5mW FMT Transmitter

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Post by Admin on Sun Apr 28, 2019 4:55 pm

dare4444 wrote:Thank you, Harry. The page looks great and thanks for printing my name on the pics. It is very satisfying Smile At the bottom of the page just add this:

Note: R10 was increased to 33K to enhance frequency stability even further. Power output fell to +10dBm or 10mW but frequency stability improved and first hour drift was less than 10KHz. I am quite satisfied by the performance of this circuit.

Hi Joy,
Sorry once more for the delay. This morning I took our last guest to the airport, but now i have updated the text with your note.

Please check it again to be sure everything is correct.

I will upload to www.sm0vpo.com when I return to Sweden.

BR Harry - EA/SM0VPO

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Post by Admin on Wed Apr 24, 2019 8:38 am

Hi Joy,
Thank you for the update. I shall add your updated text tot he page.

I will also add your name to all your pictures. I could kick myself for not doing this sooner. I do it to all mine. I offer my humble apologies.

BR Harry - EA/SM0VPO

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Post by dare4444 on Wed Apr 24, 2019 12:12 am

Thank you, Harry. The page looks great and thanks for printing my name on the pics. It is very satisfying Smile At the bottom of the page just add this:

Note: R10 was increased to 33K to enhance frequency stability even further. Power output fell to +10dBm or 10mW but frequency stability improved and first hour drift was less than 10KHz. I am quite satisfied by the performance of this circuit.


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Post by Admin on Tue Apr 23, 2019 9:54 pm

Hi Joy,
I sent you a link by e-mail:

http://sm0vpo.altervista.org/joy/joy-20mwfmtx.htm

Please review it and let me know what you think, or if you have any corrections.

Since I now have several of your projects I am opening a page just for you: http://sm0vpo.altervista.org/joy/<projectname>.htm
you projects will have their own index page, but still links from my TX page :-)

Very best regards from Harry - EA/SM0VPO

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Post by Admin on Fri Apr 12, 2019 1:10 pm

Thank you, I will make a note of that.

BR Harry

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Post by dare4444 on Thu Apr 11, 2019 4:22 pm

Thank you Harry. I did increase the resistor to 33K and output power fell to 10mW but the oscillator became even more stable.

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Post by Admin on Tue Apr 09, 2019 6:23 pm

A little info, I have downloaded the files and your correction, 1K to 15K.
All the pictures, and I will work on this ASAP.

Just spent the last three days on a seek-and-destroy for cat-pee, armed with a UV lamp.
The neighbour let all the cats in the neighbourhood into the house while we were away. They slept on our bed, sofa, dining-room table, and pee'd everywhere. They must have had a pee-fight.

When I get the little buggers in a cat box I'll show them a pee-fight - like sitting ducks!!

Can't blame the neighbour. He has been good enough to feed our 2 cats while we were away, but he wasn't to know that we only had 2 cats, not the 5 he let in the house!

Ok, will let you know when the files are on the server.
BR Harry - EA/SM0VPO

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Post by Admin on Sat Mar 23, 2019 7:32 pm

I just got one of those FM transmitters today. Cost me $0.66 from a car-boot sale.
Mine covers the whole house without any problems, but it is a simple one - dry batteries and only 4 fixed channels.
Will have to look inside it. It must be synthesised, so perhaps I can get it to cover more channels?

BR Harry - SM0VPO

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Post by zsolt on Sun Mar 17, 2019 6:49 pm

hi there
i read below that an fm transmitter for playing music around the house is needed. I had the same idea a few years ago. I did try out a bunch of one transistor circuits from an australian site called talking electromics. They did work but the best idea i had was to buy a cheep fm modulator with line input for cars . Actually i was able to find one with remote control and equaliser. Range is not so great by itself but with 10 cm of wire soldered to the circuit it covers a 25 m perimeter

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Post by dare4444 on Fri Mar 15, 2019 1:24 am


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Post by dare4444 on Fri Mar 15, 2019 12:13 am


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Post by dare4444 on Fri Mar 15, 2019 12:05 am

..


