Experiences with older project: AM receiver (0 -1.8 MHz) fro

[engel1]

Hello,

I have recently built a somewhat older project, namely the AM receiver project from February 2001 (DC to 1.8 MHz, based on a combination of SA612A and TDA1572) but I do not succeed in getting it to run properly: Although some stations can be received everything gets drowned in a "sea" of noise. I get the impression that this may have to do with the circuit board layout.

Is there someone out here that built this project too, and is willing to share his experiences? Is there anyone who had great success with it, or got the same type of behaviour as I reported?

Best regards,
Jurjen Kranenborg

[cwolthausen]

I have worked with the SA6XX family of IF strip ICs many times and they are very susceptible to having a high noise floor or even parasitic oscillations if the PC board on which the part is mounted is not layed out correctly. Firstly, you need to have a good ground plane underneath the chip. Even if your ground plane is on the opposite side of the PCB or on an inner layer, I would still put a ground plane on the top side of the PCB directly under and around the part, and use a liberal amount of vias to connect this to the main ground plane. Secondly, there are a number of pins that need to be AC-connected to ground (these pins carry a DC bias generated internally, so they can't be directly connected to the ground plane). For these pins, I would recommend using small chip capacitors (0603 or 0402) and mount them as close to the pins as possible and connect the other side of the caps as close to the ground plane as possible, and use vias near these connection points to connect to the main ground plane on the PCB. Check out the datasheets on the Phillips website for this and the other similar parts in this family for some example PCB layouts.

My other recommendation if you are using the mixer function in this chip is to have a clean crystal oscillator. If your oscillator has high phase noise, then this will also yield a high noise floor on the output. If you are providing an external local oscillator to the chip, be sure to use a high-pass filter on the LO input to the chip because the mixer section of the chip does have a lot of gain at low frequencies, so any low frequency noise that happens to get on the LO input to the chip will get amplified and leaked onto the output.

Good luck!