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 Post subject: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Wed 18, 2019 12:56 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
Sine wave output after low pass filter

https://ibb.co/Vmr9V4d

Prototype board

https://ibb.co/k1SBLvW

Schematic

https://ibb.co/HntCx2b

Frequency = 88.3MHz - 89.5MHz
with both C5 and C6 in circuit. Pout = 10mW

Frequency = 100.6MHz - 101.2MHz
when C5 is replaced with wire link. Pout = 7mW

All transistors are 2n3904.

L1 = 170nH or 14T 25AWG/26SWG on 2mm diameter plastic former. Coil is secured tightly with superglue.

L2 = 90nH or 3T 19AWG/20SWG on 8mm diameter air core. L2, C18, and C19 form a Butterworth low pass filter with a cut off frequency of 125MHz and matches 1000ohm (T4's collector) to 50ohm antenna impedance. T4 only provides about 4dB of power gain boosting 4mW from driver stage T3 to 10mW. T4 also helps increase oscillator isolation from the antenna.

T1 is a simple Colpitts Oscillator and T2 is an emitter follower buffer. The oscillator is lightly loaded with a 1pf capacitor. The amplifier chain built around T2, T3, and T4 provides good RF gain and adequate buffering of the oscillator.

RFC 1-3 = 70T 39AWG/41SWG on 10K 1/4W carbon film resistors.

VR2 is for frequency fine tune. It varies the voltage on top of the zener diode from 5 to 6 volts. A 16V 1W Zener has been used as a varactor diode. Audio quality is excellent due to pure FM modulation and 50uS pre-emphasis.

A 47ohm resistor was used as a dummy load for testing. Use 12V regulated power supply.

Enclose the circuit in an air tight plastic box for excellent long term frequency stability. Maximum stability and an output power of 10mW were obtained in the lower part of the FM band or 88 - 92MHz.

R5 is 10K.


Last edited by Dare4444 on Dec Thu 19, 2019 6:41 am, edited 9 times in total.

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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Wed 18, 2019 4:41 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
Posts: 2152
Location: Roanoke, VA
Note that in order to use this transmitter legally in the US you need to connect it to the antenna that you will be using and then measure the radiated field in intensity to satisfy 47CFR 15.239.

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


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Wed 18, 2019 7:09 pm 
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Dale H. Cook wrote:
measure the radiated field in intensity to satisfy 47CFR 15.239

Hi Dale, in a previous thread you mentioned that proper field intensity measuring device needed to be calibrated and certified. Who does the certification and how often must it be done?


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Wed 18, 2019 8:28 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 264
Location: Arlington, TX, USA
Macrohenry wrote:
Dale H. Cook wrote:
measure the radiated field in intensity to satisfy 47CFR 15.239

Hi Dale, in a previous thread you mentioned that proper field intensity measuring device needed to be calibrated and certified. Who does the certification and how often must it be done?


I doubt you'll get an answer to that question. The MO of the legalists here is to spout their bluster and storm off in a huff when others disagree or dare to question them. I have asked the same question before and the only reply was the sound of chirping crickets or vague responses like "that's the law", no specific information or citations.

There is a field strength limitation for most unlicensed radio transmissions, but I read no iron clad requirement for calibrated measurements, so using a good quality radio receiver to insure the signal doesn't travel over about 200 feet should suffice and would at least show a good faith attempt to comply with the rules. Yes, a calibrated measurement would be the best way, but is just plain impractical for a hobbyist who is just slapping together a few bucks worth of parts to transmit to a nearby receiver.

https://www.fcc.gov/media/radio/low-pow ... nformation

This link has the 200 foot rule of thumb in it.


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Wed 18, 2019 10:30 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
I appreciate Dale for keeping us informed about the rules and regulations. I'm running the transmitter into a dummy load (47ohm resistor) and getting 20 feet of range. Designing and building your own transmitter is fun!


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Thu 19, 2019 2:48 pm 
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Location: Roanoke, VA
Macrohenry wrote:
Hi Dale, in a previous thread you mentioned that proper field intensity measuring device needed to be calibrated and certified. Who does the certification and how often must it be done?

The certification is done by the manufacturer, who also generally recommends the calibration interval. As one who has used calibrated field intensity meters at work for more than forty years I do not like to see one reach ten years between calibrations, and prefer to see them calibrated at five year intervals.

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


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Thu 19, 2019 3:10 pm 
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Location: Roanoke, VA
Erich Loepke wrote:
There is a field strength limitation for most unlicensed radio transmissions, but I read no iron clad requirement for calibrated measurements ...

