Hi Gurus...

I'm just picking electronics back up after a loooong hiatus, and am scratching my head at a project.

I'd like to build an AM box that contains both Rx and Tx, but have them locked to the same freq...which can be changed by a single knob. So the two components always are on the same frequency, but that frequency can change with a single "Tuning" knob.

Does that make sense? Any help is greatly appreciated!!

Thank you!.
Chris

 

A CB radio does that.

 

Do you want to generate a modulated carrier for Tx, same as what you are receiving ?

Regards, Dana.

 

Two way radio sounds like what you need, or cb radio as mentioned ...

 

Hi...Thank you all for the replies!
A CB or Walkie-Talkies are great suggestions, however they won't work for this as I need to be able to pick up stations like NPR. I also need to be able to transmit and receive at the same time. Think of it as providing commentary, over the air, on Charlie Rose's AM Radio show. The only difference is that the transmitter won't be powerful enough for the signal to get out of my living room...er...workshop.

 

Why note use Bluetooth to handle it. You can get modules to handle
the link, and just pipe the AM out to the BT and speakers. In fact you
can get Class D amps w/BT module in them to handle speaker.

Regards, Dana.

 

wifi karaoke?

 

Ha ha...Close. The project is music related

Dana...That's an interesting thought. Though, I don't know if it'll get me where I need to be. Basically, I want to inject my own signal on top of whatever AM station I happen to be tuned to.
I thought of just injecting my signal in the output of a receiver, but my signal would come out too clean...I want to be able to hear the nastiness that is AM, over the entire output.

I don't suppose it could be something simple like sharing a varicap that handles the tuning...?

Thank you all for the replies, I do appreciate it!

 

Listen and record the radio broadcast that you want to comment on. While recording...make your comments into the recording mic.

Now, re-broadcast the recording with your comments.

Or...just record the broadcast. Then when re-broadcasting....make your comments.

 

You probably will not easily be able to talk over the station signal unless you can very accurately lock to the station frequency. Otherwise you will get a beat note between the carriers.
And I think you are getting pretty close to making an illegal radio station. Best check your regs for where ever you are at.
Some years ago, young friends on mine lost all their electronic gear after being raided because of a home made AM transmitter they were playing around with. They were lucky to not get heavy fines or jail time!

 

You cannot transmit on any broadcast frequency without the proper license to do so.

 

You cannot transmit on any broadcast frequency without the proper license to do so.

Not true. It's done all the time. Ever see those gadgets that you plug a CD player into and then tune your car radio to appropriate channel (usually the device offers a few options) to listen to it. Those are all unlicensed transmitters.

In the U.S., you are generally authorized to transmit on the AM and FM broadcast bands as long as your effective range is under 200 feet. In addition, you can't cause interference to another service and must accept any interference from another service. This is known as Part 15 operation.

 

A CB radio does that.

CB radios don't send/receive on the same frequency. In the older radios (crystal controlled), you had separate TX and RX crystals for each channel, so the common switch was actually (or at least effectively via circuitry) a ganged switch switching in two crystals into two circuits. Games could be played accordingly. For instance, the FCC authorized Civil Air Patrol to operate CB radios with the Channel 10 crystals installed backwards.

 

License requirements -

https://www.fcc.gov/media/radio/low-power-radio-general-information

Regards, Dana.

 

My first CB was in 1965. It would now be considered a large walkie talky style and had 6 channels. 2 watts. 2 units.

This was well before the 70s CB fad. By the way...most say the 73 oil crisis......55mph limits caused it. I don't believe that. It was transistor power and Asian production. And skip. Skip was a big deal. Check out some of the antennas.

Before the fad and FCC part 95......CB was very different than now. People and business shared the band and there was no BS. And no one talked to anyone over 100 mi. away....when the skip was in.......it was illegal.

In '65 regulations were enforced by the FCC, with all the help of licensed holders. I never got a license....and I don't know if the FCC ever tried to find me..........but the local CBers sure did. But it's hard to tri-angulate on a couple of bicycles.

I never saw a CB that did not TX and RX on the same channel. Of the original 23 channels....there was an empty channel in every 3 CB channel. They were used for hobby R/C. If I recall correctly there were 2 empty channels between 22 and 23. They were used for diathermy in our area and played hell with channel 23.

