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 Post subject: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sat 18, 2021 4:58 pm 
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https://www.nutsvolts.com/magazine/arti ... que-radios

With loop antenna!


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sat 18, 2021 5:19 pm 
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Tried to clean up the schematic a little.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sun 19, 2021 1:00 pm 
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There are some aspects of this I like, for example the output buffer IC and the modulator circuit, which is an emitter current controlled differential amplifier. It is not a "modified Gilbert cell" as it says in the text. A Gilbert cell has additional transistors with a crossed collector configuration that you see in IC's like the MC1496 and is a 4 quadrant multiplier.

I also approve of their loop, however, for a 1 meter diameter loop, for the upper MW band, it is much better to have a 4 to 5 turn tuned loop, which has a substantially higher radiation resistance and tap into one or two turns of it.

The other issue, it would be much nicer to have the oscillator as a clean sine wave oscillator with low harmonics, which can easily be made with a crystal and an MPF102 jFet.

Yes, overall it is a lot better than many of the AM transmitter projects about, but it also lacks audio compression and peak limiting which I insist on for my pantry transmitters.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sun 19, 2021 1:38 pm 
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Location: Montreal, Quebec
But if you have a crystal, and divide it down, you're more.likely to find a cheap crystal on a frequency you want.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sun 19, 2021 2:27 pm 
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ACORNVALVE wrote:
There are some aspects of this I like, for example the output buffer IC and the modulator circuit, which is an emitter current controlled differential amplifier. It is not a "modified Gilbert cell" as it says in the text. A Gilbert cell has additional transistors with a crossed collector configuration that you see in IC's like the MC1496 and is a 4 quadrant multiplier.

I also approve of their loop, however, for a 1 meter diameter loop, for the upper MW band, it is much better to have a 4 to 5 turn tuned loop, which has a substantially higher radiation resistance and tap into one or two turns of it.

The other issue, it would be much nicer to have the oscillator as a clean sine wave oscillator with low harmonics, which can easily be made with a crystal and an MPF102 jFet.

Yes, overall it is a lot better than many of the AM transmitter projects about, but it also lacks audio compression and peak limiting which I insist on for my pantry transmitters.


Your expert thoughts on vectronics 1290 modification I posted?


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sun 19, 2021 8:54 pm 
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Location: Colorado Springs, CO 80917
Quote:
But if you have a crystal, and divide it down, you're more.likely to find a cheap crystal on a frequency you want.


I have numerous crystals in HC-6 size cans for the AM broadcast band. PM me if interested.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sun 19, 2021 8:56 pm 
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With 5 crystals and eight DIP switches, we have eight different crystal controlled medium wave band frequencies. CD4069 oscillator driving a CD4040 divider which in turns feeds CD4013 flip flop with 2F signal to produce the required frequencies in 0 and 180 degrees phase shift to drive two 2n7000 or BS170 in pushpull mode and modulated by a LM386. Secondary winding of feritte rod is tuned by a variable capacitor and steps up the voltage to match the high impedance of a 6 foot wire antenna.

Your comments please? No VFO, No PLL but still has 8 different MW frequencies with no drift and easy to build! Four ICs and two mosfets.

How's this design?


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IMG_20210920_003626.jpg [ 512.18 KiB | Viewed 680 times ]
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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sun 19, 2021 9:06 pm 
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Dare4444 wrote:

Your expert thoughts on vectronics 1290 modification I posted?


I cannot remark very well on this type of design with a modulated switching output stage, as I am less familiar with it. I would have to make one, one day and try it out.

My preferred method to make a pantry transmitter includes a crystal oscillator at the transmission frequency, a linear amplitude modulator prior to the output RF amplifier (an offset 4 quadrant multiplier) and not modulating the output stage directly. Also processing the audio with compression and a soft peak limiting arrangement to prevent over-modulation. Also a transmitting loop. That is the combination of design features that I have found works very well for me.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sun 19, 2021 9:09 pm 
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Modulating a pushpull class C stage produce an amazing sounding audio. Pushpull 2n3904 gave hifi audio but saturated at 75%. 2n7000 should provide 95-100% modulation.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sun 19, 2021 9:16 pm 
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Location: Montreal, Quebec
My comment was to explain the square wave.

