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 Post subject: My four transistor FMTX
PostPosted: Mar Wed 13, 2019 12:08 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
I wasn't satisfied with the old design. This new design is mega stable thanks to added buffer stages. Frequency drift in the first hour was less than 5KHz. Output power can be lowered by increasing R10.

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 better 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 (maximum voltage 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 you'll find this circuit interesting. Increasing R10 to 33K should lower the output power to 10mW level. Stability seemed to be even better with the 33K resistor. The oscillator becomes heavily isolated with higher R10 values.

Note: I used 2n3866 for T3 and T4. You can use S9018 transistors instead of 2n3866. They are cheap on Ebay and shipped from China. With R10 = 33K frequency drift was minimum and output power was around +10dBm. It covered my 2 bedroom apartment with good signal strength.


Last edited by Dare4444 on Mar Thu 28, 2019 3:23 pm, edited 8 times in total.

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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Wed 13, 2019 4:44 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
Posts: 1958
Location: Roanoke, VA
Dare4444 wrote:
The 10p capacitor across the emitter resistor of T4 can be removed to lower RF output to 5mW or so.

Note that Federal regulations for unlicensed FM transmitters do not specify maximum output in transmitter power, they specify it in field intensity.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


Last edited by Dale H. Cook on Mar Thu 14, 2019 1:35 pm, edited 1 time in total.

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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Wed 13, 2019 4:47 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
Yes. I was transmitting into a 50ohm dummy load and got a range of 15-20 feet. Not using an external antenna is the safest way for a low power homebrewed FM transmitter. This article is written for a magazine in asia.


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Wed 13, 2019 10:11 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
Posts: 1958
Location: Roanoke, VA
Dare4444 wrote:
Not using an external antenna is the safest way for a low power homebrewed FM transmitter.

That is a wise and sufficient precaution.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


Last edited by Dale H. Cook on Mar Thu 14, 2019 1:36 pm, edited 1 time in total.

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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Thu 14, 2019 1:47 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
Dale H. Cook wrote:
Dare4444 wrote:
Not using an external antenna is the safest way for a low power homebrewed FM transmitter.

That is a wise and sufficient precaution.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html" -="window.open(this.href);return false;" -="window.open(this.href);return false;


Yes I don't want to be fined by the FCC or cause unwanted interference.


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Thu 14, 2019 6:31 am 
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Joined: Nov Sun 04, 2018 8:27 am
Posts: 7
Location: India
Dare4444 wrote:
Yes. I was transmitting into a 50ohm dummy load and got a range of 15-20 feet. Not using an external antenna is the safest way for a low power homebrewed FM transmitter. This article is written for a magazine in asia.


I would think a single 2n3904 with an extra antenna buffer stage should be enough for transmitting into that distance or even more, if not 100ft. Using 2 stages of 2n3866 power transistor(1W capable) to achieve this appears like a severe design flaw or overkill. I have difficulty in understanding the whole design.


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Thu 14, 2019 10:47 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
transistor495 wrote:
Dare4444 wrote:
Yes. I was transmitting into a 50ohm dummy load and got a range of 15-20 feet. Not using an external antenna is the safest way for a low power homebrewed FM transmitter. This article is written for a magazine in asia.


I would think a single 2n3904 with an extra antenna buffer stage should be enough for transmitting into that distance or even more, if not 100ft. Using 2 stages of 2n3866 power transistor(1W capable) to achieve this appears like a severe design flaw or overkill. I have difficulty in understanding the whole design.


Yes it's bit of an overkill but in my experience the less power is drawn from an oscillator the more stable it becomes. In my circuit the oscillator is loaded by a 15 - 33K resistor. The 2n3866 can be replaced by BF199 but they are obsolete these days so I had to use 2n3866. They work well in small signal applications.

R10, T2, and T3 heavily isolate the oscillator from amplifier stage T4. Moving my body closer to the antenna pulls the frequency by only 2-3 KHz or so and long term stability at room temperature is good. Etching the coil on PCB and replacing oscillator capacitors with NP0 type should improve it even further. Most of the LC based designs available on the internet drift by a few hundred KHz.


Last edited by Dare4444 on Mar Thu 14, 2019 3:10 pm, edited 2 times in total.

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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Thu 14, 2019 1:34 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
Posts: 1958
Location: Roanoke, VA
Dare4444 wrote:
Yes it's bit of an overkill but in my experience the less power is drawn from an oscillator the more stable it becomes.

That is entirely true whether the oscillator is solid state or vacuum tube. The higher the load impedance the more stable the frequency and amplitude characteristics are.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Thu 14, 2019 2:41 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
..


Last edited by Dare4444 on Mar Thu 14, 2019 10:37 pm, edited 1 time in total.

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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Thu 14, 2019 3:21 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
transistor495 wrote:
Dare4444 wrote:
Yes. I was transmitting into a 50ohm dummy load and got a range of 15-20 feet. Not using an external antenna is the safest way for a low power homebrewed FM transmitter. This article is written for a magazine in asia.


I would think a single 2n3904 with an extra antenna buffer stage should be enough for transmitting into that distance or even more, if not 100ft. Using 2 stages of 2n3866 power transistor(1W capable) to achieve this appears like a severe design flaw or overkill. I have difficulty in understanding the whole design.



The other time when 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 a few KHz. 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 subject: Re: My four transistor FMTX
PostPosted: Mar Sat 16, 2019 5:35 am 
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Joined: Nov Sun 04, 2018 8:27 am
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Location: India
Dare4444 wrote:
Yes. I was transmitting into a 50ohm dummy load and got a range of 15-20 feet. Not using an external antenna is the safest way for a low power homebrewed FM transmitter. This article is written for a magazine in asia.


