We used to build circuits like these, when the first digital stuff came to market. An easy way to calibrate an analog dial. There wasn't any digital readouts yet. At least affordable readouts.

http://www.solorb.com/elect/hamcirc/amcalib1/

Just a 25KHz square, near a front end will usually suffice all thru HF. Handy markers for sweeping front ends, IF, or any bandpass.

But I hadn't seen it done with a can osc like that.

 

None more amazed than I am that it even sort of worked.

It's not all that difficult. If you have an oscillator module that will run on anything from 3.3 V to 5.3 V (for easy math), and you drive the power pin with 1 volt p-p audio sitting on 4.3 Vdc, then the output will be a 1 MHz square wave with about the same modulation depth - depending on loading, output circuit headroom, how close it can swing to GND, etc. Probably breaks every unlicensed rule on the books, but it does work.

ak

 

We used to build circuits like these, when the first digital stuff came to market. An easy way to calibrate an analog dial. There wasn't any digital readouts yet. At least affordable readouts.

(Some text removed for clarity)

It was in the evening in the late 1960's when I was experimenting with a transmitter based on the uL914 NOR gate when our neighbor poked his head into my bedroom window and said "Can you stop that for about an hour -I'm trying to watch the game."

 

It was in the evening in the late 1960's when I was experimenting with a transmitter based on the uL914 NOR gate when our neighbor poked his head into my bedroom window and said "Can you stop that for about an hour -I'm trying to watch the game."

One of the first examples of "broadband".

ak

 

I see the circuit posted and it will never work. The reason is that the small oscillator module is a system that requires a constant supply voltage and it delivers a constant output amplitude and frequency. Read the manufacture's specification to understand that. But that same oscillator module can drive an amplifier whose output amplitude is varied by modulating the supply voltage.

But you need to know that in order to legally transmit a useful signal at that frequency, which is in the middle of the Broadcast band, you need a license, at least in the USA. In other countries the requirements may vary.

The legal part is a big problem. And the license even a bigger problem. In some countries you can get away with transmitting, in some no.

 

The legal part is a big problem. And the license even a bigger problem. In some countries you can get away with transmitting, in some no.

OK, so I was incorrect with this answer, fairly much. I never claimed to be infallible. And it may be that the digital module used in this case is more tolerant of a variable supply voltage than those that I am experienced with. But perhaps while the signal was understandable on an AM receiver it is probable that the frequency was not always within the specifications.
For a stable frequency and the best fidelity you do need to have a stage of amplification that can accept the modulating signal much closer to 100% modulation, and to avoid transmitting a whole lot of harmonics, which would interfere with other communications, you must have additional filtering before the antenna. But that same filter network will also serve to match the output stage to the antenna more efficiently. At it's simplest, it would consist of a fairly high Q tuned circuit resonant at your intended frequency.

 

@John_2016

Hi rananthan3

1.- would you please be so kind to clarify what receiver did you use to ascertain what you wrote above

.. you can hear an audio signal being broadcast wirelessly ..

so when you say it 'worked' because you see the LM consuming, it has to 'work' on the right direction.

2.- What was the distance from the circuit? where you in the same room?

1. I used an AM receiver
https://www.amazon.com/Memorex-MR4240-Portable-Pocket-Radio/dp/B01DFZ8K9O

2. I was in the same room and the range was approximately 3-4 meters from the transmitter with decent quality audio.

Sorry for the late reeply

 

@MrChips thanks for verifying. I had no idea that a square wave was being ouput thanks @wayneh. The oscillator was purchased from Jameco

https://www.jameco.com/z/OSC1-1-MHz-Full-Can-Crystal-Oscillator_27861.html

 

@MisterBill2 thanks for your advice; that makes sense. I will try to implement a high Q tuned circuit for a cleaner signal.

 

@BR-549 yes it worked. I posted the site where I purchased the can but there was no datasheet and I also have not taken one apart.