I never saw his LOWfer scrapbook, but Ken Cornell did have some articles in 73 on the topic, and they favored using higher frequency crystals, then divided down.

The one broadcast band crystal have is for 1MHz. From RCA, it's meant for a broadcast transmitter, even has a heater for temperature stability.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Sun 19, 2021 9:21 pm 
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Audio quality is high fidelity due to the pushpull final design and it keeps distortion low.

T2 collector = 26T Center tapped and secondary = 4T

A 50 ohm resistor terminates the ouput and provides a stable impedance. An ATU can be used to match the 50 ohm output impedance to that of a 6-10 feet wire antenna for maximum range.

The circuit was tested with a DDS VFO at 1.5 MHz.
The 2.2k variable pot at the input should be adjusted for proper drive level. Output power of the transmitter should stop increasing at optimum drive level. Leave the pot setting at that. I used an oscilloscope to monitor the output waveform and adjust the 2.2k pot until no increase in the output power was seen. Input power to the final pushpull amplifier should be less than 100mW making it part 15 compatible. The pushpull final also helps clean up the even order harmonics. Use thin magnet wire to wind the transformers. BN43-202 ferrite baun cores were used.

T2 is 42.25 : 1 step down transformer.

2115 ohm impedance is presented to the pushpull transistors.
Output power = 36/2115 X 2 = approx 30 mW

T1 is 1:1 transformer. It's an old circuit. I think I measured the modulation at 75% and really high fidelity audio. A pushpull stage when modulated always produces high fidelity audio.


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E0437-D8-D-234-A-4-AF7-A483-AB1-CFF2-E0-B7-A.jpg
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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Mon 20, 2021 12:31 pm 
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How does a modulated push-pull RF stage contribute to lower distortion compared to a properly tuned, single-ended RF stage?

Dave


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Mon 20, 2021 1:08 pm 
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ACORNVALVE wrote:
Yes, overall it is a lot better than many of the AM transmitter projects about, but it also lacks audio compression and peak limiting which I insist on for my pantry transmitters.


I solved that issue with my part 15 two tube transmitter by using a program called Stereo Tools on my PC since I was already planning on using the PC to feed audio to the transmitter anyways.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Mon 20, 2021 1:47 pm 
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Tube Radio wrote:
ACORNVALVE wrote:
Yes, overall it is a lot better than many of the AM transmitter projects about, but it also lacks audio compression and peak limiting which I insist on for my pantry transmitters.


I solved that issue with my part 15 two tube transmitter by using a program called Stereo Tools on my PC since I was already planning on using the PC to feed audio to the transmitter anyways.


That could work, if it does actually work. Some of the software compressors I have tried are hopeless. The best example would be one built into iPods.

I found the better way to do it was with two NE571 compander IC stages cascaded so the average volume level becomes not proportional to the square root of the absolute level, but the 4th root. So the effect for all playback material from mp3 files of widely differing levels is like somebody is adjusting the volume control to convince your senses that the audio volume is the same for every song.

Also, because of the wide dynamic range in the music material, a soft peak limiter is essential (if you have a linear modulator at least) to prevent the carrier being modulated out on peaks. If stereo tools can do this, then very good. However my AM transmitters are self contained with the circuitry in them and don't require computer support, you plug the iPod directly into the unit which is very convenient.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Mon 20, 2021 2:10 pm 
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Dave Doughty wrote:
How does a modulated push-pull RF stage contribute to lower distortion compared to a properly tuned, single-ended RF stage?

Dave


It's wideband, the two transistors generate two symmetrical sidebands with no tuning parts and audio quality is very close to the original. Loading the output with just a 50ohn resistor does the trick!


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Mon 20, 2021 2:12 pm 
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ACORNVALVE wrote:
Tube Radio wrote:
ACORNVALVE wrote:
Yes, overall it is a lot better than many of the AM transmitter projects about, but it also lacks audio compression and peak limiting which I insist on for my pantry transmitters.


