Colpitts oscillatoroutput power for Am transmitter

[Francesco_bre]

Hi everyone, i'm new to electronics and to this forum. I'm trying everyday to learn more about electronics. In these days i'm experimenting with the colpitts oscillator and am transmitters. I successfully built a colpitts oscillator which resonates around 800 kHz. I also modulated the wave with an audio signal and received the signal with an am receiver. Obviously the range was very low, about 3 feets. My question is, how to increment the output power in order to transmit at higher distances? I also attached an antenna to the output of the colpitts but it wasn't much effective. I also noticed that if i change the inductor to some smaller value, in order to increase oscillating frequency, i get lower output in terms of peak to peak voltage of the produced sine wave. Can someone explain also this behaviour? Thanks in advance.
I leave the circuit diagram of the colpitts and the Ltspice file attached to the post.

 

[Francesco_bre]

What is your actual goal for power ?

Take a look at this for getting a few W :

Some more ref reading :

QRP Classics The Best QRP Projects from QST and the ARRL Handbook : Free Download, Borrow, and Streaming : Internet Archive

QRP Classics The Best QRP Projects from QST and the ARRL Handbook

archive.org

Regards, Dana.

--- Updated Sep 12, 2023 ---

Running sim lots of power lost in output coupling C, so here its
eliminated :

View attachment 184924

Note the above output transistors runnimg way over their Pdiss limits, so those
should be picked for higher power, and heatsink. Note using a P-P output direct
coupled creates a lot of static Pdiss.

I think you have to focus first on matching load to output drivers, 3904 and 3906 (higher
power versions of those.). Using a toroid.


Regards, Dana.

Thank you for your help Dana. My goal for output power is actually the maximum i can do with general purpose transistors that cannot handle much power. I just want to push the range a little farther (just some meters)with what i have. I think the max i can do is with bjt's which handles maximum 1W dissipation,as stated in datasheet.
How can i adapt your schematic to lower the power enough for the transistors to handle it but also enough to have power amplification with respect to the simple colpitts+buffer?
Moreover, can you explain how the impedance matching is done with a toroid, what's the explaination behind this technique. Thanks for your help

 

[danadakk]

https://www.qsl.net/wa1ion/bev/bb_antenna_matching.pdf

http://www.gnarc.org/wp-content/uploads/The-End-Fed-Half-Wave-Antenna.pdf

https://qsl.net/nf4rc/2020/ResonantEndFedUnUnConstructionManual.pdf

https://www.youtube.com/results?search_query=antenna+matching

Regards, Dana.

--- Updated Sep 13, 2023 ---

Maximum power transform :

Maximum power transfer theorem - Wikipedia

en.wikipedia.org

Regards, Dana

 

[Francesco_bre]

https://www.qsl.net/wa1ion/bev/bb_antenna_matching.pdf

http://www.gnarc.org/wp-content/uploads/The-End-Fed-Half-Wave-Antenna.pdf

https://qsl.net/nf4rc/2020/ResonantEndFedUnUnConstructionManual.pdf

https://www.youtube.com/results?search_query=antenna+matching

Regards, Dana.

--- Updated Sep 13, 2023 ---

Maximum power transform :

Maximum power transfer theorem - Wikipedia

en.wikipedia.org

Regards, Dana

Thanks for the reply. I've read the files you shared with me and i'm starting to understand the concepts of impedance matching and the related efficiency in power transfer. I think the best thing i may do is buying a NanoVna for example, in order to exactly measure the antenna impedance and the output impedance from my buffer, in order then to perfectly do a matching network. But i think that i'll buy a NanoVna later on, so maybe the best thing i can do without measurements, is finding docs or calculators for approximating the impedance of my wire antenna. I also need to approximate the output impedance of the buffer, maybe i can do it with Ltspice, given my complete circuit, even if i have some doubts about measuring it in simulation, should i take out the buffer from the circuit and measure the impedance as this or should i leave the complete circuit together?
I leave attached the current circuit i'm using, which can transmits in all the points of my house, more or less. In simulation, i'm using as modulation audio a 1 kHz sine wave that in the real circuit is audio coming from my pc jack.

