60-160pf

I know thats a huge range right there. It still doesn't cover the whole fm band though. Only about 3/4 of it. If I Wanna move it up I have to unwind the ferrite slug in the coil.

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as I understood the tda7000 is good for 75ohm antena, I need to make it for 50ohm antena, how can I do it???

 

I wouldn't worry about it, I doubt it's got any low impedance matching inside the chip - it's probably only specified at 75 ohm because that's what's used for FM radio aerials.

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Doing some matching may not be a bad idea as can be seen from the plots. Without matching the there's quite a bit of loss and bad return loss.

With matching the input loss is 4dB less and the return loss very reasonable in the passband. Also the added selectivity will make a difference in performance.

Attached Images

tda_7000.gif (23.5 KB, 492 views)

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How did you arrive at the circuits and plots?, there's no suggestion in the TDA7000 datasheet that the input impedance is 75 ohms? - which is what I suggested in my pervious post.

The internal block diagram shows it has a balanced input, with a 700 ohm resistor from each input to a 1.4V reference. The second input is simply grounded via a capacitor in the application note.

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I found an application note that gives the input model of the TDA7000 and worked from there. My plot seem to agree with the one in the application note.

Attached Images

tda_7000_1_131.gif (6.4 KB, 472 views)

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thanks for helping....
I can't find aplication notes for TDA7000. does someone know where can I get it?
I read that this chip working on if=70MHz, is it right? how can I change it to other if???


thanks for help...

 

http://www.alldatasheet.com

NO! - it's only 70KHz IF, which will give it absolutely pathetic image rejection - particularly with an untuned front end. This chip is basically designed for low performance cheap radios - not really a 'proper' superhet at all!.
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I understand, and if I need if=11-12Mhz with sensitivity 0.5uV, for 50ohm antena.
what chip do I need to use????

 

The standard IF for VHF radio is 10.7MHz, and ceramic filters (and conventional IF transformers) are freely available for those frequencies.

As I mentioned earlier, the TDA7000 doesn't appear to have a 75 ohms input impedance, it's not mentioned anywhere in the specs, nor would I expect the input of a chip to do so.

You appear to be wanting a very high spec VHF receiver, as such I would recommend a 'proper' superhet design, with at least one tuned RF stage (preferably more), a seperate mixer, a seperate oscillator - all using discrete devices. Then follow this by a 10.7MHz FM IF strip, using a chip, which could also contain the FM demodulator - I can't remember the number off hand, but there's an excellent one that's been used in FM radio projects for years.

As you have a requirement for 50 ohm input impedance, I presume this isn't for a broadcast receiver?, as such it's more likely an NBFM (Narrow Band FM) receiver, in which case you should do a second conversion down to a 455KHz second IF, to get a decent level of audio out.

The aerial input impedance is completely down to your own design, simply on the tuned circuit at the front end - you tap the coil for the impedance you require.

What EXACTLY are you trying to do?.

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Here is the application note for NB use
http://www.qsl.net/yo5ofh/data_sheet..._reception.pdf

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thanks again for the help.
I want to transmit voice at NB at freq 30-80Mhz, when the freq I choose by digital way, with IF I want(10-12Mhz).
what chip can I use???

 

The 4m amateur band is in the middle of that, you might try searching for 4m amateur projects. If you're wanting a synthesised transceiver for that wide range (30-80MHz) you're probably talking a pretty complicated device - never mind the highly illegal nature of it!.

BTW, 4M behaves fairly similar to 2M, basically line of sight - on 2m with a 1W handheld radio I've easily managed 70-80 miles from the top of a hill.

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The MC3361/2 I have used a lot in the past
http://www.alldatasheet.co.kr/datash...LA/MC3362.html
http://www.datasheetcatalog.com/data...3/MC3361.shtml
There are still a few suppliers selling it.
Another option is to use a NE605/6 615
http://gw.hb9ww.ampr.org/ftp/pub/Dat...lips/NE615.pdf, but the 3361/2 is much easier to use.
The following supplier keep some nice chips
http://www.oselectronics.com/ose_p45.htm
Check MC13175, MC2833, MC3357P

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Thanks for helping....
I choose mc3362.
I have some questions:
1. at page 4, there is LC connected to pins 21,22. the note saied it should be 10.7Mhz above or below the RF in. The LC tuned to 6.4MHZ below, why 6.4 and not 10.7?????
2. The varicap at pin 23, what dela F LO I can get with it????
3. How can I use PLL wuth this chip?

does someone can tell me a number of a RF amp chip???

thanks again....