I solved that issue with my part 15 two tube transmitter by using a program called Stereo Tools on my PC since I was already planning on using the PC to feed audio to the transmitter anyways.


That could work, if it does actually work. Some of the software compressors I have tried are hopeless. The best example would be one built into iPods.

I found the better way to do it was with two NE571 compander IC stages cascaded so the average volume level becomes not proportional to the square root of the absolute level, but the 4th root. So the effect for all playback material from mp3 files of widely differing levels is like somebody is adjusting the volume control to convince your senses that the audio volume is the same for every song.

Also, because of the wide dynamic range in the music material, a soft peak limiter is essential (if you have a linear modulator at least) to prevent the carrier being modulated out on peaks. If stereo tools can do this, then very good. However my AM transmitters are self contained with the circuitry in them and don't require computer support, you plug the iPod directly into the unit which is very convenient.


Stereo Tools gives me broadcast quality audio that I would say is at least on par with if not a little better than the best sounding AM stations.

Plus it has a plethora of settings you can use to tweak the audio to however you want it to be.

If you've ever seen any of my topics where I demonstrate how one of my console radios sound with a video, you've heard Stereo Tools and my part 15 transmitter.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Mon 20, 2021 4:57 pm 
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Dare4444 wrote:
Dave Doughty wrote:
How does a modulated push-pull RF stage contribute to lower distortion compared to a properly tuned, single-ended RF stage?

Dave


It's wideband, the two transistors generate two symmetrical sidebands with no tuning parts and audio quality is very close to the original. Loading the output with just a 50ohn resistor does the trick!
That makes sense...Thanks! It takes very careful tuning to make the sidebands of a single-ended RF output stage symmetrical.

Dave


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Mon 20, 2021 6:34 pm 
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Dave Doughty wrote:
Dare4444 wrote:
Dave Doughty wrote:
How does a modulated push-pull RF stage contribute to lower distortion compared to a properly tuned, single-ended RF stage?

Dave


It's wideband, the two transistors generate two symmetrical sidebands with no tuning parts and audio quality is very close to the original. Loading the output with just a 50ohn resistor does the trick!
That makes sense...Thanks! It takes very careful tuning to make the sidebands of a single-ended RF output stage symmetrical.

Dave


Yes, it was clear from initial experiments that audio quality was very high, almost like listening to the original. But modulation depth was limited to 70%. If two pushpull amplifiers are cascaded and both are modulated, I tell you 100% modulation is obtainable with amazing natural sounding audio. It's just different, I mean the audio quality, it's sweet and you can't tell the difference if its being played on CD player or on AM radio.


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Mon 20, 2021 8:15 pm 
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What limits the AM modulation depth to 70% in a push-pull RF amp?

Dave


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 Post subject: Re: BUILD AN AM TRANSMITTER FOR USE WITH ANTIQUE RADIOS
PostPosted: Sep Tue 21, 2021 1:37 am 
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Dave Doughty wrote:
What limits the AM modulation depth to 70% in a push-pull RF amp?

Dave


Dave Doughty wrote:
What limits the AM modulation depth to 70% in a push-pull RF amp?

Dave


Even single transistor class C stage saturates at 70%. Their power gain is limited between 1x and 2x voltage where as mosfets fair better. In an AM TX a voltage rise of x2 quadruples the output power resulting in 100% modulation. Pushpull amplifier with transistors follow the same rule. A 2n7000 pushpull class C amplifier would modulate deeper. See this attachment. Output power was around 350mW at 7MHz. The 2n3866 driven with low impedance secondary. All transistors suffer from this problem. Therefore the driver stages are also modulated.

This 7MHz amplifier, the final class C stage comprising 2n3866 gave an output power of 350mW with modulation peaks reaching 70% only. The driver stage wasn't modulated.


Attachments:
IMG_20210921_060906.jpg
IMG_20210921_060906.jpg [ 477.72 KiB | Viewed 536 times ]


Last edited by Dare4444 on Sep Tue 21, 2021 2:00 am, edited 4 times in total.
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