 

I am wondering a lot about the opened can in the picture, does it still function? and is the frequency the same as it was?

 

@MisterBill2 this picture was from wikipedia

https://en.wikipedia.org/wiki/Crystal_oscillator

 

@MisterBill2 this picture was from wikipedia

https://en.wikipedia.org/wiki/Crystal_oscillator

OK, that explains why there was no additional information about it. Thanks for clearing up the mystery.

 

Hi Rananthan3

1.- why don't you try a Raspberrypi?

you will be able to modulate AM, and if you then want to try other modulations this link shows details

https://www.instructables.com/id/Raspberry-Pi-Radio-Transmitter/

https://www.rtl-sdr.com/transmitting-fm-am-ssb-sstv-and-fsq-with-just-a-raspberry-pi/

http://www.southgatearc.org/news/20...erry_pi_transmitter_software.htm#.W-o6xZOeSUk


2.- Alternatively, you can use an Arudino to generate AM signal

https://forum.arduino.cc/index.php?topic=88422.0

3.- Comparing Raspberrypi and Arduino paltforms

https://www.makeuseof.com/tag/arduino-vs-raspberry-pi-which-is-the-mini-computer-for-you/

 

1.- why don't you try a Raspberrypi?
2.- Alternatively, you can use an Arudino to generate AM signal

And why would you want to use a digital processor to generate an analog signal when a simple analog circuit will do the job? Digital isn't best for *everything*.

 

Ylli poses my question perfectly. The code to generate a complex waveform is not trivial by any means, and the ability of those packages would not deliver a very good clean signal, at best. There are applications where pure analog is the best choice and this is certainly one of them. Also, it seems to me that the goal of the TS is to be able to transmit something beside a simple tone, such as music or speech. Yes, there are digital systems that can do it, but far more complex and expensive and using more power as well.

 

Hello

@John_2016 thanks for sharing I never knew that.

 

OK, when I say "never work" I am thinking "in the manner that it was designed to work." The manufacturers will probably not claim that their little module will meet any of the published specifications unless it is supplied with the specified voltage.

Hi,

I have to agree that it will never work very well and because of that it should be done a different way.
Back in the mid 1970's some time i used a crystal and two TTL nor gates to create the RF AM carrier signal, then modulated it with a transistor. I was able to get out about two miles with a small indoor antenna on the second floor. I used to make my own radio station so i could listen to it on the way to work

Another interesting side point is that since these oscillators are all square wave, we get some carrier frequencies at odd multiples of the base frequency. So a 1MHz crystal generates 3MHz, 5MHz, 9MHz, etc., probably useful up to maybe 11th harmonic depending on how you do it. A small filter helps narrow down the frequency you want to use. Could probably even reach up into the FM broadcast band for short distances.

 

Hi,

I have to agree that it will never work very well and because of that it should be done a different way.
Back in the mid 1970's some time i used a crystal and two TTL nor gates to create the RF AM carrier signal, then modulated it with a transistor. I was able to get out about two miles with a small indoor antenna on the second floor. I used to make my own radio station so i could listen to it on the way to work

Another interesting side point is that since these oscillators are all square wave, we get some carrier frequencies at odd multiples of the base frequency. So a 1MHz crystal generates 3MHz, 5MHz, 9MHz, etc., probably useful up to maybe 11th harmonic depending on how you do it. A small filter helps narrow down the frequency you want to use. Could probably even reach up into the FM broadcast band for short distances.

The other thing about harmonics is the fact that they will cause interference to other uses of the radio spectrum, and here in the USA you may suddenly find that the FCC is asking you a whole lot of questions.

 

The other thing about harmonics is the fact that they will cause interference to other uses of the radio spectrum, and here in the USA you may suddenly find that the FCC is asking you a whole lot of questions.

Hi,

Yes, and that's why the filter has to be good enough to reject everything else that you dont want to get out there.
Without a filter though there will be a lot of frequencies going out