Transmitter question

[zachtheterrible]

criminey. an oscilliscope will help me some though, right? I took it over 2 a friends house that has a 100 MHZ oscilloscope like i mentioned, and it seemed to work just great.

 

[Nigel Goodwin]

criminey. an oscilliscope will help me some though, right? I took it over 2 a friends house that has a 100 MHZ oscilloscope like i mentioned, and it seemed to work just great.

Yes, a scope is a great help in electronics, but less for VHF frequencies than lower ones - if it will show the output of your transmitter it will have too low a horizontal resolution to measure the frequency (too many cycles across the screen).

 

[zachtheterrible]

But my friend did it!!!!!! He zoomed in so that there was two or three waveforms on the CRT. his is just a regular ol' 100 MHZ. This oscilliscope im planning on buying has a little LED display on it. im guessing thats to tell u the frequency of the signal?

If i got a 200 MHZ oscilliscope, would it be better suited 4 RF?

 

[audioguru]

Hi Zach,
You're planning to buy an expensive piece of test equipment and you don't know its features nor spec's? Maybe the display is a clock!
If your transmitter is at 100MHz, and if the 'scope has a tolerance of 5%, which is pretty good, it will show anything from 95MHz to 105MHz, which is one-half of the entire FM band! A cheap frequency counter will be at least 10 and maybe even 100 times better.
A 100MHz 'scope will show the carrier voltage at 70.7% of what it really is. A 200MHz 'scope will show about 80% to 85%. Neither will show clipping nor distortion at 100MHz. That's not very accurate.
I frequently use my 5MHz kit 'scope that I built about 40 years ago, and souped it up a bit for rock-solid sync. It's great for audio, generators and CMOS circuits. I have made a fair number of RF circuits but never needed to see the carrier on a 'scope.

 

[zachtheterrible]

mayb i will just get a frequency counter

 

[John Sorensen]

From the website above,

In every transmitter there are present apart from the main output frequency various harmonics that usually have a very short range. In order to make sure you haven’t tuned on one of them ...

Is it possible for the intended frequency and it's harmonics both to be in the FM range at the same time?

I was messing around with a 2 transistor transmitter once trying to tune in around the 88 MHz end when I discovered I had an awesome signal on the other end of the dial...

j.

 

[audioguru]

Hi John,
The first harmonic (actually called the 2nd harmonic) of 88MHz is it doubled, which is 176MHz. The 3rd is 264MHz. Probably his radio was overloaded by his nearby transmitter and was producing a spurious signal. That could be caused by his transmitter carrier beating with another transmission that is a multiple of his radio's IF frequency (10.7MHz) apart.

 

[John Sorensen]

Wow, that's complicated :!:

Best left to the experts...

j.

 

[zachtheterrible]

Audioguru, u may have just solved my transmitter woes. Wenever im testing the thing out, i have the radio lilke 2 feet from my transmitter. how far apart should they be?

Also, what do u mean by my transmitter "beating" with another frequency? Is this the same as mixing?

Maybe my transmitter is transmitting @ 20 MHZ and im getting harmonics on my radio cuz i get sometimes 88 MHZ and 108 MHZ. thats y it would be very helpful if an oscilliscope could tell me my base frequency, then i could know for certain.

 

[audioguru]

Hi Zach,
Yes, your noise problem all over the FM band may be caused by your radio's RF input stage being overloaded by your nearby transmitter. Move them apart a few feet and see if the problem disappears. Then you can easily find your base frequency.
Use a quality hi-fi FM tuner, not a cheap radio. A quality tuner usually tunes the input and output of its input RF amplifier. A cheap radio may not tune the signal before the input RF amplifier and therefore will easily be overloaded by any frequency.
When the input RF stage of a radio is overloaded it becomes a mixer that generates new "beat" frequencies that are the sum and difference of input frequencies. If you have many FM stations nearby, a cheap radio's output will be a big mess.

 

[zachtheterrible]

hmm, ill try that

 

[zachtheterrible]

I tried moving the receiver away, and that did help quite a bit. The only receiver that I have is an older clock radio. I also have the car radio but can't drive yet :evil:, just got my permit. Could anyone recomend a small pocket hi-fi reciever like audioguru was recommending? I need one that can fit in me pocket and has a headset jack. Is a car radio hi-fi? Also would a stereo that's about 3 or 4 years old be hi-fi? I have one in storage. Thanx

 

[audioguru]

Hi Zach,
You don't need a quality "hi-fi" tuner because it sounds good. Your simple FM transmitter won't sound good anyway. It will sound something like an AM radio, or like a hi-fi that has its treble control turned all the way down.

You need a quality tuner that doesn't overload all the time, and has very good selectivity and sensitivity, so that it picks-up your transmission far away. Clock radios and most pocket radios are junk. Many "stereos" are junk too, it depends on the quality.
Most car radios are good, and up here, Radio Shack sells quality Grundig portable radios, but I haven't tried one.
You know what quality means: $$$$$, and sometimes, namebrand too.

 

[zachtheterrible]

Is there any way of telling what is a good radio just by looking @ what the box says? Or do I just have to open it up and test it?

 

[audioguru]

Hi Zach,
You can't always believe what it says on the box, it won't give any details. It might print a fantastic sensitivity (maybe for only one far-away station without any other stations), but won't say that it overloads all the time if there are any other stations.

 

[Nigel Goodwin]

Most car radios are good, and up here, Radio Shack sells quality Grundig portable radios, but I haven't tried one.
You know what quality means: $$$$$, and sometimes, namebrand too.

Just a point, Grundig went bust last year! - so I don't know what happened to the car radio side, but the entire company folded.

In the UK the Dixon's group bought the rights to use the Grundig name (actually well BEFORE Grundig folded), so the name lives on - basically to replace the Matsui name on their cheap crap!.

At one time Grundig were supplying radios to Vauxhall, but I seem to recall they started taking other makes instead.

As for the rest of the world, who knows? - presumably the receivers sold the trade name in other countries as well?.

 

[zachtheterrible]

well then, looks like im gonna be building my own if nobody knows of any good pocket receivers that i can buy. Ne good schematics ne1 know of??

 

[audioguru]

Hi Nigel,
Thanks for the info. On this side of the Atlantic I didn't hear that Grundig went bust.
I'm curious, so I'll go to Radio Shack to see where and when their Grundig radios were made.

Hi Zach,
On the web, I found the projects page of the author of your circuit!
**broken link removed**
He has many FM transmitter projects and sells kits and PCBs. Check out his printed coils. Since wiring methods are critical at high RF frequencies, maybe you should buy from him.
He also has a simple TDA7000 one-IC FM tuner project. It has pretty good spec's. Try it, or maybe Philips has a newer and better IC now. The tuner will need a small amplifier such as an LM386 to drive your headphones or a speaker.
Since the author is Swedish, all parts are european, which may be difficult for you to obtain.

 

[zachtheterrible]

Wow . thats a pretty cool little receiver circuit, really simple. and you say that this will help me get way better range? If i tried a different IC like you said audioguru, wouldnt it change the whole circuit and i would basically have to start from scratch? And where would i find a newer updated IC?

 

[Vaibhav Birit]

FM receiver

You can build a much better and easier FM receiver. Visit this webpage

**broken link removed**

and then go to Radio Shack Special (RSS) project. The author of this project is Patrick Cambre. He is very helpful.

Go ahead and build the magnificient FM receiver