Audio Guru FM transmitter capacitor and inductor enquiry

[ckchew8]

hello there audio guru,
I have some enquiries regarding the types of capacitors and inductor which can be used for your fm transmitter. I'm trying to make a pcb out of smd components with pour copper to reduce em and also reduce high frequency noise.

The following are the trimmer capacitor which i plan to use:
**broken link removed**

The following shows the inductor that was planned to be used:
**broken link removed**

as for the ordinary capacitors, i would go for ceramic capacitors...any opinions or advice on this? would be advisable to make your own inductor? if so how can you ensure that it is the correct rating and how do i make it? [unfortunately i do not have access to expensive equipment to measure inductance ]

btw, how was tuning done using audio guru's fm transmitter? i mean how would you know which frequency the transmitter is transmitting at by tuning the trimmer capacitor? do i need to use a spectrum analyser to tune everytime itry and change the frequency of operation of the circuit? please do advice!thanx in advance

 

[audioguru]

The trimmer capacitor you selected will be perfect.

The inductor you selected has a very high resistance and its self-resonant frequency is too close to the high end of the FM broadcast band.

I made my inductors with enamel-insulated 1mm wire from a speaker's crossover coil. I wound them tightly on a 3mm drill bit, spaced them a little above the circuit board and used a knife and sandpaper to remove emamel from the ends for soldering.
Their resistance is extremely low and their self-resonant frequency is very high.

Since my FM dial is full of stations, I tuned my FM radios to a low power foreign language station on the other side of my huge city. Then I tuned my transmitter so it could be heard. Cheap FM radios are overloaded by the transmitter when it is close, causing the transmitter to appear all across the dial. My Sony Walkman radio has an antenna attenuator switch (local, distant) that allows tuning with the transmitter close.

Here is my home-made coil:

 

[ckchew8]

hi there audio guru,
Thanx for the information on the inductor...based on your diagram, you have 9 turns of a 1mm wire right? so the following can be calculated :
1mm=0.0393700787402 inches
3mm=0.1181102362206 inches
length of coil=0.4 inches (to be exact 9 turns X 1mm = 9mm)
9mm=0.3543307086618 inches

using the formula:
L= (d^2 * n^2)/(18d+40l)

where:

L is inductance in micro Henrys,
d is coil diameter in inches,
l is coil length in inches, and
n is number of turns.

then the inductance of the coil would be=0.06933uH
This value deviates from the 0.1uH of your original circuit. Would this affect anything?
on the other hand, you mentioned that the resistance is too high and the self resonance is too close, what about this one?we basically try and avoid self resonance as the impedance would be very high at resonance right?

what about this one with 0.26 ohm dc resistance and 1310MHz resonant frequency:
EPCOS|B82498B3101J|INDUCTOR, 0805 CASE, 100NH | element14 Malaysia

thanx in advance for the great help!

 

[mvs sarma]

Better use this calculator **broken link removed** select aircore.

try loading 0.1uH, dia (wire centre to wire centre) .4mm, coil length as 10mm, you get 8.7 turns and WIRE #17AWG. wire length is almost 11cm
these readings are very close to the spec prescribed by AudioGURU. also it is dam cheap to workout from even recovered wire. adds to experience. Your mony in RM is useful for other components.

 

[audioguru]

I simply copied my coil from other FM transmitter projects. I don't know if the coil is actually 100nH because nobody ever measured its tiny inductance.
The EPCOS coil is fine and is much better than the other one that is manufactured.

 

[ckchew8]

haha, i see...many thanx for the expert advice audio guru and mvs sarma, i will post up the smd board later hehe

 

[ckchew8]

here this is the circuit as mentioned earlier, please ignore the comments haha, didn't have time to clean that up, what do you think?

Circuit:
**broken link removed**

PCB Layout:
**broken link removed**

 

[audioguru]

Please attach your schematic to your post here (click on Go Advanced), instead of over at yfrog.

You use 1uf for C1 and C2. Their size is so big that their inductance makes a very poor bypass at 100MHz. I used a small 1000pF ceramic disc capacitor that is excellent as a bypass at 100MHz.

You use an ordinary huge 7805 voltage regulator that fails to regulate when the 9V battery quickly drops to 7.5V and less. I used a small low-dropout regulator that regulates perfectly until the 9V battery drops to 5.5V.

I didn't look at your pcb.

 

[ckchew8]

hi audio guru, thanx for the comments! i never noticed my mistake haha, the capacitors were meant to be 1nF as 1000pF=1nF, mistaken haha, thanx or pointing it out! regarding the low-dropout regulator, i never considered the dropout voltage before! learnt something new! hehe thank you so much for your help!...will upload the latest version soon! Have a nice day and thank you again!