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Post by dare4444 on Thu Mar 14, 2019 3:05 am

Take your time Harry. I would be excited to see my new project on your website.

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Post by Admin on Wed Mar 13, 2019 8:31 pm

Ok, no problems Joy,

As I said before, I have limited time at the moment so I have not been able to do anything.
I will download the pics and info probably tomorrow and work from the memory stick.

Really appreciate your work. BR Harry - SM0VPO

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Post by dare4444 on Tue Mar 12, 2019 8:44 pm

Hi Harry I changed the circuit and text again. This should be final. I have increased the 1K resistor to 15K to heavily isolate the oscillator.

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Post by Admin on Thu Mar 07, 2019 10:53 pm

Hi Joy,
That looks great  Very Happy
I will see if I can put it all together over the weekend, if not then I will do it in my lunch breaks from work.

I really liked this script, good intro, well written and easy (for me) to understand. But I will look more closely tomorrow.

Thank you very much. BR Harry - SM0VPO

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Post by dare4444 on Wed Mar 06, 2019 9:13 pm

Prototype Board Ugly Bug Construction: https://ibb.co/DffC4jf

Schematic: https://ibb.co/0n5tnF0

Close up of coil L1: https://ibb.co/YZhVb5v

20mW FM Transmitter

I needed a low power FM transmitter to broadcast music around the house. It should be frequency stable and easy to build without complex PLL circuitry. After a week of hacking around the board I came up with the following circuit. It uses four commonly available transistors and offers superior frequency stability in comparison to other designs available on the Internet.

It all begins in T1 which is wired as a simple colpitts oscillator with C6 and C7 providing the feedback. The base is lightly coupled to L1 with a low value capacitor C5. Trimmer capacitor VC1 (12pF - 66pF) sets the operating frequency from 91MHz to 108MHz. The value of emitter resistors R8 and R9 were carefully chosen to set the current through T1 to 6.5ma.

The oscillator coil L1 is secured with superglue. It is 12-14 turns of 26SWG enamelled copper wire tightly wound on a 2mm diameter former. I used a piece of Johnson cotton earbud. Evenly apply a layer of superglue all over the finished coil and let it dry.

T2 acts as a buffer and isolates the oscillator from the rest of the amplifier chain. It is fed directly from emitter of T1 via a 15K resistor (R10). The 15K resistor ensures that the oscillator is lightly loaded and very little power is drawn from it. This greatly helps in frequency stability and is a crucial part of this design.

The signal level is still very low. It is further amplified in T3 and T4. They are both high frequency 2n3866 transistors and offer good RF gain. T3 is an emitter follower which is driving the PA stage built around T4. T4 is biased for class A linear operation with 25ma of standing current and delivers 20 milliwatts of RF power output between 90MHz and 95MHz into a 50ohm load. During testing a quarter wavelength piece of wire was used as an antenna. It is recommended to operate the transmitter in the lower part of the FM band or 90 to 95MHz to get the maximum RF out.

The prototype was constructed on a piece of copper clad board. The large ground plane is ideal for VHF circuits. T3 and T4 do not require a heatsink. R1 and C1 add 50uS pre emphasis. C1 is five 100n capacitors in parallel. Change R1 to 15K for 75uS pre emphasis. Audio quality is excellent due to pure FM modulation and no AM. A 16V 1W zener is used as a varactor diode in the circuit and offers good performance. Voltage divider R4 and R5 keeps the voltage at a constant 4.8V on top of the zener. R4 can be replaced with a 22K resistor and a 10K variable pot in series with it. The 10K pot can then be used to vary the zener voltage from 5V to 6V. This will fine tune the frequency by about 1.5MHz. If the circuit is enclosed in a small metal or plastic box then the pot can be fitted on the outside for frequency control.

