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Radio FM

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TheOne

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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.
 

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Nigel Goodwin

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Zener_Diode said:
thanks for helping....
I can't find aplication notes for TDA7000. does someone know where can I get it?

https://www.alldatasheet.com

I read that this chip working on if=70MHz, is it right? how can I change it to other if???

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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Nigel Goodwin

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Most Helpful Member
Zener_Diode said:
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?.
 

Nigel Goodwin

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Most Helpful Member
Zener_Diode said:
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.
 

TheOne

New Member

Zener_Diode

New Member
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....
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Zener_Diode said:
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?????

I don't see anything about 6.4MHz?, the first IF is 10.7MHz (standard for VHF radio) - so the first local oscillator needs to be either 10.7MHz above, or below, the incoming signal. The LC on pins 21/22 are the tuning components for the first local oscillator.

2. The varicap at pin 23, what dela F LO I can get with it????

It will depend on the LC circuit you use, and the range of the varactor diode.

3. How can I use PLL wuth this chip?

It's got a control voltage input, and a buffered oscillator, that's all you need to add a PLL tuning system - it's designed to be used with one.

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

Generally you would probably use a discrete RF amp, tuned input and output would be a good idea as well for any sort of decent receiver.
 

Nigel Goodwin

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Most Helpful Member
Zener_Diode said:
L=0.41uH
C=33pF

f=1/2*pi*sqr(LC)=1/2*pi*sqr(0.41*10^-6*33*10^-12)=43.258Mhz
Frf=49.67MHz

frf-f=49.67-43.25=6.41MHZ

why 6.41 and not 10.7?????

Probably because you haven't used the correct value for C!, what about the varactor diode in parallel with it?.
 
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