Two crystals were needed to change channels. One was a subH of TX F and one was a subH of converter F(for IF).

Then I got a real radio.

 

CB radios don't send/receive on the same frequency. In the older radios (crystal controlled), you had separate TX and RX crystals for each channel, so the common switch was actually (or at least effectively via circuitry) a ganged switch switching in two crystals into two circuits. Games could be played accordingly. For instance, the FCC authorized Civil Air Patrol to operate CB radios with the Channel 10 crystals installed backwards.

Not true. Yes the black box that is the radio has filters and mixers to do the work.
However, the actual carrier frequencies for AM TX and RX are the same. Well close enough to the same due to tolerances.

Real World Example:
You might as why general aviation radios use AM, as opposed to the higher fidelity of Fm.
The answer is we want to hear weaker signals in the presence of stronger signals, as is the case of an aviator in distress.
FM, radios essentially lock on to the stronger signals and eliminate the weak signal, not good for our distressed aviator.

So yes, what ChrisSh is asking is entirely asking is possible and easily done. (Just don't transmit so powerfully that it interferes with the licensed broadcaster.)

redrok@redrok.com

 

Not true. Yes the black box that is the radio has filters and mixers to do the work.
However, the actual carrier frequencies for AM TX and RX are the same. Well close enough to the same due to tolerances.

Yes, I badly mispoke. The actual carrier frequencies are the same, but the send and receive channels are not slaved to each other by a single circuit with a single tuning device (at least in the older crystal-controlled radios), which is the point I was trying to make. The send channel used one crystal and the receive channel used a different crystal. For instance, Channel 10 is listed as 27.075 MHz, which is the transmit crystal frequency, but the receive crystal's frequency (for most radios in the 1970's and 80's) is 26.620 MHz (455 kHz lower). The transmit side ran at the TX crystal frequency, but the receive side used a separate circuit with it's own crystal to general a local oscillator frequency that was 455 kHz lower so that the received signal could be mixed with the local oscillator to general a signal at the intermediate frequency.

The fact that it was only the difference that mattered meant that you could physically swap the TX and RX crystals and operate on a frequency below the CB band. Of course, this was illegal unless authorized. Civil Air Patrol is allocated frequencies in the military band (not the citizens or amateur bands), but most CAP radios are member-owned and it is highly desirable to let them buy the much less expensive and available civilian radio gear. So the FCC authorized CAP to operate on 26.620 MHz, which is in the military band but can be reached by swapping the Channel 10 TX and RX crystals with each other.

Similarly, on the 2 meter band, CAP was allocated frequencies just above and just below the 144 MHz to 148 MHz amateur band (namely 143.9 MHz and 148.15 MHz) specifically so that members could use amateur radios with the appropriate crystals because the hardware could virtually always operate that small amount out-of-band without difficulty.

Unfortunately, with newer technology (synthesized frequencies) and much tighter frequency tolerances, these kinds of tricks went by the wayside. On the other hand, the general cost of radios that could be made to operate, sometimes out of the box and sometimes with a firmware upgrade, came down enough to make the point moot.

 

The solution for transmitting and receiving an AM signal on either exactly, or else "very close" frequency lies in using what is normally called a direct conversion receiver, and using the very same oscillator to generate the transmitter frequency. CB radios do it by having two oscillators, the one providing the local oscillator for the first conversion stage of a standard super-heterodyne receiver, and then the second oscillator being at a fixed frequency equal to that same first IF frequency. That is a lot more parts and circuit than a direct conversion system. So there you have two real options, including one very adequate one, the direct conversion scheme. And there are quite a few circuits published so it will just be a matter of adapting them to whatever frequency you choose.
BUT you certainly DO need to verify that the frequency you choose is legal for you to use with whatever license you may have. And the power level may also be regulated by various and sundry laws. Good Luck on the project.

 

Thank you all for the replies! MisterBill2, I'll look into the direct conversion option
I'm not worried about transmitting any real distance...I might even box the Tx and Rx in a Faraday Cage so my Tx signal doesn't get out, and have an antennae for picking up exterior AM. So, hopefully, the FCC and I will remain unacquainted.

Thanks again.