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 Post subject: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Wed 22, 2018 12:44 am 
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https://ibb.co/nysS0z
https://ibb.co/jbipDK

Uses 4 x 10 mhz crystals in parallel and two
In4007 as varicap diodes. Output = 90 mhz. Range = 25 feet.


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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Wed 22, 2018 4:34 pm 
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Location: Somers, CT
What is the reason for 4 crystals in parallel?

Are the 1N4007 diodes able to produce adequate modulation?

I doubt it is street legal, and the harmonics must be horrendous.

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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Wed 22, 2018 5:15 pm 
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Four crystals form a super vxo and the frequency deviation is greater. IN4007 r not proper varactors but work kinda okay. Sound is low but music sounds okay.

Two diodes in parallel give better
capacitance ratio. A very narrow band pass filter would be needed to filter out the 90 mhz target
Frequency. Harmonics are only 10 mhz apart so it would be difficult but not impossible. I also published a FM TX with 30 mhz crystal and a proper isv149 varactor diode. Audio quality is great even for music! And band pass filter would be a lot easier as harmonics are 30 mhz apart.


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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Wed 22, 2018 6:27 pm 
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Peter Bertini wrote:
I doubt it is street legal ...

Peter makes a strong point, as the only way to tell if the power output of an FM broadcast band transmitter is legal in the US is with measurements made with a calibrated field intensity meter or with a calibrated spectrum analyzer using an antenna of known gain.

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


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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Wed 22, 2018 6:34 pm 
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But range is only 20 feet with a sensitive receiver. I recommend a band pass filter at the output and use a spectrum analyzer to make sure all harmonics are below 60 dbc or more


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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Sun 26, 2018 5:17 pm 
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That doesn't make it FCC compliant.

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


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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Sun 26, 2018 5:58 pm 
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Dale H. Cook wrote:
That doesn't make it FCC compliant.

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

But do we really care?

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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Sun 26, 2018 6:15 pm 
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Throughout the years that the FCC rules have been in place, has anyone ever done and published a "real world" test as to how far a transmitter can be reasonably heard with an average receiver at certain frequencies.Just a ballpark estimate test.

Example, the testing showed decent reception with approved measures at 30 feet. Compared to Joe Schmoe's which gave decent reception out to 100 feet with a similar antenna. Obviously Joe would now know he is not in compliance as far as power output is concerned.


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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Sun 26, 2018 7:36 pm 
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35Z5 wrote:
Dale H. Cook wrote:
But do we really care?

Some of us do, as our careers depend upon not running afoul of the Enforcement Bureau, who have in recent years been extremely active in pursuing FM transmitter operators who are in violation.

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


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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Sun 26, 2018 9:21 pm 
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Alright I dismantled the transmitter. It was just a test setup to test varicap capabilities of IN4007 diodes as normal varactor diodes are getting obsolete these days. It also worked fine with one 24 MHz crystal and only one IN4007 diode (96 Mhz transmit frequency). Building and designing transmitters is fascinating. Sound is better with 24 mhz xtal and one in4007. Now i want to add narrow BPF and observe the spectrum on an usb analyzer.


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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Sun 26, 2018 10:41 pm 
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Dare4444 wrote:
Alright I dismantled the transmitter.

Did you bury it in a irritating neighbor's backyard?

Might be a good idea to scatter the pieces in different rivers, better yet, mail a few parts to fictitious addresses in different cities. FCC will never figure out what's going on. :?

With all the noise generated by various items containing switching power supplies, worrying whether it's "legal" is beyond ridiculous.

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 Post subject: Re: 90 mhz fm transmitter schematic and prototype pic
PostPosted: Aug Mon 27, 2018 5:09 pm 
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35Z5 wrote:
Dare4444 wrote:
Alright I dismantled the transmitter.

Did you bury it in a irritating neighbor's backyard?

Might be a good idea to scatter the pieces in different rivers, better yet, mail a few parts to fictitious addresses in different cities. FCC will never figure out what's going on. :?

With all the noise generated by various items containing switching power supplies, worrying whether it's "legal" is beyond ridiculous.
:lol: :lol: :lol: :lol: :lol:


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