Which magazine? Is it a national or local magazine?


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Sat 16, 2019 10:51 am 
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Posts: 169
transistor495 wrote:
Dare4444 wrote:
Yes. I was transmitting into a 50ohm dummy load and got a range of 15-20 feet. Not using an external antenna is the safest way for a low power homebrewed FM transmitter. This article is written for a magazine in asia.


Which magazine? Is it a national or local magazine?


Electronics For You www.electronicsforu.com published in India


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Sat 16, 2019 12:19 pm 
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Joined: Nov Sun 04, 2018 8:27 am
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Dare4444 wrote:

Electronics For You http://www.electronicsforu.com" -="window.open(this.href);return false;" -="window.open(this.href);return false;" -="window.open(this.href);return false; published in India


I do follow EFY from late 90's onwards..Not much nowadays, because I think the magazine lost the analog charm which it had during 2000 period.

And I found few of your articles online(mw TX,door lock arduino etc).


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Sat 16, 2019 10:14 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
transistor495 wrote:
Dare4444 wrote:

Electronics For You http://www.electronicsforu.com" -="window.open(this.href);return false;" -="window.open(this.href);return false;" -="window.open(this.href);return false;" -="window.open(this.href);return false;" -="window.open(this.href);return false; published in India


I do follow EFY from late 90's onwards..Not much nowadays, because I think the magazine lost the analog charm which it had during 2000 period.

And I found few of your articles online(mw TX,door lock arduino etc).


Thank you and yes these days it's all about digital circuits. It's Arduino craze. I don't like it.


Last edited by Dare4444 on Mar Sat 16, 2019 10:24 pm, edited 1 time in total.

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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Sat 16, 2019 10:17 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
Two years back I published this GSM home home alarm system and it's getting quite famous.

https://electronicsforu.com/electronics ... ity-system


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Sat 16, 2019 10:30 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
EFY never published my 88 - 108MHz PLL FM Transmitter on their website. It covers the entire FM band in 0.1MHz steps. Coil L2 is unmarked. It's a BN-43-202 binocular ferrite core and power output is around 150mW. Here's PDF

https://docdro.id/XIJ9RPg"


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Sun 17, 2019 6:01 am 
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Joined: Nov Sun 04, 2018 8:27 am
Posts: 7
Location: India
Dare4444 wrote:
transistor495 wrote:
The 2n3866 can be replaced by BF199 but they are obsolete these days so I had to use 2n3866. They work well in small signal applications.


You could try S9018 in place of BF199 requirements, they are cheap and excellent transistors at VHF FM frequencies.

Generally, I think the non-PLL circuits will have some degree of drifts over time, but that mainly depends on quality of parts used(temperature coeff) in the frequency determining section. For eg, below cases must be addressed at least-

> NPO ceramics can be preferred
> Metal film resistors in place of carbon(because carbons are worst at temp changes)
> Highly regulated power supply
> JFET buffers in place of BJT(J310,BF245 etc)
> Shielded oscillator(to prevent interference from nearby objects)
> Glued frequency determining components(coils etc)

ALso with best stable oscillator topology can help reduce the drifts to a minimum level, if not completely eliminates it.


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Sun 17, 2019 12:26 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 169
transistor495 wrote:
Dare4444 wrote:
transistor495 wrote:
The 2n3866 can be replaced by BF199 but they are obsolete these days so I had to use 2n3866. They work well in small signal applications.


You could try S9018 in place of BF199 requirements, they are cheap and excellent transistors at VHF FM frequencies.

Generally, I think the non-PLL circuits will have some degree of drifts over time, but that mainly depends on quality of parts used(temperature coeff) in the frequency determining section. For eg, below cases must be addressed at least-

> NPO ceramics can be preferred
> Metal film resistors in place of carbon(because carbons are worst at temp changes)
> Highly regulated power supply
> JFET buffers in place of BJT(J310,BF245 etc)
> Shielded oscillator(to prevent interference from nearby objects)
> Glued frequency determining components(coils etc)

ALso with best stable oscillator topology can help reduce the drifts to a minimum level, if not completely eliminates it.


Excellent points. I'll keep them in mind if I redesign the circuit. I've ordered some NP0 caps from Ebay. Isolating the oscillator with a resistor 15K or more eliminates drift due to loading. Rest of the drift is from temperature changes.. and overnight drift was 30KHz when room temperature changed by 3F. I am planning to design a proper PCB and have the coil etched on it. Let me look up S9018 on Ebay.. will order some today. Excellent suggestions on your part. I was thinking of using JFETs but these components are not easily available in India where I publish my projects. Electronic and radio amateur hobby went downhill after the advent of social media.

Thank you for letting me know about the S9018. It's Ft is 800MHz and it would make an awesome replacement for the 2n3866s which I have been using for small signal work. Placed an order on Ebay.. 30 pieces for $3.5


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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Sun 17, 2019 6:39 pm 
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Location: Perrysburg, OH, U.S.A.
transistor495 wrote:
NPO ceramics can be preferred.

Be aware that a lot of tuned L-C circuits use N750 caps as part of the "C" to offset the slight positive inductance increase of the "L" with temperature. You state that the frequency changed by 30KHz with a 3°F temperature change. If the frequency decreased with a rise in temperature (an effective increase in capacitance), that's a positive tempco and an N750 cap would help. The same is true if the frequency increased (an effective decrease in capacitance) with a drop in temperature.
John

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 Post subject: Re: My four transistor FMTX
PostPosted: Mar Sun 17, 2019 6:49 pm 
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It decreased by 30KHz overnight when room temperature changed from 65F to 62F and started to increase during daytime.


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