 

[danadakk]

I think your audio should be applied to the base, as it is now its looks like
almost a dead short to ground (AC wise) because of 12V Vsource and the inductor
are essentially 0 ohms.

Note Zin of emitter follower falls off as we approach 1 Mhz. That de-Q'S the Colpitts quite
a lot, eg. more drift, tradeoffs always tradeoffs.

Regards, Dana.

 

[BradtheRad]

In simulation, i'm using as modulation audio a 1 kHz sine wave that in the real circuit is audio coming from my pc jack.

Amplitude modulation is different from your green waveform. Your green waveform is additive in the sense it adds the carrier to the fundamental. You might hear nothing from your radio or you might hear soft tones.

Here's an easy method to produce louder tones from your radio. Tune it to any AM station. Tune your transmitter close to that identical frequency. Their combination generates a beat frequency. When the frequencies are closer than 20kHz, the beat frequency is in the audio range and is heard as an audio tone coming from your radio.

--- Updated Sep 14, 2023 ---

 

[Francesco_bre]

I think your audio should be applied to the base, as it is now its looks like
almost a dead short to ground (AC wise) because of 12V Vsource and the inductor
are essentially 0 ohms.

View attachment 184939

Note Zin of emitter follower falls off as we approach 1 Mhz. That de-Q'S the Colpitts quite
a lot, eg. more drift, tradeoffs always tradeoffs.

Regards, Dana.

Thank you for the reply Dana. I should make a more reliable buffer with higher input impedance, yes it is always tradeoffs.
I've tried to put the audio in the base of emitter follower but when i did that, the output of the emitter follower just collapsed to 0V, while if i leave the circuit as it is, i can see the modulated output with still 8 volts peak to peak.
I used the inductor on the supply and the capacitor on the audio signal because i've seen a guy doing that to modulate the signal. He said that it was a bias T configuration, so that the capacitor let only the ac signal to pass, while the inductor blocked the sine wave to disturb the supply.
I have a question about the power amplifier, especially when the load is the antenna. I mean, from the colpitts i get 9 volts pk to pk, that is like a voltage amplifier, then i send it to the emitter follower to buffer it and to have the same voltage on output,but with higher current, but, that would work perfectly if the load was actually a 10 Ohm resistor between the emitter and the ground, so that it would "take" all the current. But in my case, i just have an antenna connected to the output on the emitter, which is not shorted to ground, as if it was a simple resistor. So, even if i had another amplifier stage after the buffer, capable of delivering high current with high voltage, so high power, that would not be "taken" by the antenna because actually it is not a simple resistor connected to ground? Is that's why there's always needed an impedance match between tx and antenna?
Those can be stupid questions given by the fact that i'm still a newbie trying to learn more, i'm sorry for that.

 

[danadakk]

Yes, the Z match is basically to create a complex conjugate so all the generated
power is delivered to the load.

Quite a few concepts (s parameters, smith chart) in this video but he does a good job.

Conjugate Matching vs. Reflectionless Matching in Electronic Circuits

Impedance matching is employed as either conjugate matching or reflectionless matching depending on the requirements of maximum power transfer or minimum reflections, respectively.

resources.system-analysis.cadence.com

Other discussions - https://www.youtube.com/results?search_query=antenna+matching

End fed long wire antenna - https://palomar-engineers.com/tech-...-antenna/End-Fed-Long-Wire-Antennas-c25706168


Regards, Dana.

 

[Francesco_bre]

Yes, the Z match is basically to create a complex conjugate so all the generated
power is delivered to the load.

Quite a few concepts (s parameters, smith chart) in this video but he does a good job.