A specific field intensity limit for Part 15 FM stations is specified in 47CFR 15.239. FCC field engineers always measure the field intensity of FM pirate stations to prove that they are in violation and they always use a calibrated field intensity meter - no other means of measurement will satisfy the FCC.

Erich Loepke wrote:
but is just plain impractical for a hobbyist who is just slapping together a few bucks worth of parts to transmit to a nearby receiver.

That is why, for FM, I use a commercially manufactured Part 15 FM transmitter with the manufacturer-supplied antenna, and bearing an FCC ID number, which is required for all Part 15 FM transmitters sold in the US. The manufacturer has made appropriate field intensity measurements with a calibrated FIM to secure that certification.

Erich Loepke wrote:
This link has the 200 foot rule of thumb in it.

It is a useful rule of thumb, but 47CFR 15.239 must still be met. Feel free to ignore the law, but as one whose profession is, in part, to make certain that licensees meet the FCC rules and regs, and as one who has held an FCC commercial operators' license for more than 50 years, I have no choice in what I do. I would note that FCC enforcement actions against illegal and overpowered unlicensed FM stations have been on the rise in recent years. While it is unlikely that someone who honestly tries to restrict the range of their unlicensed FM station will ever get a visit from Uncle Charlie, nowadays it doesn't take much more than a formal complaint of interference from a licensed broadcaster or the FAA to prompt a field investigation. YMMV.

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


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Fri 20, 2019 12:43 am 
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Looks like FCC commercial operators' license is for aircraft and ships.
https://www.fcc.gov/wireless/bureau-div ... se-program

DM


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Fri 20, 2019 1:03 am 
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Joined: Apr Thu 12, 2007 3:24 am
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Location: Milwaukee WI
The way I read that link is that license is needed to perform operations on ships and aircraft. But nothing states that is only thing the license holder is licensed to do.

Kind of like a driver with a higher CDL rating can drive multiple types of trucks.


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Fri 20, 2019 4:38 pm 
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Location: Roanoke, VA
devilsmist wrote:
Looks like FCC commercial operators' license is for aircraft and ships.

It is now, but when I started in radio more than fifty years ago every station had to have someone with at least a third class license on duty at all times, and had to have at least one holder of the first class license as chief engineer. Directional AM stations had to have first class licensees on duty at all times. I got my third more than fifty years ago, and my first more than forty years ago. Many station owners still want a chief engineer who passed the first class exam, even though operators licenses are no longer required in broadcasting. I have held commercial licenses continuously for more than half a century and will not jeopardize that.

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


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Fri 27, 2019 5:28 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
https://ibb.co/wKqmW9P

New updated schematic. Output is now taken from the collector of colpitts oscillator instead of emitter and Frequency stability improved further. Total drift in 24 hour period was 7KHz (2Khz upward during daytime at higher room temperature and 5KHz downward during night at lower room temperature).

The board was covered with a plastic bowl but it was not air tight.


This design outputs 10mW (88-92MHz).
All transistors are 2n3904. No hard to find RF transistors have been used.

Update: Emitter resistor of T4 increased to 100ohm for improved stability.


Last edited by Dare4444 on Dec Tue 31, 2019 5:58 pm, edited 1 time in total.

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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Sat 28, 2019 11:01 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
"If you physically match the diameter against the length. You should be able to get the temperature coefficient to be zero. if the coil is short and fat, the increase in diameter will increase the inductance as the temperature rises, if its long and narrow the inductance will decrease as the coil gets longer."

Is the above statement true? It explains the drift in my FM TX. As the temperature rises the frequency goes up as well. My coil is long 1.5cm in length and only 2mm in width.


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Tue 31, 2019 4:54 pm 
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Joined: Sep Fri 14, 2018 7:56 pm
Posts: 54
Location: Switzerland
Thank you for that great transmitter!
I built it „ugly construction“ on a piece of double sided pcb with drilled soldering islands.
Attachment:
File comment: First try
stable fm tx_rev1 built V1.0.jpg
stable fm tx_rev1 built V1.0.jpg [ 587.95 KiB | Viewed 1619 times ]

The coil has 6 turns 1.2 mm blank copper wire (nearly SWG 18), is 1.2 cm in length and 6 mm in diameter. The Zener is a BZX79 C16.
The transistors are unknown pnp (supposed to be similar to 2N3703). Most parts came out of the junkbox.
At first the oscillator didn’t work, probably due to a lower transition frequency of the used transistors. I tried some minor changes of C6 and C7 and then put a 2N3906 as oscillator and it worked. A 70 cm wire was coupled directly to the lp filter. The range is 20 m inside the house, perfect and stable.
I have no possibility to exactly measure such a high frequency resp. such low inductances.
The stability has been tested by increasing the temperature of the coil with a soldering iron from 68 to around 100 °F. Of course all the frequency determining parts nearby also warmed up. Measuring device was a ReVox B760 tuner. I could center again increasing the tuner by 25kHz.
Next I’ll try another coil ….


