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Two-Stage Mixing vs. Single-Stage Mixing [Shortwave Radio] (Read 2126 times)

jchong89

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Two-Stage Mixing vs. Single-Stage Mixing [Shortwave Radio]
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?

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aaron_do

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Re: Two-Stage Mixing vs. Single-Stage Mixing [Shortwave Radio]
Reply #1 - 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

there is no energy in matter other than that received from the environment - Nikola Tesla

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Two-Stage Mixing vs. Single-Stage Mixing [Shortwave Radio] Reply #2 - 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.
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