Conjugate Matching vs. Reflectionless Matching in Electronic Circuits

Impedance matching is employed as either conjugate matching or reflectionless matching depending on the requirements of maximum power transfer or minimum reflections, respectively.

resources.system-analysis.cadence.com

Other discussions - https://www.youtube.com/results?search_query=antenna+matching

End fed long wire antenna - https://palomar-engineers.com/tech-...-antenna/End-Fed-Long-Wire-Antennas-c25706168


Regards, Dana.

Thank you. I've seen that video just yesterday and it was clear, also the smith chart, probably i'm loosing too much power just on the impedances not matched.
I tried do build a Unun with a toroid, i wanted a 9:1 unun but i choose the wires too big and got only 7 wounds, and i connected it as seen in a video that i will link.Unfortunately that didn't work, actually it worked worse than before, but it was probable given that i have a random length wire of 2.5 meters without knowing the exact impedance.
I have a question, if i step up the voltage, so the amplitude of the sine wave enough, is it possible to reach higher distances? I'm thinking to trade current with voltage using a transformer to step up the voltage and to feed the antenna on the secondary. I've tried with some transformers i had,attaching the primary to the output of buffer, where i have 9 volts pk to pk, but actually the output voltage was lowered to 2 volts or less and not magnified. Why is that happening? I also tried the transformer alone by connecting the primary to my pc audio output, giving a 1 khz sine wave with 3 volts pk to pk and the signal was greatly magnified on the secondary, so the transformer works by itself.

 

[danadakk]

I've tried with some transformers i had,attaching the primary to the output of buffer, where i have 9 volts pk to pk, but actually the output voltage was lowered to 2 volts or less and not magnified. Why is that happening? I also tried the transformer alone by connecting the primary to my pc audio output, giving a 1 khz sine wave with 3 volts pk to pk and the signal was greatly magnified on the secondary, so the transformer works by itself.

If the transformers you were using were magnetically wound on laminated or steel
cores they are only good for low freq stuff, like audio. So they would totally be lossy
at the 1 Mhz area. Ferrite cores are the milk of impedance transformations and matching,
and when you get to 100's of Mhz and above arena throw those out and wave propagation
physical structures matter.

Trying the UNUM on 2.5 M wire antenna, w/o measuring what the antenna present in terms of
complex impedance, is like shooting at a duck with a pea shooter 100 miles away. RF is a discipline,
which I still am trying to learn. Its good you found those transformers sucked, I learned a lot thru
wrong path approach, highly recommend it.

There are ways of using substitution of known elements to measure impedance. A resource for
old period design prior to VNAs and Network Analyzers and spectrum analyzers can be found
here spread across many issues :

https://worldradiohistory.com/ Tons of material. Also search for old HP and Tektronix archives.
Lots of measurement stuff with primitive instruments and methods.

Do you have a scope ? If so model, make ?


Regards, Dana.

 

[Francesco_bre]

If the transformers you were using were magnetically wound on laminated or steel
cores they are only good for low freq stuff, like audio. So they would totally be lossy
at the 1 Mhz area. Ferrite cores are the milk of impedance transformations and matching,
and when you get to 100's of Mhz and above arena throw those out and wave propagation
physical structures matter.

View attachment 184958

Trying the UNUM on 2.5 M wire antenna, w/o measuring what the antenna present in terms of
complex impedance, is like shooting at a duck with a pea shooter 100 miles away. RF is a discipline,
which I still am trying to learn. Its good you found those transformers sucked, I learned a lot thru
wrong path approach, highly recommend it.

There are ways of using substitution of known elements to measure impedance. A resource for
old period design prior to VNAs and Network Analyzers and spectrum analyzers can be found
here spread across many issues :

https://worldradiohistory.com/ Tons of material. Also search for old HP and Tektronix archives.
Lots of measurement stuff with primitive instruments and methods.

Do you have a scope ? If so model, make ?


Regards, Dana.

