
-60dB attentuation at 5MHz
-20dB attentuation at 2MHz.
Again use 10uH inductors.

It will output a nice sine wave to drive a broadband (pushpull?) 2n3904 with 50ohm input impedance.


Attachments:
IMG_20200814_094505.jpg
IMG_20200814_094505.jpg [ 79.09 KiB | Viewed 623 times ]
IMG_20200814_094531.jpg
IMG_20200814_094531.jpg [ 210.67 KiB | Viewed 623 times ]
Top
 Profile  
 
Post New Topic Post Reply  [ 24 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 12 guests



Search for:
Jump to:  


































-->


Privacy Policy :: Powered by phpBB