Messages

1

The RF Workbench / Re: Simple but effective FM transmitter help VCO

« on: January 07, 2014, 2348 UTC »

I found the problem and now the VCO works perfect.  The issue was the 15pf capacitor in series with the tank circuit.  It was the incorrect value, I didnt notice when I put it in that it's actually a 1.5pf as opposed to 15pf.  I just couldnt see the point in there til I got a closer look.  With the new change the VCO is working awesome and sweeps across the band no problem with very good audio quality.

Thanks for the advice guys.  I figured the C930 would be a better choice vs a new 2N3904 or something a long those lines since it's an RF transistor. I suppose the original part called for would maybe have a bit more gain?  The plans say the VCO should do 30 meters roughly on a 70cm wire and this does maybe 7 or 8.  Although granted I'm trying it with a lot obstructions around.  Should try it on a clearer area. 

Not sure if I'll be building the rest of it yet as I need to source some more components that are going to be tricky to get, mainly the cap trimmers.

Thanks again. 

2

The RF Workbench / Re: Simple but effective FM transmitter help VCO

« on: January 07, 2014, 0757 UTC »

Thanks for the advice.  

I made my first pcb ever and it turned out really great.  I didnt use photo transfer, I did it the cheapest way I could with what I had.  Which was sharpie marker drawn traces and chloride.  PCB turned out perfect!  I was pretty surprised as I was expecting a lot of pinholes or for the sharpie to give out and let the copper get etched, but even the fine writing turned out wonderful.

Now I'm having some issues.  Here's what I built.  This is a cropped version of the original schematic, since I only built the VCO I'm posting this for simplification.  Original is here, http://projectsforece.blogspot.mx/2012/01/long-range-fm-transmitter.html



I built this VCO with a few differences.

The first thin would be that I didnt use the varicaps indicated as those seem impossible to find.  Instead I used what I found inside an old wireless phone, I felt blessed to even find that.

I replaced them with two MV2105 varactors.  See their data sheet here.

http://www.datasheetarchive.com/dlmain/Datasheets-16/DSA-315502.pdf

It's very hard to find info on the original varicaps used on this vco but the best info I could find on them seems to indicate that it's a 4-45pf device.  Meaning in total this would be 8-90pf.

I dont know why this image says the oil is 24nh, the original schematic says 5 turns on 4mm former using 0.6mm wire.  I plugged in the L1 coil data into this calculator http://deepfriedneon.com/tesla_f_calchelix.html
and it tells  me it's a .082uH coil.  Using the original varicap capacitence this calculatorr http://www.deephaven.co.uk/lc.html tells me that it has a range of 58.58-196.5mhz

Using the same information and calculator I get that using my varicaps with up to 9v reverse current would go roughly 12pf-30pf (24-60pf in total) and I should get a range of 71.7-113.4mhz which is actually closer to the FM band than with the other varicaps.


The other replacement that I did was probably the more damaging one as instead of using BF199 I ended up using C930 out of an old FM tuner front end. Datasheet here, http://www.paco-electronics.com/pdf/2SC930.pdf

So the project is built and read, no other values where substituted at all and the thing will not work.  I kept the traces as short as possible and I checked for any shorts or mistakes and I cant find any.  Could this have to do with my transistor substitution?  I cant imagine this having to do with my varicap substitution as the LC calculations say it's fine, if not better.

Any help?  

Greatly appreciate any help.
Thanks.

3

The RF Workbench / Re: Simple but effective FM transmitter help

« on: December 05, 2013, 0755 UTC »

Good news is I just sourced chemicals for etching, opens up many more possibilities!

Yes I enjoyed Harry's site, his V7b with optional synth is really cool and versitile, that's good thinking right there!  Also see a TV transmitter on there too!

4

The RF Workbench / Re: Simple but effective FM transmitter help

« on: December 04, 2013, 2240 UTC »

Thank you guys very much.

I have to stay away from kits because of shipping costs (and the parts of the kits themselves) I'm on a tight budget and my location isnt the best for receiving mail. 

Etching boards is something I'd wish to avoid but I guess if i have to then I have to.   I was hoping on possibly just hand etching a board out of copper clad.    Of course there are problems with that, the biggest being sometimes circuits involved etched in coils. 

I was looking at this circuit here.

http://www.electronics-lab.com/projects/rf/001/

But as you can see it includes an etched in coil, L4.  Can I have you guys opinion on trying to build this on a hand etched board and using a wound coil rather than an etched in one?  I dont really know however, how I would go about determining the proper size and turns of the coil.

Another I'm currently looking at is the following,

http://www.circuitstoday.com/2-km-fm-transmitter

Of course a much smaller power output but that's a pretty okay circuit for experimenting.  4 watts like in the other one is a bit more than I initally was looking for anyway, I was thinking more around the area of 1 watt to start up with.


I liked the Harry's Homebrew page

I found this circuit and the related ones rather interesting

http://www.sm0vpo.com/tx/v7_proj.htm

The transistors seem like more of something I could get my hand on.  I'll give it a closer look and I'll write more to you guys about it later on.

Again thank you for the help!

5

The RF Workbench / Simple but effective FM transmitter help VCO

« on: December 04, 2013, 1019 UTC »

Hello everyone,

I've been wanting to build a small FM transmitter for a while now but I've always gotten sidetracked and soon forgotten about the projects.  One of the main reasons is availability of parts.  I'd like to build a transmitter using commonly available parts.  Passive components aren't really an issue as much as the active components, mainly transistors or mosfets.  

I'm not looking for a great deal of power I'm mainly just wanting to build a transmitter that goes beyond those DIY type projects that you see everywhere.

Anyone have any good schematics to show me?  I'd really appreciate.

Also thanks for allowing me to be part of this board,  I'm a big radio fan and I'm happy to be among my own kind. Thanks.