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Re: Receiver basics question



Tony,
>If one was to build a receiver strictly for detecting weak signals, and
>adjacent signal interference was nonexistent,  that is, under ideal
>conditions would there be any reason to use double or quad conversion
>designs?  Wouldn't a single conversion receiver offer the least amount of
>loss and a lower noise floor?
>...
>1:  The double conversion rig seemd to have a slight edge in sensitivity and
>a slightly lower noise floor.
>       This seemed to hold true for both double conversion rigs.
>
>2:  The quad conversion rig seemd to have more noise and hiss at the noise
>floor which I'm assuming is due
>       to the additional stages that make up the quadruple conversion design.


Gain and sensitivity is easy to attain through a 10 dollar low noise 
transistor.  What the quad conversion rigs do is add dynamic range to the 
receiver.

Intermodulation products and fundamental overload can be a problem on the 
dual conversion rig, but the extra stages in the quad conversion rig will 
add to the dynamic range.

The extra noise is indeed circuit noise from the addition of the extra 
stages, where every component adds to the total noise figure.  One good 
thing is that the noise is generally constant and an adaptive DSP unit will 
remove most if not all of the thermal noise.

Most dual conversion rigs use 455k and 10.8M as IFs, and therefore suffer 
from birdies and mixing products; on HF from strong MW frequencies mixing 
with the local oscillators, and on VHF and UHF from paging and land mobile 
systems.

I think you knew the answers already, but were looking for confirmation.

Anecdotally, I had an FT990, which I traded with my 726 for an FT847.  The 
990 is a quad conversion rig, and had the best receive section of any rig 
I've ever owned or listened with.  It blows the socks off the 847, except 
the 847 is probably marginally more sensitive.

When I first got the 847, I wondered why Yaesu opted to put the DSP in the 
audio section, instead of the IF.  After playing with the rig for a year 
and a half or so, it's easy to say. With all the birdies the dual 
conversion allows, there is no way you can keep ALL of them out of the ham 
bands, so put in a DSP notch which will remove the heterodyne.  Instant 
cure to a messy problem.

Early on you heard people complain about the birdies on the 847, well, they 
are still there, the tech just tweaked the LOs to put the birdies where 
they don't bother the op!

Good food for thought Tony!

73, Mike




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