PS: thanx for the tip on how to attach the schematics!^^

 

[audioguru]

I use the terms "pF" and "uF" for ceramic capacitors ands use the term "nF" for film capacitors. I use film capacitors to couple audio.

 

[sanjubluerock3]

and also you should thank MVS sarma for sharing pcb design



https://www.electro-tech-online.com/threads/audiogurus-fm-tx-troubleshooting.111783/

 

[mvs sarma]

Perhaps that is not what we expect, after all the members should clear their doubts and become strong in design and manufacture their projects with proper and logical technical documentation. It gives satisfaction. Equally we expect a feed back that an OP has used and got over his/her design or technical issues .I find many people don't do it. this is NOT to hurt any ones sentiments.or EGOs
I am an admirer of AudioGURU myself and don't expect any thanks what so ever.


.

 

[ckchew8]

hi audio guru, may i ask why you would prefer usinig film than ceramic to couple for audio? are these refering to capacitors C14 and C12?

 

[ckchew8]

oh hi mvs sarma, sorry i didn't notice your message, only saw page 1, thanx for helping, your advice has significantly guided me in my project!

PS: No offense but i did not use your circuit or pcb design as i don't trust people in designing circuits and pcb layouts in eagle for my projects as everyone is fallible. i only saw it once in eagle and that's all. Well it's a matter of personal preference though. thanx for sharing! cheers!

 

[audioguru]

hi audio guru, may i ask why you would prefer usinig film than ceramic to couple for audio? are these refering to capacitors C14 and C12?

C12 and C14 couple the 100MHz radio signal, not 50Hz to 15kHz audio. C12 and C14 should be ceramic.

C1, C3 and C4 couple 50Hz to 15kHz audio and should be film.

Plastic film capacitors produce no distortion to audio. But they cannot pass radio signals well.
Ceramic capacitors change their capacitance a little with voltage level so they produce audio distortion. They are excellent at radio frequencies.

 

[sanjubluerock3]

PS: No offense but i did not use your circuit or pcb design as i don't trust people in designing circuits and pcb layouts in eagle for my projects as everyone is fallible. i only saw it once in eagle and that's all. Well it's a matter of personal preference though. thanx for sharing! cheers![/QUOTE]

You don't trust mvs sarma????you are the only one person who has said something awful about him...till now..any way forget it...

 

[ckchew8]

erm, i'm trying to be cautious on my side and by NO MEANS trying to insult his or anyone's wisdom or knowledge and i appreciate his guidance and help...besides, do you usually do your projects based on other people's circuit design without any verification or reasoning? Sources from the internet forums and etc may or may not be acurate at times and i'm pretty sure you would've done something to ensure you don't waste any materials and money. Having said that, my preference in verification is by designing and understanding something from scratch not just grab a circuit and build it. Do pardon me if it seemed like i was insulting anyone.

C1, C3 and C4 couple 50Hz to 15kHz audio and should be film.

Audio guru, isn't that on the power source side of the circuit? How is it coupled to the audio? similarly, why do you say that

C12 and C14 couple the 100MHz radio signal, not 50Hz to 15kHz audio.

isn't it connected directly to the audio source? please do enlighten me from your expert point of view. Thanx in advance!

 

[audioguru]

Audio guru, isn't that on the power source side of the circuit? How is it coupled to the audio? similarly, why do you say that.....

We are confused because you took my schematic and changed all the part reference numbers.
My schematic has C1, C3 and C4 for audio coupling but yours has them at the voltage regulator.
Mine has C12 and C14 for RF coupling but yours has then for audio coupling.

I didn't copy your schematic because it is pastel and is posted over at yfrog who sometimes takes a long time to open.
Also your schematic has no dots showing joined wires so it is confusing to look at.

 

[ckchew8]

oops, sorry about that haha, it was so much easier when i refer to your original circuit, humblest apologies...btw, regarding the tuning of the circuit, since Q2 is the radio frequency oscillator, this means i have to tune C6 and L1 (of your circuit) right? What about the L2 C13 pair? does it require any specific tuning settings? oh ya, can your circuit actually transmit stereo signals such as songs directly from mp3 players and etc? Thanx in advance!

 

[audioguru]

C6 tunes my schematic to the radio frequency you want. C13 tunes the output very broadly because the low impedance of the antenna loads down the tuned circuit. If C13 is peaked at 98MHz in the center of the FM broadcast band then there is still plenty of output at 88MHz and 108MHz.

My FM transmitter circuit is only mono. It is easy to mix stereo into mono. A completely different circuit (BH141x FM stereo transmitter IC) is used for stereo and it has a crystal controlled phase-locked-loop frequency synthesizer.