Demodulator plans?

#366211

Can anyone point me to some plans to build a demodulator as referenced by 543Fortworth in this post?
http://www.cbradiotalk.com/vi ... or#p352728
I found one on utube* but he doesn't have a parts list so not much help for me there.
Any help would be greatly appreciated, Thank you
hank
*wont link it cuz he may have a business

#366217

Hey Hank...

Best thing to do is to PM 543 and see if he has any more info.

Bombero

#366219

Bombero wrote:Hey Hank...

Best thing to do is to PM 543 and see if he has any more info.

Bombero

I've actually tried that but my pm's wind up in the outbox, don't know how to fix.

Deleted User 14541 · #366244

I got your messages hank. It's been a busy weekend. Here is a diagram and picture of what I'm using. It is for low power only. The most it ever sees is 100 pep from an HF rig. I'm using a double male pl259 to connect it to the back of the radio.

#366248

Thank you Sir!

#366251

thanks for sharing 543

#366253

Excellent little box there 543.

3's

Greg

Deleted User 14541 · #366254

The second photo is how I originally built it and the 2.2k resistor is not there. Before adding the resistor the audio in the headphones sounded a little rough around the edges. I also added more wraps of mag wire because the radio it stays connected to only does 8-10 pep and it wasn't very loud.

#366883

Almost done, I used a ratshack enclosure however speak of the devil I have hit a snag. R shack wants $6.99 to ship a $2.96 diode and I'm not goin to do it.

Deleted User 14541 · #366888

Looking good. Almost any rectifier diode will do the trick. I don't think I paid over $6 for the 50 or so 1n6263 diodes that are in my bin. Radio Shack is pricy on their components because you're paying for the convenient packaging. It's hard to tell by the pic but the coil isn't shorted against the wire its wrapped around is it? The 10k pot might make the adjustment touchy but should work fine otherwise.

#366906

Thanks! I used shrink tubing on the ends of the coax and rubber faucet washers to isolate the coil from the coax. I will look for a 1k pot, maybe Fryes has them or that computer junk store on 303.

-- Sunday, 04 January 2015, 16:32 PM --

Ok I found a work around on the diode, ordered it off of ratshack's fleabay and got free shipping hehe. Also ordered a 1k pot from another 'bayest. While I was waiting I found an old door opener and salvaged the diode out of it and made a new coil smaller diameter and more turns and now I have a working demodulator. These are fun to build, get off the fence if your thinking about it. I don't want to add a talkback board on my TRC-485 but do wish to play with echo mike so now I'm all set!

-- Sunday, 04 January 2015, 17:17 PM --

I hooked it up to the computer to get a sample with my wife's help. The hum is an issue with my old sound card.

#367075

Another thing that you could add is a tap for a counter or scope, just need a BNC connector (panel type) and a 10pf cap, take one leg of the cap to the center of the BNC, the other to the center of the SO239, insulate it so it doesn't short to the windings.

3's

Greg

#373028

I added a little bit to 543 drawing,

This will give you a tap for a frequency counter on the resistor BNC (top BNC) and a tap for an oscilloscope on the cap BNC (bottom BNC).
I started building it tonight, got this much done,

I'll be adding the components tomorrow if I get the time. More pics to come.

3's

Greg

Deleted User 14541 · #373034

Cool. I have a jack on mine with a 10 or 15 pf cap to feed a cheap sdr receiver. It works good for a scope pick up too.

#373035

543FtWorth wrote:Cool. I have a jack on mine with a 10 or 15 pf cap to feed a cheap sdr receiver. It works good for a scope pick up too.

Now that is neat, never thought of it. Now can you key up and does your transmit show up on the SDR or would it be too much signal and overload it. I'm pretty sure the input on one of those radios is less than 40mV.

3's

Greg

#373039

I gotta say that is a fun to build and rewarding little project, Thanks guys!

Deleted User 14541 · #373043

MDYoungblood wrote:
543FtWorth wrote:Cool. I have a jack on mine with a 10 or 15 pf cap to feed a cheap sdr receiver. It works good for a scope pick up too.

Now that is neat, never thought of it. Now can you key up and does your transmit show up on the SDR or would it be too much signal and overload it. I'm pretty sure the input on one of those radios is less than 40mV.

3's

Greg

The sdr I have will overload even if no antenna is attached to it. That's the only complaint I have about it but it's just a $20 USB stick. In the past I used a relay inside the radio to disconnect the sdr antenna coax and ground it when I key the radio. The sdr receiver isn't very sensitive going through the cap but it works ok for local. I just like to be able to see the whole band or monitor a few channels at once.

#373046

Update,

I didn't think the volume was good enough so I capped it up to give it a little cleaner audio,

Test it out some more later.

3's

Greg

Deleted User 14541 · #373048

Did the caps make it louder? The .01 cap just filters rf after the diode. On the last one I built I found that the volume was louder if I tacked the ground side if the coil to the case and only made one layer of wraps.

I was making one pass, covering it with shrink tube and making a second or third layer. It seemed to be counter productive. I do hear a bit of distortion when the volume knob is turned down. It's not very noticeable with a normal radio but with wideband audio it is. I've been thinking of using the pot to pull the audio to ground vs adding resistance inline....haven't got around to it yet.

Using a higher value resistor or trying it without one will make it louder too.

#373052

It didn't increase the volume much but it did clean up the distortion. I was using a CB HT that doesn't have a lot of modulation to begin with, I'll be playing with it some tomorrow with a AM mobile I'm working on, it produces loads of modulation and wattage to excite my frequency counter, don't have a scope yet to see how that part works.

3's

Greg

#378840

I hate to rez the dead thread here but i'm building this thing!

I would like to know if i should add an RF ground-path wire directly to both so-239 connectors (from the outer conductor), and what about rf shielding issues or is the tin enough?

#378841

LeapFrog469 wrote:I hate to rez the dead thread here but i'm building this thing!

I would like to know if i should add an RF ground-path wire directly to both so-239 connectors (from the outer conductor), and what about rf shielding issues or is the tin enough?

Use a metal box and there will be no issues with RF leakage. I still haven't tried the tap I made for the scope yet, don't have one.

3's

Greg

#378928

I know this isn't a demodulator but... I made a simple pickup for my scope with a coax "T", I unscrewed the pin from one connection and shortened it so it wouldn't make contact. Works fine and the pin is threaded, so it's adjustable.

#383411

Look's good, 343.
Thank's for the head's up on this post, Greg.
I'm going to have to manifangle one of there too.
C-ya Trapp.

#383412

I built mine for about $20, most expensive thing was the aluminum box ($14), I've seen similar on that auction site for over $200.

3's

Greg

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