AM/FM receiver using the TDA7088

[emuman100]

Hello,<p>For years, the magic of radio waves has intruiged me how with a simple circuit can pick up radio waves and strip the audio from them. When I first started to get actively involved in electronics not too long ago, I always wanted to design and build my own AM/FM receiver.<p>Then after I got more familiar with electronics after an EET degree, I've noticed the super cheap "autoscan" FM radios. Most of them use a D7088 IC, which is a Chinese clone of Philips' TDA7088. This IC is a narrow-band FM receiver for low cost portable applications. It requires a minimum of parts, and uses a 70kHz IF, so RC filters are used to tune the IF stages. Data is available at http://www.semiconductors.philips.com/pip/TDA7088.html<p>Philips claims an AM application can be achieved, and on page 9 there is an AM/FM application schematic. I'm not an EE and I have not had any real education in receiver designs. I contacted philips and they gave me nothing because it was obseleted and they claimed they had no information. <p>The FM part uses the FLL of the IC, with the 70kHz IF. The AM part uses a TRF AM receiver IC, the MK484, despite what philips labels it as.<p>This is why I'm contacting you guys. I thought this would be a cool project for the magazine because it's not a conventional receiver design. The AM antenna is a standard AM antenna you find in any AM radio, but maybe not because it's one winding serves as the FM RFC. The tuning condensor is the type you find in an AM only receiver, with two ganged condensors. They are also found in AM radio kits, etc. I think you know what I'm talking about.<p>The "AM IF" transformer according to the schematic is the 1:1 I think, it only has 4 pins for both windings. Maybe not 1:1, I'm not sure. The one by the band switch in the upper left corner is what I'm not sure of, but it's windings are not the same as the AM IF transformer.<p>The FM IF coil in the upper left is a few turns of wire, like the ones you see in FM radios. The Audio transformer I have no clue about, it's probably to match headphone impedance.<p>The CC7642 is I think an MK484. The AM section is seperate from the IC, using the MK484, and FM using the TDA7088. I realize the performance of this receiver will be poor, but it will work with strong stations if properly aligned, but still work better than an FM superregenerative receiver. I think it'll be a cool project to have working, and really cool for the magazine.<p>I'm writing you guys because I can't find anything on this type of design with the TDA7088 and would appreciate any advice or help you can give me, or even put this in a future issue. <p>Thanks so much.<p>Jonathan

[jollyrgr]

Here is a site for the MK484 type project:
http://www.thebest.net/wuggy/mk484.htm<p>And here is a listing of more chips like the TDA7088 and MK484:
http://www.oselectronics.com/ose_p81.htm

[emuman100]

The project I want to do is to use a MK484 as an IF amp and detector in a superhetrodyne receiver thats in the schematic. I want to do exactly that in page 9 of the TDA7088 datasheet, but I'm not sure about the one IF transformer and the audio transformer. Also the FM indictor in parallel with the tuning gang, what do you think the wire guage and turns should be? I know you collaps or spread this inductor as part of FM alignment, but I'm not really sure. <p>I never did any RF design before, so I'm not 100% sure.<p>Thanks.<p>Jonathan