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Design >> RF Design >> Two-Stage Mixing vs. Single-Stage Mixing [Shortwave Radio]
https://designers-guide.org/forum/YaBB.pl?num=1300100596

Message started by jchong89 on Mar 14^th, 2011, 4:03am

Title: Two-Stage Mixing vs. Single-Stage Mixing [Shortwave Radio]
Post by jchong89 on Mar 14^th, 2011, 4:03am

Hello, I am designing a shortwave AM radio and I am trying to receive a station at 9.74 MHz. I'm having trouble understanding the mixing stages. I'm also wondering about the advantages of using two stage mixing over single-stage mixing. I am trying to mix down to an intermediate frequency of 455 kHz.

So far, I understand that I want to attenuate image frequencies that show up after the mixing. So far, I have tried to use 4 MHz as a medium frequency; so I'll be mixing 9.74 MHz to 4 MHz, then in the 2nd stage, I want to mix from 4 MHz to 455 kHz.

What I want to know is why can't we just mix from 9.74 MHz to 455 kHz? What is the advantage of two-stage mixing from 9.74 MHz to 4 MHz to 455 kHz over the single-stage mixing from 9.74 MHz to 455 kHz?

Title: Re: Two-Stage Mixing vs. Single-Stage Mixing [Shortwave Radio]
Post by aaron_do on Mar 14^th, 2011, 7:49am

Hi,

its probably a matter of image rejection. Your band is from 9.4 MHz to 9.9 MHz right? That means to mix directly to 0.455 MHz, you could use a tone from 8.945-9.445 MHz. But doing this, your image frequencies would be between 8.49 and 8.99 MHz. So before filtering, you would need a filter which passes 9.4-9.9 MHz while rejecting 8.49-8.99 MHz (with some rejection, say 60 dB, but I don't really know what you need). Obviously the requirements on such a filter are quite stringent, so its easier to mix down to 4 MHz first. In that case the image frequencies are much further away and can be easily rejected.

cheers,
Aaron

Title: Re: Two-Stage Mixing vs. Single-Stage Mixing [Shortwave Radio]
Post by loose-electron on Mar 14^th, 2011, 1:52pm

Agree with Aaron -

You probably want to read up some on classic double conversion receivers - Might be stuff on the amateur radio web sites or the ARRL web sites.

Run some search engine look arounds on "double conversion receivers, IF frequencies" and similar.