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 Post subject: AM Transmitter Design
PostPosted: Dec Tue 19, 2017 8:21 pm 
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I'm making a model AM Transmitter with a custom circuit and I'd like some feedback on it.

This is quite a basic circuit and I expect it to work but I'm also quite the newbie to the subject and you guys may see something I don't

Schematic in attachments


Attachments:
keri.pdf [28.53 KiB]
Downloaded 43 times


Last edited by Gip-Gip on Jan Thu 25, 2018 4:12 am, edited 3 times in total.
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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Wed 20, 2017 1:00 am 
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Here's the PCB and a crude 3d sketch. Almost entirely made of SMDs but I'm able to get away with a smaller board size, putting high-qual US-based PCB services in my price range

The images of the PCB were screen-capped from OSHPark, my PCB manufacturer of choice

As for the sketch, I slapped together a model in FreeCAD and it should give a general idea. The PCB's the grey blob in the middle suspended by the 4 other grey blobs that sit atop the big grey blob.

I'll order the parts by new year and I'll have test results by mid January

Like before, I'd appreciate some feedback.

EDIT

Updated the PCBs a bit. Changed the trimmer to a (in many ways better) SMD, freeing up a bit of board space

Attachment:
Screen Shot 2017-12-19 at 06.48.31 PM.png
Screen Shot 2017-12-19 at 06.48.31 PM.png [ 42.22 KiB | Viewed 2046 times ]

Attachment:
pcbt.png
pcbt.png [ 41.2 KiB | Viewed 2029 times ]

Attachment:
pcbb.png
pcbb.png [ 50.73 KiB | Viewed 2029 times ]


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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Wed 20, 2017 5:58 am 
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Gip-Gip wrote:
I'm making a model AM Transmitter with a custom circuit and I'd like some feedback on it.

This is quite a basic circuit and I expect it to work but I'm also quite the newbie to the subject and you guys may see something I don't

Schematic in attachments


Welcome to the forum! Might I suggest you post an image of the schematic? PDFs can contain malware, I don't know you yet, and I'd need to see your schematic to comment on it. Makes sense?


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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Wed 20, 2017 7:10 am 
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Location: Black Hills, SD 57745
Welcome to ARF! Nice little design you came up with. Here's a screen grab of the pdf schematic, rearranged a bit to make an ARF-friendly jpeg.
-Ed


Attachments:
keri.jpg
keri.jpg [ 46.72 KiB | Viewed 2009 times ]
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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Wed 20, 2017 5:03 pm 
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Macrohenry wrote:
Welcome to the forum! Might I suggest you post an image of the schematic? PDFs can contain malware, I don't know you yet, and I'd need to see your schematic to comment on it. Makes sense?


Understood!

Ed in SoDak wrote:
Welcome to ARF! Nice little design you came up with. Here's a screen grab of the pdf schematic, rearranged a bit to make an ARF-friendly jpeg.
-Ed


Thanks for the screen grab and the complement! I already updated the schematic/pcb to use a panel-mount switch and jack, so here are the updated images:

Attachment:
Screen Shot 2017-12-20 at 10.59.08 AM.png
Screen Shot 2017-12-20 at 10.59.08 AM.png [ 69.2 KiB | Viewed 1985 times ]

Attachment:
pcbt.png
pcbt.png [ 53.08 KiB | Viewed 1985 times ]

Attachment:
pcbb.png
pcbb.png [ 29.01 KiB | Viewed 1985 times ]


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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Thu 21, 2017 5:05 am 
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I've been working on ways to allow switching frequencies and the one I chose is to use a SPDT switch to bypass(or not bypass) a few axial inductors. This allows the transmitter to operate at 1260, 890, and 727 kHz

I added such configuration to the schematic below

Attachment:
Screen Shot 2017-12-20 at 10.33.19 PM.png
Screen Shot 2017-12-20 at 10.33.19 PM.png [ 78.84 KiB | Viewed 1941 times ]


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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Fri 22, 2017 5:50 am 
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Added a few TVSs and the schematic/pcb is near completion! For those interested I'll work on the base this weekend, and I'm quite excited to get this thing together!

Attachment:
Screen Shot 2017-12-21 at 11.47.00 PM.png
Screen Shot 2017-12-21 at 11.47.00 PM.png [ 60.79 KiB | Viewed 1867 times ]


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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Sat 23, 2017 6:29 am 
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The PCB's ready to ship! I'll wait till after Christmas to order, but otherwise no more amendments will be made to the design!

Attachment:
pcbt.png
pcbt.png [ 50.72 KiB | Viewed 1817 times ]

Attachment:
pcbb.png
pcbb.png [ 46.99 KiB | Viewed 1817 times ]


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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Sat 23, 2017 7:35 pm 
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Location: Mount Pleasant UT 84647 (from Coventry, UK)
It is always good to see another transmitter design. I am not smart enough to comment on the design, but I do have one question. Considering that a lot of people will feed it with an iPod or PC, both of which have stereo outputs, would it not have been a good idea to use a stereo input jack and sum the channels on the board? Just wondering.

