96 MHz FMTX schehstic + spectrum Analyzer shot

First topic message reminder :

.

Hi again Joy, 

http://213.114.131.21/temp/96mhzfmtx.htm

If you need any corrections or additions then let me know before I link it on the homepages. At the moment it is in a temporary directory, but the final link location will be "http://213.114.131.21/_visitors/index.htm#tx" on www.sm0vpo.com.

Incidentally, 4mW (+6dBm) into a modest and resonant antenna (such as my http://213.114.131.21/antennas/vpole0.htm) will give you a good 500 metres if mounted outside "in the clear" (on the roof of the house). With a good domestic receiver you should be able to get 2 km or more.

Thank you Glenn. I used your suggestion and made a JPG from a screen dump of the e-mail address.

Hope all is well.
BR Harry

Yes Harry pls include the email address.

Admin wrote:Hello Joy,
Sorry for the delay. I have been sick (again) and had a few other "urgent" panics. yesterday I had a 3D x-ray of my head ;-)
...
Can I include your e-mail address so I can direct any questions to you?

BR Harry - SM0VPO
...

Hi Harry

Hope you have recovered/will recover.

Suggestion: Please make a "screenshot" of the email-addresses and embed these (tiny) images, so it is not directly script harvestable?

best regards,

Glenn

Hello Joy,
Sorry for the delay. I have been sick (again) and had a few other "urgent" panics. yesterday I had a 3D x-ray of my head ;-)

But I have copied all your text and the pictures, so I can create the HTML file. I will upoad the finished file to the altervista mirror and give you a link when it is done.

Can I include your e-mail address so I can direct any questions to you?

BR Harry - SM0VPO



PS - I hoped to do it at work, but www.sm0vpo.forumotion.com has been blocked by my employer  
Something to do with the companies "code of ethics". Perhaps they don't want engineers to have access to a technical Q&A forum ?

Hi Harry I edited the text and made some changes in the tuning procedure. Please use the updated text. Thanks

I updated it again today. Use the latest pics and description. Thanks Harry!

Thank you very much, I have not forgotten about you.

Maj-Lis is leaving for Spain in the morning, so my evenings are rather spoken for. But I hope to put the page on the server over the weekend and I will send you a link for your review.

Thank you once again.

BR Harry - SM0VPO

Hi Harry, here are the final images and text description.

https://ibb.co/jjnMgz (24MHz to 500MHz spectrum analyzer shot. 5dB per division)

https://ibb.co/f8fk8e

https://ibb.co/gi9OuK

The crystal locked FM transmitter presented here avoids the use of a LC tuned tank circuit or complex PLL (phase locked loop) method for FM carrier generation. Instead it uses a fixed frequency crystal and IN4007 as varactor diode. The 24 MHz crystal frequency is quadrupled for a final transmit frequency of 96 MHz. The audio fidelity is surprisingly good.

Circuit and Working

The circuit is built around a low-power audio amplifier using LM386 (IC1), transistors T1 and T2 (2N3904), 24MHz crystal (XTAL1), rectifier diode IN4007 (D1), and a few other components. A low power colpitts oscillator is built around T1 along with XTAL1, biasing resistors R3, R4, and R5, and capacitors C5 and C6 for feedback. The oscillator’s frequency is set to 24MHz by XTAL1. T2 is fed directly from emitter of T1 and acts as a buffer isolating the oscillator from the antenna. T2's collector is rich in harmonic energy and this makes it an ideal frequency multiplier. The tank circuit comprising inductor L1, C7, and trimmer capacitor VC1 is tuned to four times the crystal frequency or 96MHz. The fourth harmonic of 96MHz is boosted while all the other harmonics including the fundamental frequency of 24MHz are suppressed by 21 dB or more. L2 and C11 is a series resonant circuit tuned to 72 MHz and helps trap the third harmonic to ground thereby attenuating it.

A 50-Ohms level is taken off at 1/4 turn from top of the collector tuning coil and coupled to a 78cm long wire antenna for transmission. The signal can be picked up by an FM receiver tuned to 96 MHz.

The audio amplifier built around IC1 has a gain of 20. R1 and C3 removes noise and suppresses IC1’s tendency to self-oscillate. The amplified audio from the LM386 frequency modulates the oscillator by varying the capacitance of diode D1 which is connected in series with XTAL1. No hard to find varactor diode has been used. The IN4007 gives good performance. C2 prevents RF from getting into IC1. The trasmitter accepts audio input from an external equipment, such as a computer, music player, or your mobile phone. The external volume level should be set for best audio quality without distortion.


Construction and Testing

Keep all leads short. It is a standard practice at RF and aids in stability. The prototype was built on a piece of copper clad board using ugly bug construction method. Apply 12V power from a SMPS regulated power supply. Turn on S1 and tune VC1 for voltage reading at the point marked as ‘V’. It should be around 130mV. Carefully tune the 50 pf trimmer capacitor for the highest peak reading. All unwanted harmonics are now down below -24 dBc or more. A spectrum analyser may be used to confirm that the fourth harmonic has been boosted. If you don't have access to a spectrum analyser then the correct tuning of VC1 can be confirmed by connecting a wire antenna and testing for maximum range on an handheld FM receiver by walking away from the transmitter. Correct tuning of VC1 will give maximum range on 96MHz. L1 and L2 are 90nH air core inductors.

Turn off S1 and connect a 78cm wire antenna. Use shielded cable for input audio connection. The current consumption of the transmitter is 20ma and delivers an RF power output of +6dBm (4mW) which is more than adequate to cover my two bedroom apartment with good signal strength. An external VHF amplifier may be used to boost the power output to several hundred milliwatts along with a matched ground plane or dipole antenna for 1Km range.

L1 = 3T 20SWG enamelled copper wire wound on 8mm diameter air core. Tap at 1/4 turn from V+

L2 = 3T 20SWG enamelled copper wire wound on 8mm diameter air core.

Variable trimmer capacitor = 50 pF (12-66pf variable)

100uH RFCs used are readymade moulded chokes to isolate the oscillator and buffer stage and help reduce unwanted harmonic content at the output. L1 and L2 are 2cm apart and 90 degrees to each other. This arrangement gives the best harmonic suppression. A low pass filter with a cut-off frequency of 110MHz may be used to further clean up the output signal.

Joy Mukherji
Joy226010@gmail.com

Willl add final text & schematic. Give me a day. Thanks . Yes it looks ugly but works great.

Hi Dare4444,
Yes, or course  will accept it in the visitors circuits section.
Interesting that this method of construction looks really messy, but most often out-performs a custom PCB 

Is this the final text you want in the article? Or do you want to add more explanations, purpose, reason, motivation, etc? E-mail address and name?

BR Harry

Hi Harry would you accept this circuit for publication under visitor circuit section of your website?

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