VR1 sets audio deviation. It should be adjusted for loud and clear sound without distortion. VC2 should be adjusted for maximum RF power into a 50ohm load or at the point marked as ‘V’. To further enhance frequency stability the coil can be etched on a piece of 1in. X 1in. PCB. This would remove frequency drift due to room temperature changes and 'microphony' from circuit board vibrations as well. NP0 capacitors can be used in the oscillator section. The power supply should be tightly regulated. I used a 7812 regulator IC and a 19.5V laptop DC adapter as the power supply source for the voltage regulator chip.

I hope your readers find this project interesting. The range of the transmitter should be around 100 meters with a quarter wave wire antenna. For more range add an additional 2n3866 class AB amplifier stage to boost the 20mW output to over 200mW and use a ground plane or dipole antenna mounted 10-20 feet above ground. Don’t forget to add a low pass filter to remove harmonic energy. I can be reached at joy226010@gmail.com for questions and comments.


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Post by Admin on Wed Mar 06, 2019 6:19 pm

Hi again,
Thank you for the updated information.
Only 15kHz (0.15%) is good. Your TX can be used for long periods without having to re-tune to follow the transmission.

You are correct, 1K0 should be about right. I will think about building the project myself, but as usual I have a lot of projects "on the go" and so little time to do anything about them.

I shall look forward to your text. Very best regards from Harry - SM0VPO

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Post by dare4444 on Wed Mar 06, 2019 4:47 am

Design final

https://ibb.co/zxkR0qT
https://ibb.co/dBPvCky

How do you like the final design Harry? Stability improved further after I added a class A amp for 80mW RF out. Overnight 8 hour drift was 15KHz. As you can see the oscillator is buffered by a 1k resistor. I'll write the text and submit it for publication's on your website. Wrapping up the experiments now. One thing I would like to try is to etch the oscillator coil on a 1in. X 1in. PCB and replace the hand wound coil with it.

One question: Is the 1k resistor good enough to properly isolate the oscillator? I didn't want to use BF199 as they are hard to find in India where I publish my projects. The capacitive reactance of a 1.7p capacitor is about 1k at 100MHz so very little power is drawn from the oscillator. 1k resistor is therefore equivalent of a 1.7p capacitor in my circuit.

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Post by dare4444 on Wed Mar 06, 2019 1:54 am

Thank you Harry for taking out the time to reply. I'm learning a lot from you these days. It's the copper coil and it's no good. There is this guy who knows how to design pcbs.. I'll give the layout to him and he will design it for me with the coil etched on it. Do you think I need the BF199 buffer when I am taking the output from the emitter of the oscillator via a 1k resistor? The 1k resistor pretty much isolates the oscillator and prevents any significant loading. I’m using a 16V 1W Zener. You mean to say that using three 5.6V Zener diodes with a 470k resistor across each diode would be more stable than a single 16V Zener?

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Post by Admin on Tue Mar 05, 2019 11:01 pm

Hi Joy,
1 - the drift on my transmitters (not the V4) was measured at around 15kHz. I attributed this to the PCB form of coils. The V4 had no buffer stage, so it drifted by something like 150kHz to 250kHz when you moved you hand near it. But the point of the V5 was the buffer stage that removed the antenna from the oscillator.

2 - I get my NPO caps from Elfa.com or Mouser.com. They can be a bit pricy, but they are well worth the expense.

3 - The V7 dis drift a bit, but that was due to the zenerI used as a varicap. I found that if you use 3x 5.6V zeners in series, with 3x 470K across them to equalise the voltage drops, then the stability improved tremendously. I didn't bother to publish the data since there were never any complaints.

4 - As I pointed out, copper has a really large expansion coefficient, so if you can use a PCB coil or use iron wire, then things will improve. It is just to experiment and see if your frequency is dependent on voltage, temperature or other effects. Then take the appropriate action.

I have a big bag, probably about 200 pieces, BF199. The problem is that I have moved house from Lunda to Spain, then an apartment hotel, then to Märsta 54 then to Märsta 4. They are in a polythene bag, but not sure if they are in Sweden or the UK  Embarassed  I will search for them and if I find them I will send you a few. I got them from a junk sale in Eskilstuna about 10 years ago. You are more than welcome to them (foc) if I can put my hands on them.

