RF Front end for a ADC

[ma]

Hello,

I like to do build or buy an RF front end for an ADC to be able to
capture RF signal and process with them.



Perfectly I like to cover the frequency range of FM and AM radios (150MHz).

The RF would have an antenna interface and a heterodyne stage. The ADC that
I have can sample up to around 100 MSpS but I like that my sampling rate be
around 20 to 30 MSpS so the heterodyne should down convert to 10MHz.

For AM radio receiving I think I don't need the heterodyne stage and I can
sample at the rate of 1o MSpS.



And my question: Where should I start? Can I buy anything such as this? My
budget isn't very high so I can't buy anything more than 100pound or so on.



Any help appreciated.



Best regards

Mansour

 

[Dave Platt]

I like to do build or buy an RF front end for an ADC to be able to

capture RF signal and process with them.

Perfectly I like to cover the frequency range of FM and AM radios (150MHz).

The RF would have an antenna interface and a heterodyne stage. The ADC that
I have can sample up to around 100 MSpS but I like that my sampling rate be
around 20 to 30 MSpS so the heterodyne should down convert to 10MHz.

For AM radio receiving I think I don't need the heterodyne stage and I can
sample at the rate of 1o MSpS.

And my question: Where should I start? Can I buy anything such as this? My
budget isn't very high so I can't buy anything more than 100pound or so on.

A lot of off-the-shelf FM radios use a double-conversion system... an
RF stage, oscillator, and mixer, with the output of the first mixer
being centered at 10.7 MHz. There's usually a tuned or crystal filter
stage of some sort (bandwidth of perhaps 100 kHz) with some
amplification, and then a second oscillator and mixer which creates a
lower-frequency (often 455 kHz) IF.

You could probably start with an off-the-shelf battery-powered AM/FM
radio (a several-decades-old one would probably be easier to work
with), tap off the signal at the output of the first mixer, and
amplify it enough to give you a useful input for your ADC (a
Minicircuits or similar monolithic amplifier would be an easy way to
get gain). You might be able to use the existing IF strip and its
amplifiers, if you jumper around the crystal or tuned-transformer
filters and convert it to a broadband amplifier.

Google for "software radio" - there are a number of projects working
on RF front ends and ADCs like this, and there might be some which
have a bandwidth as wide as you are looking for.

 

[Joerg]

Hello Dave,

A lot of off-the-shelf FM radios use a double-conversion system... an
RF stage, oscillator, and mixer, with the output of the first mixer
being centered at 10.7 MHz. There's usually a tuned or crystal filter
stage of some sort (bandwidth of perhaps 100 kHz) with some
amplification, and then a second oscillator and mixer which creates a
lower-frequency (often 455 kHz) IF.

Most of them are single conversion. 10.7MHz into a filter (or whatever
they nowadays call 'filter'), then into the FM demodulator. The 455kHz
section that runs in parallel is for the AM band(s).

The only radios with any decent performance I have seen are the ones
over $1000 or the old tube sets.

Regards, Joerg

 

[ma]

Hello,
I aonly need the antena amplifier and hytrodyne section. No
demodulation. I want to do demodulation in software.
so my questos are:
1- How can I design a simple antenna amplifer that connect to an antena from
one side and amplify the signal to a suitable level for a adc (1Vpp).
2- How can I design a hetrodyne section that can convert from 100MHZ to
10MHz?

best regards

 

[Joerg]

Hello ma,

I aonly need the antena amplifier and hytrodyne section. No
demodulation. I want to do demodulation in software.

You could still use an old radio. Just tap off at the mixer output.

so my questos are:
1- How can I design a simple antenna amplifer that connect to an antena from
one side and amplify the signal to a suitable level for a adc (1Vpp).
2- How can I design a hetrodyne section that can convert from 100MHZ to
10MHz?

I suggest to get a copy of the ARRL Handbook. That shows in great detail
how it's done.

Regards, Joerg

 

[Dave Platt]

so my questos are:

I suggest to get a copy of the ARRL Handbook. That shows in great detail
how it's done.

"Experimental Methods in RF Design", also available through the ARRL,
would also be a good reference source. Lots of detail there about the
design of local oscillators, various sorts of mixers, IF amplifiers,
and so forth.

 

[Joerg]

Hello Dave,

"Experimental Methods in RF Design", also available through the ARRL,
would also be a good reference source. Lots of detail there about the
design of local oscillators, various sorts of mixers, IF amplifiers,
and so forth.

And then there is always the easy way out: Whip out the credit card, buy
a Mini Circuits mixer and a nice synthesizer chip from AD. But that kind
of spoils the sportsmanship.

Regards, Joerg