Colin


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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Sat 23, 2017 7:53 pm 
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Colin Ames wrote:
It is always good to see another transmitter design. I am not smart enough to comment on the design, but I do have one question. Considering that a lot of people will feed it with an iPod or PC, both of which have stereo outputs, would it not have been a good idea to use a stereo input jack and sum the channels on the board? Just wondering.

Colin


Can't say that didn't slip out of my head :oops:

Easy fix though, will do

I'll go ahead and cobble up a basic theory, wouldn't hurt if people understood my design as well


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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Tue 26, 2017 9:09 pm 
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Parts have been ordered and will be here within a week or so! Looking forward to testing the performance!


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 Post subject: Re: AM Transmitter Design
PostPosted: Dec Sat 30, 2017 2:49 am 
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Parts from mouser arrived today! PCBs just got in shipment, expecting them midweek at the most!

Here you can see the bags of components and the somewhat crude frequency select switch
Attachment:
20171229_202328.jpg
20171229_202328.jpg [ 134.35 KiB | Viewed 1623 times ]

Attachment:
20171229_181901.jpg
20171229_181901.jpg [ 99.09 KiB | Viewed 1623 times ]


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 Post subject: Re: AM Transmitter Design
PostPosted: Jan Mon 08, 2018 7:37 pm 
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Thanks to recent weather, the transmitter's circuit board is taking longer than expected to ship. I did print out a copy of the board through my inkjet printer and all the parts fit their footprints, which is a good thing.

I will update when the PCB arrives


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 Post subject: Re: AM Transmitter Design
PostPosted: Jan Fri 19, 2018 2:56 pm 
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It works! :D

The carrier's perfectly sinusoidal, the power consumption's low, I can't hear any distortion, it all seems to be working out...

Other than it can barely transmit a foot without fading into the static. But it has had a few hacks and deliberate power limitations, so once I fix those I'll post the results.

I also need to make a few documents explaining the theory of the whole thing, update schematics, etc. etc.

Expect a semi-large update in the near future!


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 Post subject: Re: AM Transmitter Design
PostPosted: Jan Sat 20, 2018 5:36 am 
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Good that it works. Always feels good. Looking forward to your explanation of the mysteries you've presented.


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 Post subject: Re: AM Transmitter Design
PostPosted: Jan Wed 24, 2018 12:45 am 
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I've built a base for it so it looks less ugly. The solder joints are worse than I wish them to be, but it's my first true project, so I'll give myself a break. Unfortunately I don't have my good camera on me so the photos are kinda blurry.

Attachment:
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0123181755.png [ 112.35 KiB | Viewed 1235 times ]


Attachment:
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0123181755a.png [ 157.55 KiB | Viewed 1235 times ]


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 Post subject: Re: AM Transmitter Design
PostPosted: Jan Thu 25, 2018 3:14 am 
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As you have probably observed in my other topic, the transmitter is turning out to be very stable, and to be honest it doesn't surprise me too much. The only thing to "vastly" change spec with temp is the comparator and not only is it not getting hot but the spec only determines the efficiency of the transmitter. As I found, altering the timing pot (tuning pot in current schematic) doesn't affect frequency. I'm still trying to figure out why but this theoretically allows very simple and stable frequency-agile transmitters.

Once the final schematic is up I encourage someone with better equipment than an USB oscilloscope to validate my design; I need someone to crush my dreams :)

I'm still working on documentation, trust me


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 Post subject: Re: AM Transmitter Design
PostPosted: Jan Thu 25, 2018 4:19 am 
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Finished the revision 1 schematic, download the PNG here or the PDF on the first post

Attachment:
Screen Shot 2018-01-24 at 10.16.41 PM.png
Screen Shot 2018-01-24 at 10.16.41 PM.png [ 67.31 KiB | Viewed 1175 times ]


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 Post subject: Re: AM Transmitter Design
PostPosted: Jan Fri 26, 2018 12:18 am 
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Location: Saskatoon
When I first looked at your schematic, I had assumed that the frequency was determined by an RC network which are notorious for drifting. That was the reason for my concern in the other discussion. I see now that the frequency is determined by an LC network, which should be just as good as other LC tuned transmitters that have been presented in this forum, and give good performance. It looks as if the LC network could also be replaced with a crystal or ceramic resonator, if the builder wants the simplicity of a fixed on-channel frequency.

I'm looking forward to seeing your discussion of the circuit, the modulation scheme in particular.


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 Post subject: Re: AM Transmitter Design
PostPosted: Jan Fri 26, 2018 1:14 am 
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I guess I don't want to wait for your write up. I was looking at the modulation scheme, too. It looks like on the positive swing of the audio wave, the comparator flips to high, creating an instantaneous pulse fed to the LC network through D1. The pulse would then last the duration of the time the peak of the audio wave exceeds the voltage on the other input of the comparator. I can see who this would pulse width modulate the output.

I assume the fast rise time of the pulse causes the LC network to ring, producing RF similar to the spark gap of a Tesla Coil. The quality of the ringing thus is dependent on the Q of the inductors and the repetition rate of the audio. If this is the case, what happens when there is no audio? It looks like no carrier would be present? Am I reading this wrong?

BTW, purely curious, how do you pronounce Gip-Gip? Gip like "The Gipper" or Gip like "Gyp?" How did you come up with that moniker?


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