Best regards from Harry - SM0VPO

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Post by dare4444 on Tue Mar 05, 2019 12:09 am

In my quest to develop a minimum drift FMTX I've found that a 1k resistor is the perfect load for the oscillator. I've reduced the current through the oscillator transistor to 5.7ma and it is feeding a emitter follower buffer (2n3904) via a 1k resistor in series. The third stage, a 2n3866 emitter follower is now giving only 3mW output but drift seems to be minimum now. It's so tricky to stabilize a free running LC oscillator but a light load on the oscillator and temperature control by enclosing the board in a plastic box seem to help in stability. Next step is to design a PCB with the coil etched on it.

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Post by dare4444 on Sun Mar 03, 2019 2:07 pm

One question.. my design is sound right and everything else in the circuit looks okay and frequency drift should minimize once I use a coil etched on PCB? I turned on central heat and now the frequency increased by 9KHz. So it must be the coil. Also no NP0 capacitors have been used since they are so hard to find.

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Post by dare4444 on Sun Mar 03, 2019 12:51 pm

How much did your 40mW FM transmitter drift over a 8-12 hour period? I guess you can't ignore the benefits of a coil etched on PCB when it comes to frequency stability. I would also like to know the amount of drift on your 150mW WBFM V7 transmitter. I'm thinking of designing my own PCB with a coil etched on it. Then it would be easy to replicate the project if I decide to publish it in a magazine. My main goal is frequency stability and after that the output power.

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Post by dare4444 on Sun Mar 03, 2019 12:43 pm

Hi,

It's 51KHz drift after 8 hours now. No it didn't happen in the first 30 seconds.. it's drifting slowly in one direction. It could be the coil or I'm still taking too much power from the oscillator. BF199 are hard to find these days.. I'll make a buffer but with two 2n3866 transistors. It was the only thing that controlled drift in my previous circuit many years ago. I might use the coil on the PCB of your high power FM mic.. will order some boards. A PLL would have been much simpler but the challenge and the learning process trying to stabilize a free running LC oscillator is fun.

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Post by Admin on Sun Mar 03, 2019 12:00 pm

Hi Joy, I will throw in an idea. Maybe stupid, but ...

Today you can get really cheap SYNTHESISED domestic FM radios. If you look at the output of the ratio detector you have a DC voltage, modulated with the recovered audio. The DC voltage is proportional to the frequency error. If you were to filter out the audio you could perhaps use the DC level to lock your TX frequency to the receiver synthesizer? 

Some of these radios use a free-running oscillator but have a frequency counter, they are not stable. You will need one that has a real PLL synthesizer, eg.:

$10:
https://www.aliexpress.com/item/TECSUN-PL-600-Digital-Radio-Tuning-Full-Band-FM-MW-SW-SBB-AIR-PLL-SYNTHESIZED-Stereo/32760205704.html?src=google&albslr=227878876&src=google&albch=shopping&acnt=494-037-6276&isdl=y&slnk=&plac=&mtctp=&albbt=Google_7_shopping&aff_platform=google&aff_short_key=UneMJZVf&&albagn=888888&albcp=1713454486&albag=67456279139&trgt=296904914040&crea=en32760205704&netw=g&device=c&gclid=EAIaIQobChMI7YmO7Obl4AIVlRsYCh1uVwDDEAQYAyABEgKFc_D_BwE&gclsrc=aw.ds

$9:
https://www.banggood.com/TD-V26-Portable-Mini-Stereo-FM-Radio-MP3-Speaker-Music-Player-Support-Micro-SD-TF-USB-p-1060014.html?rmmds=buy&cur_warehouse=CN
(I have one of these and it is good)

$1.63:
https://www.ebay.co.uk/itm/192651017283
(they claim it to be PLL synthesised, but seems hard to believe)

If the radio tunes in 50kHz steps then you can also use this to change your TX channel after you have got a frequency lock :-)
Just a little idea plucked out of the air.

