Hello everyone,
I want to build an AM receiver using a ne612 and the famous TA7642 (MK484). So these are my 2 questions;

1) As you can see in the datasheet of the ne612, it suggests the use of an sfg455a3 filter. But these 3pin filters, usually made by MURATA, are expensive and hard to find. On the other hand, these 2pin filters are extremely cheap. Are they ok for the job?

2) In order to be able to tune in the whole AM spectrum (lets say 500-1500KHZ), the oscillator must have a minimum frequency of 50 KHZ to a maximum 1050 KHZ. The question is; can the ne612 oscillate at so low frequencies?

Thank you.

 

Hello,

This page has some ideas how to use the 2-pin resonators in filters:
http://www.agder.net/la8ak/12345/n19.htm
(you will find the drawings about halfway down the page).

Bertus

 

Thank you, but I really need some help understanding the ne612 datasheet. For example, it includes some oscillator circuits but it doesn't give you any formula for their frequency.

 

Hello,

In the example under mixing is used (oscillator frequency lower as the reciever frequency).
You van also use upper mixing (oscillator frequency higher as the reciever frequency).
In your case the reciever frequency is 500 to 1500 kHz.
The oscillator frequency will be from 955 kHz (500+455) to 1955 Khz (1500+455).

More info can be found in this PDF :
AM transmitters and receivers
From page 5 and on you will see the example.

Bertus

 

As I recall the 2 pin filters act like a series resonate circuit while the 3 pin act like a parallel resonate circuit and have a input and output impedance of near 3KΩ.
I think the SA612 series is pretty good using the small transistor IF transformers also.
I didn't notice your location, but I have an excess of my needs of ceramic 455KC filters. The ones I have are 6 element but they are 20KC wide at 6db down. If your are in the U.S. and are interested I could send you a copy of the data sheet.