Thanks for the reply. I didn't consider the material and also the high frequency. I'm doing a lot of trial and error but I'm learning lots of things, also with the help of this forum and all the answers i received. I'll check the methods for measuring impedance with different approaches without using Vna.
I'm learning about Vna and how can be used, and i have a doubt: is it possible to measure the output impedance of my total circuit if i had one of those? I searched on the internet about measuring output impedance but didn't find much, many people were speaking about the risk of damaging the vna.
If it is possible to measure output impedance with vna, what is the proper way to do it? Should i simply connect my circuit to the supply and let it work and connect the vna on the output?

 

[danadakk]

VNA is fundamentally a 50 ohm device, so one is always concerned about power
dissipation in its input circuits (principally mixer). Best practice is always out a 50
ohm 20 db attenuator to the VNA inputs, and obviously be aware of the dbM the
inoput can handle. Amatuer radio folks deal quite often with anything from mW
to KW, and reflected power can smoke a VNA if not attenuated in an instant.

Also in high SWR environments one can generate HV on the antenna, that can
fry stuff.

Protect Nanovna

https://people.engr.tamu.edu/spalermo/ecen689/sparam_agilent_tutorial.pdf Relation of Zo to s params

https://electronics.stackexchange.com/questions/6917/measuring-output-impedance Substitution Method

https://download.tek.com/document/70W_60918_0_Tek_VNA_PR.pdf

Course

<p>In this course, discover what a vector network analyzer does and the theories behind the network parameter measurements.</p> <p>Learn:</p> <ul> <li>What a vector network analyzer measures</li> <li>Understanding network parameters (H, Y, Z, S) </li> <li>How to calibrate a network analyzer</li>...

learn.keysight.com

https://cas.web.cern.ch/sites/default/files/lectures/ebeltoft-2010/caspers-s-parameters.pdf

https://indico.cern.ch/event/361988/contributions/1775741/attachments/1159237/1668205/C2_RF_Measurement_Tutorial.pdf

Regards, Dana.

 

[Francesco_bre]

VNA is fundamentally a 50 ohm device, so one is always concerned about power
dissipation in its input circuits (principally mixer). Best practice is always out a 50
ohm 20 db attenuator to the VNA inputs, and obviously be aware of the dbM the
inoput can handle. Amatuer radio folks deal quite often with anything from mW
to KW, and reflected power can smoke a VNA if not attenuated in an instant.

Also in high SWR environments one can generate HV on the antenna, that can
fry stuff.

Protect Nanovna

https://people.engr.tamu.edu/spalermo/ecen689/sparam_agilent_tutorial.pdf Relation of Zo to s params

https://electronics.stackexchange.com/questions/6917/measuring-output-impedance Substitution Method

https://download.tek.com/document/70W_60918_0_Tek_VNA_PR.pdf

Course

<p>In this course, discover what a vector network analyzer does and the theories behind the network parameter measurements.</p> <p>Learn:</p> <ul> <li>What a vector network analyzer measures</li> <li>Understanding network parameters (H, Y, Z, S) </li> <li>How to calibrate a network analyzer</li>...

learn.keysight.com

https://cas.web.cern.ch/sites/default/files/lectures/ebeltoft-2010/caspers-s-parameters.pdf

https://indico.cern.ch/event/361988/contributions/1775741/attachments/1159237/1668205/C2_RF_Measurement_Tutorial.pdf

Regards, Dana.

Thanks for the clarification and links.
I think i'll buy myself the nanovna with the attenuators. Probably for my low power application i would not need necessarily an attenuator, i tried to estimate the output power by connecting dummy loads, and with 50 Ohm load the power across the resistor it's about 7 mW. I know it's not so precise but maybe for such low power it's not risky.

 

[danadakk]

Safe practices :

You might post at https://groups.io/g/nanovna-users and ask for any training on safe
practices. Some are DC coupled, so pay attention to specs. I make it a habit to always
start with a 20 db attenuator and then if I am down in the noise use incremental lower
attenuation to get good displays.

Lots of fun, and cheap instrumentation that used to have a barrier cost of > $10K.


Regards, Dana.