I just built my first radio transmitter today using a crystal oscillator that I hooked up to my computer through a printer port and made a quick VB program to control it. It sounds more advanced than it is. It just takes one data pin (+5V) and the ground pin from the port and uses it to turn the crystal on or off.
After reading some posts on this site, I found out that these crystal oscillators produce a lot of harmonics. I verified that mine does with an old police scanner. Someone mentioned using a loading coil as an antenna or on the antenna (not sure what it is yet). It didn't sound like a very good solution though. I was wondering if there is any kind of simple circuit I can make that will filter out all of the harmonics. I talked to someone that knows a little more than me about electronics, but not radio and they said that maybe an inductor or diode of some kind could do it. Would that work?
Currently, my transmitter only goes about 20, maybe 30 feet. I was planning to extend the range a bit using a transistor (don't worry, I will keep it small to stay within the legal bounds of my town). I also live in a very rural area so I am not too constrained. Would hooking up the output of the crystal to a transistor amplify the signal or would I need a complex curcuit?
Thanks for reading all of this!
Hi Synaps3..
We need to know if you are AM or FM
AM 1000Khz. It's just a crystal oscillator, so I don't think it's possible to do FM with it.
Is there a reason why I have to be approved to reply, but can start a thread without?
First, Is there a reason why I have to be approved to reply, but can start a thread without?
Fixed. You shouldn't have this problem again.
Regarding your experiments, this is a great way to learn radio technology. Given that the range of your oscillator is very limited you are probably OK as is for experiments but if you want to develop this into a real transmitter some more circuitry is a good idea.
A "final" amplifier transistor can be used to boost the signal and, optionally, an intermediate stage used as a buffer between the oscillator and the final is a good idea.
The output "final" is easy to run in what is known as Class C and this will provide some filtering of harmonics when tuned properly.
A filter can be used with your oscillator as well as with any amplifier you add. A good choice is a filter made from inductors in series and capacitors in parallel to ground. I have used such a filter with two inductors and three capacitors and this gives harmonic suppression beyond that required for Part15 AM operation. Here's a link to the schematic on one of my transmitters. The output filter is C14, L2, C15,L3, and C16. The final stage is Q1, L1, C13 and R12. This design uses a crystal oscillator and the IC U2 serves as the oscillator and buffer. The rest of the circuitry is the audio and modulator system. This is probably more complicated than you want for your experiments but it should give you some ideas on what you can try.
An antenna with a loading coil can attenuate harmonics but this is very difficult to tune for proper operation and I suggest you put this off for now. If you want more information about this please ask.
Here's a link to a very long and nice thread where a transmitter was designed by one of our members, Carl, with the help of several of us. It is a long read but it contains discussion about the whole design process and you might get some ideas from reading this.
This is a lot to throw at you all at once but it should help you to at least know what questions to ask. If you have questions, ask them here and we will try to help. Be as specific as you can and keep asking!
Neil
A good choice is a filter made from inductors in series and capacitors in parallel to ground. I have used such a filter with two inductors and three capacitors and this gives harmonic suppression beyond that required for Part15 AM operation. Here's a link to the schematic on one of my transmitters. The output filter is C14, L2, C15,L3, and C16. The final stage is Q1, L1, C13 and R12.
Thanks for all the info. That circuit is definetly way over my head.Is this filter designed for a specific AM frequency? Is this a low or high pass filter?
My plan is to keep it simple for now and just use one transistor and a filter like you described. Is it best to filter it before or after the transistor? I think the output directly from my crystal is about 20mW (not sure yet). Does your filter require a certain output wattage for it to work?
So just to be clear, if I just bought the right kind of transistor and hooked up the three leads properly, it would amplify the signal?
Synaps3, when you add the transistor with the crystal you will have an "oscillator", and it will be interesting to hear the more knowledgable radio people explain how this is different from an amplifier.
The filter is a low pass filter, it allows the fundamental frequency of operation to pass, and attenuates all upper harmonics. Under Part 15 Rules the harmonic suppression must be 20dB or more below the fundamental frequency.
The Part 15 Rules also limit the power level to 100mW, but with just one transistor you won't have anywhere near that much.
The filter goes on the output of the transmitter, and drives the antenna, which is limited to 10-feet by Part 15.19.
The filter will cause a small power loss, but it will work at very low power.
Synaps3, when you add the transistor with the crystal you will have an "oscillator", and it will be interesting to hear the more knowledgable radio people explain how this is different from an amplifier.
Well, I already have a cystal oscillator. I got lazy and started calling it a crystal instead of a crystal oscillator. Maybe you misunderstand me.
The Part 15 Rules also limit the power level to 100mW, but with just one transistor you won't have anywhere near that much.
Doesn't that depend on the type of transistor?
It changes the picture to know that your first stage is a crystal/transistor/oscillator.
Adding a second transistor will make it a 2-stage unit, and I'm not sure how much gain would take place or whether that would be a sufficient circuit to reach 100mW or more.
Let's see what others think.
Maybe we should start a thread on this.
Sometimes there are interested people who come
here that are trying to get on the AM BCB.
I can think of a few ways:
1) Get on E-Bay and look for a "Talking House"
type of transmitter. Or:
2) Don't laugh too hard. I built a Ramsey AM-1
and modified it for crystal control. The signal
went out about 1/2 mile. With crystal control
the transmitter was stable and didn't FM.
(A 1000 kHz crystal costs next to nothing if
that channel is clear in your area.)
Cost for all of that is about $50.00.
3) Get Phil's AMT-3000 kit. My Choice. Or:
4) Look in this board's archives for circuits
you can build.
For what it's worth.
Bruce, The DOGRADIO Group
(I had meant to start this as
another thread - but I goofed
and it's part of this one.
That's OK.)