Got a couple questions regarding this fm transmitter

[D0T-C0M]

Hello everyone , I just joined here from NB Canada. I just purchased the Micromitter stereo FM transmitter kit based on the following circuit. see url below.
http://electronics-diy.com/schematics/BH1417_fm_transmitter.htm

I had a couple questions if you may,

First, has anyone tried removing the attenuator and if so what range can I reasonbly expect to gain with it removed?

Secondly on another page on diy that comments the BH1417 chip,  I read a comment saying the following,

BH1417 chip may also be used a stand alone stereo encoder. The advantage of that is that you have full freedom of using a transmitter & amplifier of your choice. You will still have a pre-emphasis, limiter, stereo encoder and low pass filter in one small package because very few external components are required for these blocks. PIN 5 is MPX output that can be directly connected to an external FM transmitter through a 10uF cap.

Has anyone tried amplifying this circuit or does anyone know of an amplifying circuit that would work with this kit? I would like to achieve about 500ft of range with this unit. I would appreciate any suggestions.

Many thanks for reading my post.

 

[audioguru2]

Hi Dot-com,
Welcome to our forum. ;D
The Micromitter is intended to transmit an MP3 player to a car radio with a range of only about ten feet. If you remove the attenuator at its antenna, use a strong 12V supply (8 AA cells at least), use an 31 inch long antenna wire and a very good FM receiver with an equally good antenna, then its range should exceed 200 feet. It would need a single transistor RF amplifier for its range to exceed 500 feet.

 

[D0T-C0M]

wow thanks for the super fast response. I am not very good at designing circuits myself.  200ft would probably not be good enough as I would like to use this transmitter in my truck and have it transmit the fm signal over my property especially in the garage and in my house. Also would like to be able to hear the signal when I mow the lawn etc..

Do you have any idea what would be involved to modify the existing circuit? Do I just remove the existing components connected on pin11 on the bh1417 chip and connect this pin to a seried 10pf ceramic capacitor and run this to the input on the attached amplifier? I got that attached amplifier circuit here http://electronics-diy.com/BH1417_PLL_Stereo_FM_Transmitter.php Theres a FAQ section near the bottom of this page with a link to this amplifier. On the amplifier page it says that the input should be at least 100mW to achieve 1W output. I would assume that the bh1417 would push 20mW max. This should give me about 200mW right? I dont want to alluminate the whole country side. LoL Maybe I could put a variable attenuator on the output of this amplifier? What are your thoughts?

ps I would be using Q1 Transistor: 2N5109, MRF227, 2N4427, 2N3866 in the amplifier circuit below.


One other side question, what are your thoughts on the bandpass filter on the output? This bandpass filter is made by Soshin Electronics Co. labelled GFWB3. It is a small 3-terminal printed bandpass filter and operates in the 76-108MHz frequency band. It has very steep cutoffs and was wondering if I should bother changing this exiting LC circuit with this? Is it worth it or not in your oipinon?

 

[audioguru2]

Hi Dot-com,
You found a very old RF amplifier circuit. I looked in Digikey and the diode and all the transistors you listed are not available anymore.

 

[D0T-C0M]

Ah frig so much for the easy solution to my problem. I'm not really an expert in designing, You wouldn't by chance have a circuit that I could use?

 

[audioguru2]

Hi Dot-com,
Use the bandpass filter in the Micromitter but don't use its attenuator.

I live beside Toronto where there are more than 80 FM stations. So using a 1W RF amplifier would cause radio and TV interference and would cause complaints. It would be very illegal. So I am sorry that I don't have a modern RF amplifier circuit for you.

 

[D0T-C0M]

ok thanks Audioguru for your very quick responses.  I've read so many different forums and you seem to be on all of them and you seem to be so knowledgable. Thanks for taking the time to answer my questions.

I've been searching for 4 days now on the internet for some site that has a decent 100Mhz amplifier. Why is it so rare to find??  If you don't have any amp circuits maybe you could tell me what exactly I should look for when choosing a good high quality amp circuit? How much power would I need to push a clear signal to at least 500ft?

 

[D0T-C0M]

I just found this
DIGITAL DISPLAY MICROPROCESSOR CONTROLLED PLL SYNTHESIZED FM STEREO TRANSMITTER MODULE
sold by EDM electronics


you can get a switchable 10/100mW rf output. They use the bh1415 chip. Whats the difference in this chip as compared to the bh1417?