BR Harry - SM0VPO

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Post by Admin on Sun Mar 03, 2019 11:42 am

dare4444 wrote:50KHz drift after 6 hours and 30 minutes. Room was cold. Is this acceptable?
No, it is not good.

50kHz is 25% of the channel spacing. Some countries allocate channels in 100kHz steps, but leaving a 200kHz gap between them. (In Spain they don't allocate them, there are no local radio licenses. All local radio in Spain is un-licensed - technically illegal, but acceptable).

An FM transmission signal channel bandwidth is +/-75kHz, so 50kHz is 66% of that.

Question: What was the drift with time? Could that 50kHz drift have been within the first 30 seconds?
Suggestion: blow on it and see if it drifts. If it does then there is some power dissipation in something that decides the frequency, or some temperature dependant component.

I like the way you are looking at stability and performance Very Happy
Many designs I have seen on the web give high specifications, and I have even seen a design delivering 5 Watts from the oscillator! Modulation excellent, power good, drift really poor. 

Hope this helps.

BR Harry - SM0VPO

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Post by dare4444 on Sun Mar 03, 2019 10:59 am

50KHz drift after 6 hours and 30 minutes. Room was cold. Is this acceptable?

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Post by dare4444 on Sun Mar 03, 2019 5:13 am

Your notes helped tremendously. I reduced the current through the oscillator transistor to 6ma to keep it cool and drift in the first 90 minutes was less than 12KHz. Is this good enough? I've kept it running tonight.. will check drift again in the morning.

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Post by dare4444 on Sun Mar 03, 2019 2:06 am

https://ibb.co/0Qyvh9S (Schematic)

https://ibb.co/7YrJrYM (Ugly bug construction prototype board)

15mW FM Transmitter

I needed a low power FM transmitter without complex PLL circuitry. It should be frequency stable and easy to build. After two days of hacking around the board I came up with the following circuit.

It all begins in T1 which is wired as a simple colpitts oscillator with C6 and C7 providing the feedback. The base is lightly coupled to L1 with a low value capacitor C5. Trimmer capacitor VC1 (12pF - 66pF) sets the center frequency and variable pot VR2 allows fine tuning by about 1.2MHz. The prototype covers 94MHz to 108MHz.

Frequency is quite stable as the coil is secured with superglue. It is 12 turns of 26SWG enamelled copper wire tightly wound on a 2mm diameter former. I used a piece of Johnson cotton earbud. Evenly apply a layer of superglue all over the finished coil and let it dry.

T2 acts as a buffer and isolates the oscillator from the final stage. It is fed directly from emitter of T1 via a 470ohm resistor (R9). R9 ensures that the oscillator is lightly loaded and very little power is drawn from the oscillator. This greatly helps in frequency stability.

The final stage T3 uses a 2n3866. It is a high frequency RF power transistor and offers good gain. T2 delivers around 1.5mW of power to base of T3. RF output is taken from emitter of T3 and coupled to the antenna via a 56p capacitor. The transmitter delivers 15mW of RF power into a 50ohm load. A 1/4 wavelength piece of wire was used for testing.

The prototype was constructed on a piece of copper clad board. The large ground plane is ideal for high frequency VHF work. T3 does not require a heatsink. R1 and C1 add 50uS pre emphasis. C1 is five 100n capacitors in parallel. Change R1 to 15K for 75uS pre emphasis. Audio quality is excellent.

VR1 sets audio deviation. It should be adjusted for loud and clear sound without distortion. To further enhance frequency stability the transmitter can be housed in a small airtight metal or plastic box with VR2 mounted outside for frequency control (1 to 1.5MHz of fine tuning). The power supply should be clean and regulated to avoid frequency drift. I used a 7812 regulator IC and a 19.5V laptop DC adapter as the power supply source for the 7812.

I hope your readers find this project interesting. If a proper PCB is designed for the circuit and coil L1 etched on PCB then the frequency would become mega stable. The range of the transmitter should be a few hundred meters in the open.

