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 Post subject: My crystal locked AM transmitter
PostPosted: Mar Thu 01, 2018 1:42 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 135
Output frequency = 1250 KHz
Range = 100 meters with 6 feet wire antenna. It sounds as good as a FM transmitter! Fidelity is excellent.

“CRYSTAL-LOCKED MEDIUM WAVE TRANSMITTER
Here is a low power AM transmitter for the MW broadcast band . It is a perfect circuit for broadcasting your favorite music around the house and yard. The transmit frequency is crystal-locked to 1250KHz.
Circuit and working
It is built using a 14-stage divider and oscillator HCF4060 (IC1), an N-Channel MOSFET transistor 2n7000 (T1), low-power audio amplifier LM386 (IC2), and a few other components. A 20MHz crystal (Xtal1) is used in conjuction with IC1 to generate the carrier. The crystal frequency is divided by 16 by IC1, giving the final output frequency of 1250KHz at Pin 7. The square waves from IC1 is driving a class C amplifier built around T1. The mosfet works as a fast ON/OFF switch. RFC1 converts the short pulses into a sine wave and also blocks RF from entering audio amplifier IC2. Additional filtering is provided by RFC2 and C4. The amplified signal is coupled to the antenna via impedance matching network comprising C6,C7,C8, C10, L1 and L2. Tune C10 for good signal strength and modulation.

The AM modulation is achieved by varying the drain voltage of T1. When no audio is present, the output of IC2 is 6 Vdc. The audio modulation changes this voltage down to 1V and up to 11V during modulation peaks. Such solid-state series modulator eliminates the need of a modulation transformer. T2 (BD139) is an emitter follower whcih boosts the current output of IC2. VR1 controls the modulation depth and should be adjusted for maximum modulation without distortion. The audio should be loud and clear on a MW AM receiver.

Use of a different crystal will change the frequency of the transmitter. Crystals in the range of 10-20 MHz can be used to cover the medium-wave band from 625KHZ to 1250KHz.
Construction and testing
Assemble the circuit on a general purpose PCB and keep all leads as short as possible. Use good quality shielded cable between the audio source and the transmitter. Use a 6-feet (180cm) piece of wire for the antenna. Use of a 12V hum-free regulated power supply is recommended.”


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 Post subject: Re: My crystal locked AM transmitter
PostPosted: Mar Thu 01, 2018 3:38 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 135
https://imgur.com/a/p8BDB

Schematic


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 Post subject: Re: My crystal locked AM transmitter
PostPosted: Mar Thu 01, 2018 9:54 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3654
Nice work! I've experimented with a similar circuit. Like you, I was able to get a strong signal easily.

Question:

Dare4444 wrote:
When no audio is present, the output of IC2 is 6 Vdc. The audio modulation changes this voltage down to 1V and up to 11V during modulation peaks.

In my incarnation, when no audio is present, the output of IC2 is at 1/2 volt. This puzzles me because this low voltage appears on the pin of an electrolytic coming off Pin 5, and I would think it would be zero. Seeing your circuit is making me rethink that. Given that your circuit has no such isolation at pin 5, is this what accounts for the 6 V quiescent voltage at pin 5?

In other words, is the DC value at pin 5 normally 6 V?

I like it that T2 provides isolation. What is the quiescent voltage at the emitter of T2?

One benefit of your scheme of dividing from a much higher oscillator is that there is some flexibility to get other frequencies that can be be dialed in and even moved closer to 10KHz spacing by bending the crystal with a low pF cap either in series or parallel with the crystal.

For example, dividing by 15 gives 1333 kHz, which, by adding capacitance can be moved closer to 1330 kHZ. Just how close depends on the particular crystal.

Similarly divide by
14 = 1429 kHz
13 = 1538 KHz
12 = 1666 KHz

The latter get closer toward the top of the band where antennas are more efficient. The question is whether the bending gets enough movement to make it worthwhile.


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 Post subject: Re: My crystal locked AM transmitter
PostPosted: Mar Thu 01, 2018 11:09 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1720
Location: Milwaukee,WI
The schematic is cut off at the bottom. Any idea what the value of the RFC1 should be? And is that supposed to be 1.5 or 1-5 mh for RFC2?


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 Post subject: Re: My crystal locked AM transmitter
PostPosted: Mar Thu 01, 2018 11:14 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2222
Location: Saskatoon
Using that divider and high frequency crystal is a nice simple way to get frequencies in the broadcast band. Good work!


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 Post subject: Re: My crystal locked AM transmitter
PostPosted: Mar Thu 01, 2018 11:25 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 135
Thanks!

The output of LM386 is 6V or 1/2 the VCC of 12V in my circuit. The voltage at the emitter of T2
is 6-0.7 = 5.3V

It isolates the LM386 pretty well but can be eliminated altogether as it may induce a little distortion but it wasn’t audible to me when testing the circuit.


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 Post subject: Re: My crystal locked AM transmitter
PostPosted: Mar Thu 01, 2018 11:31 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 135
forumuser wrote:
The schematic is cut off at the bottom. Any idea what the value of the RFC1 should be? And is that supposed to be 1.5 or 1-5 mh for RFC2?



RFC1 = 1.5 millihenry (mH) moulded choke. I used 60T on a ferrite balun core.
RFC2 = 40 microhenry (uH) moulded choke. It’s value is not critical and can be a standard 100 uH
moulded choke.

The circuit sounded best on 1250 KHz during testing.
Use any power transistor in place of the BD139. For example 2n3053, TIP31, etc


Last edited by Dare4444 on Mar Thu 01, 2018 11:33 pm, edited 2 times in total.

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 Post subject: Re: My crystal locked AM transmitter
PostPosted: Mar Thu 01, 2018 11:32 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 135
BobWeaver wrote:
Using that divider and high frequency crystal is a nice simple way to get frequencies in the broadcast band. Good work!


Yes frequency remained rock stable and the 20 MHz crystal is easy to find on eBay (from China).


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 Post subject: Re: My crystal locked AM transmitter
PostPosted: Mar Sat 03, 2018 4:34 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1720
Location: Milwaukee,WI
You mention the signal going into the FET is a square wave and then gets rounded to a sine wave by RFC1. Could I drive the FET with a sine wave to begin with or will the FET not switch on and off well enough to work?


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 Post subject: Re: My crystal locked AM transmitter
PostPosted: Mar Sat 03, 2018 8:16 pm 
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Joined: Jun Sun 22, 2008 2:03 am
Posts: 1335
Location: Burke, VA
With the proper values for the inductors and capacitors in the matching network, the output stage looks more like a class E amplifier.

_________________
Binh


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