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Tue 31, 2019 5:15 pm 
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Location: Sanford Fla 32771 (USA)
Quote:
drilled soldering islands.

All your drilled islands are very consistent, what tool did you use ?

_________________
Paul of Florida ….I had my patience tested. I’m negative.
https://paulsironhorse.smugmug.com/


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Tue 31, 2019 5:44 pm 
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Joined: Sep Fri 14, 2018 7:56 pm
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Location: Switzerland
it's not my idea, see here, in german, second picture http://www.janson-soft.de/seminare/dh7uaf/ugly/ugly.htm.
Mine looks ugly anyway:
Attachment:
island drill.jpg
island drill.jpg [ 71.67 KiB | Viewed 1611 times ]

It's made from an old 10 mm drill.


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Tue 31, 2019 5:46 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
Hi,

I am very happy to see someone try my design as I have spent a considerable amount of time in perfecting the circuit. Make sure you use the new updated schematic where the output has been taken via a 1pf capacitor from the collector of the oscillator.
For the oscillator coil, I'll soon have a PCB design with the coil etched on it. You can wind a new coil by taking 26SWG (25AWG) magnet wire on a piece of Johnson ear bud. I believe it's made of paper and not plastic. Before winding coil first evenly apply a layer of superglue on the former and then tightly wind 14T. Keep pulling the wire for a few minutes and the glue should harden. Then after it dries apply another layer of superglue evenly all over on top of the coil and let it dry. Using NP0 capacitors in oscillator section will reduce the drift. I used 2n3904 they have Ft of 300MHz. For improved stability I increased emitter resistor of final stage T4 to 100ohm. Stability is now excellent and total drift was 7KHz in a day. As long as the total drift is less than 15KHz receiver retuning to follow the transmission is not necessary. Use 2n3904s if you can and enclose the board in an air tight plastic case to remove air currents from affecting the frequency.

I have also developed a one transistor 2n3866 class AB amplifier to boost the output from T4 to over 100mW. I'll publish it here but it would make the transmitter illegal in USA if connected to a wire antenna. The new amplifier design is for my article in an electronics magazine.

Keep the TX frequency between 88 and 91MHz for maximum power output and excellent Stability. Increase T4 emitter to 100ohm for good long term stability.


Last edited by Dare4444 on Dec Tue 31, 2019 8:56 pm, edited 2 times in total.

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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Tue 31, 2019 6:11 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
I don't know why but the oscillator is most stable when current through T1 is 6.5ma (800ohm emitter resistor). Increasing or decreasing this value causes drift.


Last edited by Dare4444 on Jan Wed 01, 2020 5:57 am, edited 2 times in total.

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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Dec Tue 31, 2019 8:40 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
2n3866 100mW Class AB amplifier. Standing current = 30ma. Po = 130mW without a low pass filter @ 90MHz

https://ibb.co/4KTknGB


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Thu 02, 2020 8:49 pm 
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Good work! Might I suggest your next contribution is to build a stable MW signal source that can be substituted for the crystal in the 386 and 6888 transmitters. Maybe add a 1:100 divider to your FM oscillator to get less than 70 HZ drift on MW.


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 Post subject: Re: High Stability FM Transmitter (7 - 10mW) design.
PostPosted: Jan Fri 03, 2020 8:44 am 
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Joined: Mar Thu 01, 2018 1:30 am
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Macrohenry wrote:
Good work! Might I suggest your next contribution is to build a stable MW signal source that can be substituted for the crystal in the 386 and 6888 transmitters. Maybe add a 1:100 divider to your FM oscillator to get less than 70 HZ drift on MW.


Check http://qrp.gr/minivfo/

The author says the drift is only 20Hz! It's a 3-30MHz VFO and MW band can be covered by using different capacitor values. It's a Franklin oscillator where the LC tuned circuit is coupled lightly and very little power is drawn from it. This greatly improves frequency stability. I need to try this topology for my next frequency stable FM TX.


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