Note: To make the transmitter even more frequency stable add a 330ohm resistor in series with R8. This would decrease the current through oscillator transistor T1 to 6ma. Also increase R13 from 100ohm to 220ohm. After the modification the power output dropped to 6mW but frequency drift improved further. After one hour the drift was less than 11KHz. The circuit board was enclosed in a plastic box.

6 milliwatts fed to a quarter wave wire antenna covers my two bedroom apartment with good signal strength! The original goal of frequency stability and usable power output has been achieved. I am happy with the design Smile


Last edited by dare4444 on Sun Mar 03, 2019 5:21 am; edited 2 times in total

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Post by Admin on Sun Mar 03, 2019 12:18 am

dare4444 wrote:Thank you Harry for the long explanation I really needed it. I'm saving the text for future references. The only time I got a very stable output from an FM oscillator is when I used a BF199 based buffer and a BF199 broadband output stage. Frequency drift was less than 1KHz. I tried it in 2011. Output was fed to a three transistor amplifier stage with 2n3553 as the final amplifier for 3W output. Here's the schematic of the Exciter.

https://ibb.co/d7q6K90

I have found that many published designs often use a lot of feedback in the oscillator. I usually reduce the feedback (less load on the tuned circuit "hot end") until it stops oscillating, then increase it by about 30% and check it starts reliably at the lowest supply voltage. Gives a much cleaner signal and frequency stability. Also the oscillator device must run cold. If it heats then it can change capacitance.

I wait with anticipation for the final version  Very Happy

Best regards from Harry - SM0VPO

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Post by dare4444 on Sat Mar 02, 2019 7:35 pm

I'm still tweaking the circuit. Will post final text and schematic for publication on your site in a day or two.

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Post by dare4444 on Sat Mar 02, 2019 4:01 pm

Thank you Harry for the long explanation I really needed it. I'm saving the text for future references. The only time I got a very stable output from an FM oscillator is when I used a BF199 based buffer and a BF199 broadband output stage. Frequency drift was less than 1KHz. I tried it in 2011. Output was fed to a three transistor amplifier stage with 2n3553 as the final amplifier for 3W output. Here's the schematic of the Exciter.

https://ibb.co/d7q6K90

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Post by Admin on Fri Mar 01, 2019 10:44 pm

Hello Joy,

In your experience Harry which oscillator is better in terms of ...

In my experience, and if I were to design a high stability oscillator, then I would choose the Franklin oscillator.

The franklin oscillator used two cascaded (inverting) amplifying devices, with the output capacitively fed back to the input, shunted by a parallel tuned circuit. Because the gain of the amplifier is so high, the feedback capacitive coupling to and from the tuned circuit are so low that there is less load on the tuned circuit to de-tune it. In addition, the tuned circuit is also grounded, which also increases stability.

5mW FMT Transmitter  Fig3
(the 10pf + 10pf may be a bit generous)

5mW FMT Transmitter  Fig01
(C1 and C2 typically 1pf)

If I had to choose between Hartley and Colpitts, then I would choose Colpitts. Since it uses a capacitive divider in the tuned circuit, the capacitor from which the amplifying device is fed is much larger, usually ten X the other capacitor. If capacitors with a zero temperature co-efficient are chosen then this can make a very stable circuit.

BUT ... if you choose a Hartley oscillator with a grounded coil, then use taps of the coil (not the top hot-end) of the coil then this can give a great improvement. With most oscillators, the supply voltage can cause capacitive changes in the amplifying device, so if the amplifying device is connected to the top "hot" end of a tuned circuit, then you can get frequency variations with supply voltage changes. The tapped Hartley is more stable.

5mW FMT Transmitter  Image110
(tapped Hartley)

Avoid the Hartley variant where the coil has two "hot" ends, unless you can ensure a good mechanical, inductance-free centre-tap to ground. Also the tuning cap is not grounded:
5mW FMT Transmitter  DGhk9jV
(AVOID - Both sides of the coil have "hot" ends, affected by amplifier capacitance)

No-matter which circuit you choose, you can consider the tuned circuit itself: you can choose to have a high-L and a small C, alternatively a low-L and large C. With a high-L and small capacitance, the capacitance is so small that very small stray capacitances can affect the frequency. So you should also look at the tuned circuit environment. Is it inside a stable metal tin? Can there be hand-capacitance (eg. plug-in coils)? A mechanical switch that can apply mechanical stress?

