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 Post subject: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Wed 10, 2020 1:20 am 
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Joined: Apr Tue 09, 2019 8:50 pm
Posts: 11
Note: The circuit below is not connected to any kind of irradiator element (Antenna). Any mention to antenna refers to radio receiver used on this setup.

Hi,
I making some AM experiments using so called "Wenzel" modulator, or a amplitude modulator using a differential amplifier.
http://www.techlib.com/electronics/amxmit.htm
The original circuit uses a MW band crystal, unfortunately I don't have any, instead I used a 7.160MHz crystal (40m/41m) SW band, and tested other crystals what I have.
The modulator works and have a very good audio quality.
My audio source are MP3s with high sample rate, and high frequency spectrum, making to modulate spectrum spread above 40kHz.
To reduce this baseband bandwidth, I pre processed some audio on Audacity applying a low pass filter, and get some improvement on audio quality and reduced bandwidth. But its in not practical.
Need a hardware filter.
I modified a DSL Filter, adding capacitors in parallel with original ones, and get low pass filter near 8kHz.
For my surprise, I get a more dense modulation and more "presence" on the audio.

I not connected any kind of antenna, the irradiation are made only by circuity wiring, such a power, audio cable, oscilloscope probe, etc.
At the output have a very crude attenuator, Where SDR antenna was connected.
And a portable radio only pickup the signal very close to circuit, losing signal on the noise background half meters away.

Attachment:
am_sig_gen_7160kHz.jpg
am_sig_gen_7160kHz.jpg [ 313.87 KiB | Viewed 418 times ]

Attachment:
linrad_am_sig_gen-filters.jpg
linrad_am_sig_gen-filters.jpg [ 303.9 KiB | Viewed 418 times ]

Attachment:
amodpksato20200610.png
amodpksato20200610.png [ 5.9 KiB | Viewed 311 times ]


Last edited by pksato on Jun Wed 10, 2020 11:02 pm, edited 1 time in total.

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 Post subject: Re: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Wed 10, 2020 1:25 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
Interesting experiment.
Can you send me the circuit diagram with the 7MHz crystal as you made it? Please DM me. I'm also looking for an AM modulator on SW.

What kind of a filter are you looking for exactly?

Does the output of MP3 contain 40KHz sampling signal and you want to filter it?


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 Post subject: Re: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Wed 10, 2020 1:26 am 
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Joined: Jan Sun 26, 2020 3:39 pm
Posts: 18
If I remember correctly, the SSTran 3000 AM transmitter used a differential modulator circuit.

Mike


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 Post subject: Re: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Wed 10, 2020 1:36 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
The bandwidth occupied by the AM signal is twice the maximum frequency of the signal that is used to modulated the carrier, i.e. it is twice the bandwidth of the audio signal to be carried. You need an active low pass filter built around TL071. This opamp has high slew rate and distortion is negligible and it is better than LM358, LM741, etc.

Use this calculator
https://www.electronicproducts.com/Op_A ... lator.aspx

Enter cut off frequency and a resistor value. I entered 11KHz and 10K and got 2uf and 1uf for the two capacitors.

It will work like a charm. AM bandwidth should be limited to 10KHz or so. The above filter will give you a 22KHz bandwidth. For 10KHz, you'll have to limit audio bandwidth to 5KHz only so you will need to enter 5KHz cut off frequency. Generally LPF cut off should be 15% higher than the highest frequency you want to pass without attentuation so your filter should have a cut off of 5.75KHz. TL071 will preserve audio fidelity and not add extra distortion.

Use 3.9K and 10u and 5u capacitor for 5.75KHz cutoff. Use two 10u in series for the 5u value

Hope this helps!


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 Post subject: Re: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Wed 10, 2020 12:45 pm 
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Joined: Apr Tue 09, 2019 8:50 pm
Posts: 11
Dare4444 wrote:
Interesting experiment.
Can you send me the circuit diagram with the 7MHz crystal as you made it? Please DM me. I'm also looking for an AM modulator on SW.

The oscillator is the "classic" Colpitts configuration.
Similar to Charles Wenzel configuration, replacing 1000pF capacitor with 22pF (any bellow 100pF) and add a 100pF capacitor in series with crystal (DC blocking, or trimmer to adjust the frequency).
Collector have a 22R, Base biased with 100k, emitter a 500R trimpot.
I tested this circuit with assorted crystal from 3.57MHz to 25MHz. But, don't work with 3.57MHz crystal, need to increase feedback capacitors.


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 Post subject: Re: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Wed 10, 2020 6:20 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
Thanks..


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 Post subject: Re: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Sun 14, 2020 10:48 pm 
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Joined: Apr Tue 09, 2019 8:50 pm
Posts: 11
Hi,
A build a "new" LPF using chock/coil/transformer/inductor salvaged from a DSL filter.
The cut off are near 5.5kHz.
And add a pre amplifier.
I know, can build audio filters using OAmps.
Attachment:
am_mod_lpf_20200614.png
am_mod_lpf_20200614.png [ 5.95 KiB | Viewed 232 times ]


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 Post subject: Re: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Mon 15, 2020 12:08 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
Cool circuit! Is modulation 100% ?


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 Post subject: Re: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Mon 15, 2020 9:59 pm 
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Joined: Apr Tue 09, 2019 8:50 pm
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Dare4444 wrote:
Cool circuit! Is modulation 100% ?


Can set modulation level playing with RF and audio levels, and distortion.
Very close to 100%, and have some non linearity (audio and RF source are not perfect).
Attachment:
IMG_20200615_174310225.jpg
IMG_20200615_174310225.jpg [ 187.66 KiB | Viewed 181 times ]


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 Post subject: Re: Experiments with "Wenzel" AM modulator circuit.
PostPosted: Jun Mon 15, 2020 10:08 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
pksato wrote:
Dare4444 wrote:
Cool circuit! Is modulation 100% ?


Can set modulation level playing with RF and audio levels, and distortion.
Very close to 100%, and have some non linearity (audio and RF source are not perfect).
Attachment:
The attachment IMG_20200615_174310225.jpg is no longer available


Looks good. I needed an AM modulator for SW band and I think I'll use your circuit. If the input is fed with LM386 and negative feedback applied to its pin 2 from RF out, then it should cancel out the small distortions.

Several years back I built a two transistor SW AM transmitter and 100% modulation was obtained after I added one RFC in emitter of the colpitts oscillator. The second Transistor is a buffer and 2n3053 and 2n2219 worked well so no need to use a 2n3866. The two RFCs are 10-20T on BN-43-202 core. I may build this circuit again this time with negative feedback to pin 2. Output was 40mW after the low pass filter with modulation peaks rising to 160mW. Not bad for such a simple circuit. Crystal was 7.2MHz .


Attachments:
SAVE_20200616_025318.jpeg
SAVE_20200616_025318.jpeg [ 233.95 KiB | Viewed 178 times ]
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