Ic-7800 2nd mixer design question
 

I have a question about the design of 2nd mixer in the IC-7800.
From : http://www.icom.co.jp/world/products...7800/index.htm
man could see that Icom use an oscillator with outputs in quadrature and
then sum the mixer outputs after a 90° phase shifter.

What I don't understand , is why do they do the 90° phase shift in the
analog domain ?
Using 2 mixers with a quadrature oscillator is very common in the DSP
world (IQ demodulation). But in general , it's followed by 2 A/D
converters and if phase shift is needed to reject a side band, it's done
digitaly...
I don't think that Icom choose this design just to save an A/D in the
IC-7800 ....

any clues ?

Thierry
F4DWV

Hmmm. I haven't studied 'em, but is this an image canceling mixer? I
recently ran across an article about them (don't even remember where), but
didn't look closely enough to understand it. Probably the same as the
phasing method of SSB generation. Now, where the heck was that???...

--
Steve N, K,9;d, c. i My email has no u's.


"hello" wrote in message
...

I have a question about the design of 2nd mixer in the IC-7800.
From : http://www.icom.co.jp/world/products...7800/index.htm
man could see that Icom use an oscillator with outputs in quadrature and
then sum the mixer outputs after a 90° phase shifter.

What I don't understand , is why do they do the 90° phase shift in the
analog domain ?
Using 2 mixers with a quadrature oscillator is very common in the DSP
world (IQ demodulation). But in general , it's followed by 2 A/D
converters and if phase shift is needed to reject a side band, it's done
digitaly...
I don't think that Icom choose this design just to save an A/D in the
IC-7800 ....

any clues ?

Thierry
F4DWV

hello ) writes:
I have a question about the design of 2nd mixer in the IC-7800.
From : http://www.icom.co.jp/world/products...7800/index.htm
man could see that Icom use an oscillator with outputs in quadrature and
then sum the mixer outputs after a 90° phase shifter.

What I don't understand , is why do they do the 90° phase shift in the
analog domain ?
Using 2 mixers with a quadrature oscillator is very common in the DSP
world (IQ demodulation). But in general , it's followed by 2 A/D
converters and if phase shift is needed to reject a side band, it's done
digitaly...
I don't think that Icom choose this design just to save an A/D in the
IC-7800 ....

any clues ?

Thierry
F4DWV

This may be an issue of design philosophy.

That stage is doing a conversion down to a very low IF (it's not clear
what frequency from the block diagram), and is presumably low enough that
the image is within the roofing filter passband. Narrowing the roofing
filter might be problematic (because it would limit the available bandwidth,
or because it's too costly to have a narrower roofing filter), but they need
to knock out the image caused by that second mixer. By using phasing,
it attenuates the unwanted image, in the same way that phasing techniques
can knock out the unwanted sideband when generating SSB.

Perhaps they want a linear signal, notice there is an IF stage
after the summing from that second conversion stage, and by doing it
digitally they'd have a digital signal, or be forced
to convert back to analog.

Maybe they didn't want to A/D stages?

Michael VE2BVW

Steve Nosko wrote:
Hmmm. I haven't studied 'em, but is this an image canceling mixer? I
recently ran across an article about them (don't even remember where), but
didn't look closely enough to understand it. Probably the same as the
phasing method of SSB generation. Now, where the heck was that???...


Yes that's it, often use in direct conversion receiver or digital radio
front end.


Michael Black wrote:

That stage is doing a conversion down to a very low IF (it's not clear
what frequency from the block diagram), and is presumably low enough that
the image is within the roofing filter passband. Narrowing the roofing
filter might be problematic (because it would limit the available bandwidth,
or because it's too costly to have a narrower roofing filter), but they need
to knock out the image caused by that second mixer. By using phasing,
it attenuates the unwanted image, in the same way that phasing techniques
can knock out the unwanted sideband when generating SSB.

Yes, it's why man use this sort of design.
But yet again, why not doing the phasing in the digital domain with all
the DSP power behind , it must not be a problem

Maybe they didn't want to A/D stages?

Considering the price of the rig , it seems strange.
Perhap's they need vere very very good a/d converter and so 2 will be
too expensive.
Do someone know what do they use as A/D ?
(by the way, it's amusing to see how the marketing speak a lot about the
dsp specs and not the quality of the A/D which is more important and
problematic ...)


Thierry
F4DWV