There are a lot more considerations, even the expansion properties of copper. Silver-plated iron wire (as in some really, REALLY, cheap wound transformers) has a lower temperature co-efficient of expansion, or copper tracks on a PCB can buffer metallic expansion with heat. These are all items that can affect frequency stability.

So back to your question, I would choose, in order of preference:

1 - Franklin
2 - Colpitts
3 - Hartley

If I wanted a REALLY Megga-stable variable oscillator, then I would put it in a metal tin, lined with expanded-polystyrene (frigolit), then use an overloaded 47Ω resistor to raise the internal temperature to about 55°C. A thermistor can be used inside the box to sense the temperature to maintain a constant temperature. It may take 15 minutes for the temperature to stabilise, but then the drift properties would be excellent. The zener (varicap) diode should also be in side the temperature control. I CAN be built very simple with the L, C and ZD very close, on a PCB, and just push a polystyrene jacket over it. The polystyrene only needs to be about 3mm thick, but with no possibility of air-flow. This will add about 500 mW to your power budget.

So how far do you want to go?

As regards the drawing, thank you very much. I will do what I can and give you a link. Maj-Lis is coming from Spain in 5 days, so there may be a delay. I have to "muck-out" the house; washing and cleaning. I even have to wash up the pots piled up in the sink  Wink   So I am going to be busy for a few days   Embarassed

Have I answered you question? As you are aware, if you ask a builder or an engineer a question, don't expect a short answer.  Razz

Very best regards from Harry Lythall - SM0VPO

https://ibb.co/0Vdzj3p

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Post by dare4444 on Fri Mar 01, 2019 8:24 pm

Harry I was unsatisfied with frequency drift so I redesigned the circuit. There is now a 470ohm resistor between oscillator and buffer transistor. Output power is also reduced to about 2mW at 100MHz but frequency stability is excellent and audio sounds high fidelity. Here's the updated schematic. Pls add this to the article.

https://ibb.co/0Vdzj3p

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Post by dare4444 on Fri Mar 01, 2019 3:06 pm

In your experience Harry which oscillator is better in terms of frequency drift.. colpitts or inverted hartley? I added a 10p capacitor from base to emitter and 18p across the emitter resistor to turn it into colpitts. A tapped coil is not necessary then but I think I'm seeing more frequency drift with colpitts.

My observation shows inverted Hartley is superior to colpitts when it comes to frequency drift. Power output is higher too (7mW vs 4mW).

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Post by Admin on Wed Feb 27, 2019 1:45 pm

Hello again Joy,
Thank you for the information and the files. I will do the same as last time: create an HTML file and give you a link to review.

yes, I do find this project very interesting.

BR Harry - SM0VPO

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Post by dare4444 on Tue Feb 26, 2019 11:40 pm

Hello Harry,

Here are the images for 5mW FMTX. It uses your inverted Hartley configuration but I added a 16V Zener for true FM and a variable pot to fine tune the frequency by about 1.5MHz. VC1 sets the Center frequency. The prototype covered 93MHz to 113MHz.

Frequency is quite stable as the coil is secured by superglue. It is 12T of 26SWG enamelled copper wire tightly wound on a 2mm diameter former. I used a piece of Johnson cotton earbud. Tap is at 2T from GND. With a 12V regulated power supply, output power was 6mW at 96MHz. VR1 sets audio deviation. To further enhance frequency stability the transmitter can be housed in a small metal box with VR2 mounted outside for frequency control (1 to 1.5MHz of fine tuning).

If you find this project interesting then go ahead and add it to your transmitter circuits section.

https://ibb.co/hMNzW0n
https://ibb.co/bLMxJCB

Best regards,
Joy


Last edited by dare4444 on Wed Feb 27, 2019 5:27 pm; edited 1 time in total

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