Anyone have this? what does the FM sound like? I am thinking on returning my micromitter kit and getting this one.

 

[audioguru2]

The EDN transmitter might be good. It is expensive. Is it manufactured or is it a kit? The spec for frequency response doesn't say plus or minus how many decibels. The equalization is in Rohm's BH1415 and BH1417 chips so it should sound just as good as a Micromitter.

The Micromitter kit is from Australia. Where did you buy it in Canada?

 

[D0T-C0M]

I bought it from jaycarelectronics.com which I would imagine ships out of Australia. It was pretty cheap at $36CAN plus $10 shipping. 

I just cancelled my micromitter order because I really like the 10mw/100mW selector mode of the EDM transmitter. the 10mW would be great when I am driving about but when I want the extra range, all I have to do is flick a switch.  I really dont have the expertise to fabricate a circuit to suit my application so I don't mind paying $120 for an advanced system as this. The kit is pretty much assembled. here are the steps to follow to assembly

  1.  Remove pre-programmed microprocessor from protective foil and plug into socket holder on PCB
  2. Plug digital displays into respective sockets on PCB
  3. Connect the power wires to a suitable 12VDC source.
  4. With the EP versions, install the supplied power connector and fasten with screws
  5. Solder the power wires, Red and Black to power terminals on the mounted power connector
  6. For versions without enclosures, you may construct your own enclosure or purchase from another supplier
  7. For versions without power units, you may construct your own or purchase from another supplier
  8. Follow operating instructions supplied for specific model series in manual

My only concern was it uses the bh1415 chip but from what I read its has the same specs as the bh1417 only the frenquency is PIC controlled.

this is from there site

Frequency Response: 20Hz - 15kHz
Level: Line level input via RCA type sockets (level adjustment on module)
Max Audio I/P sensitivity: < 100mVp-p (integrated limiter)
Distortion (THD): 0.2% with 1kHz tone at 75kHz deviation level (mainly 2nd, 4th)

I emailed them to see what the +/- db is.

 

[D0T-C0M]

ok I got this back from them .

20Hz - 15kHz (flatness +/- 1.5dB)

is this good?

 

[audioguru2]

20Hz - 15kHz (flatness +/- 1.5dB) is pretty good. The curves for the Micromitter show that 15kHz is down more than that because of the 19kHz filter's action. It is worse in North America due to our increased pre-emphasis.

 

[D0T-C0M]

Well I just ordered it , I should get it in 10-15 days.  the only thing is they say I might need an external antenna and move it away from the transmitter when operating at 100mW due to feedback which might cause a hummm

 

[AN920]

RF feedback is known to cause "hum". This is normally not a problem when the transmitter is mounted in a metal box and screened audio cables are used. Kits that comes with plastic boxes may have this problem.

 

[D0T-C0M]

thx i ordered the metal enclosure with it

 

[D0T-C0M]

what FM pre-emphasis setting should I ask for? I have the choice of 50uS or 75uS.

 

[audioguru2]

Hi Dot-com,
Canada and the US use 75us pre-emphasis (a big mistake). The US started FM broadcasting with plenty of pre-emphasis (too much) to reduce hiss but audio sources in those days didn't have as much high frequencies as better sources did a little later when Europe began FM broadcasts. So Europe decided to reduce their pre-emphasis to avoid overloading the high frequencies.
Here in Canada I don't notice overloading of high frequencies so they must have found ways to stop it.

An FM radio made for Europe sounds like it has too much treble when played in Canada and the US. An FM radio made for Canada and the US will sounds muffled when played in Europe.
Radios have a de-emphasis circuit (treble cut) that cuts the boosted high frequencies back down to normal, and at the same time cuts hiss that came from the radio system.

I think the pre-emphasis for the FM for TV sound is also different in the different parts of the world.

 

[D0T-C0M]

Ok so if I understand correctly I have no choice but to go with a pre-emphasis of 75uS otherwise if they set it to 50uS I will be getting muffled treble and thats certainly not what I want. correct?

 

[audioguru2]

I thought I explained that Canada uses 75us pre-emphasis.
I explained that a 50us transmitter (made for Europe) will sound muffled on a radio made for Canada.