Does a dual conversion RX produce dual images?
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Posted by billcalley on July 1, 2007, 4:15 am
Please log in for more thread options Does a dual conversion receiver produce dual image frequencies? In other words, after the 1st IF image is removed by the frontend BPF filter, won't the second conversion create a second image? If so, is it ever an issue in receiver design? Must it, too, be filtered out just before the second mixer stage, or can it be ignored? Thanks! -Bill | |||||||||||||
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Posted by colin on July 1, 2007, 5:38 am
Please log in for more thread options any mixing produces high and low images, The problem of the first stage is that the unwanted image can be the result of an adjacent chanel the oposite side of the LO frequency to the wanted signal. this unwwanted image is ofc on top of the wanted image and so must be removed by rejection before the mixer. it isnt usualy a problem at all to do this in the 2nd stage, as there are no adjacent channels as such after the IF filter. If your first IF frequency is much higher than the input frequency range it becomes much easier to reject the unwanted image, as it is further away from the unwanted image and may not need a tuneable BPF on the input. this is probaly why 2 stage is done on low freq rf upto 100mhz. however im not a specialist in rf design. Colin =^.^= | |||||||||||||
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Posted by billcalley on July 1, 2007, 11:28 am
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>
> > > Does a dual conversion receiver produce dual image frequencies? In
> > other words, after the 1st IF image is removed by the frontend BPF > > filter, won't the second conversion create a second image? If so, is > > it ever an issue in receiver design? Must it, too, be filtered out > > just before the second mixer stage, or can it be ignored? >
> > Thanks!
>
> > -Bill
>
> any mixing produces high and low images, > The problem of the first stage is that the unwanted image can be the result > of an adjacent chanel the oposite side of the LO frequency to the wanted > signal. this unwwanted image is ofc on top of the wanted image and so must > be removed by rejection before the mixer. > > it isnt usualy a problem at all to do this in the 2nd stage, > as there are no adjacent channels as such after the IF filter. > > If your first IF frequency is much higher than the input frequency range it > becomes much easier to reject the unwanted image, as it is further away from > the unwanted image and may not need a tuneable BPF on the input. > this is probaly why 2 stage is done on low freq rf upto 100mhz. > > however im not a specialist in rf design. > > Colin =^.^= Thanks Colin. I wonder, too, whether the image noise would be a problem at the 2nd IF, as it is at the 1st IF. I'll have to see if I can find any info on that somewhere! Best Regards, -Bill | |||||||||||||
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Posted by Don Bowey on July 1, 2007, 11:59 am
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On 7/1/07 8:28 AM, in article 1183303726.664632.172060@i38g2000prf.googlegroups.com, "billcalley" >>
>> >>> Does a dual conversion receiver produce dual image frequencies? In
>>> other words, after the 1st IF image is removed by the frontend BPF >>> filter, won't the second conversion create a second image? If so, is >>> it ever an issue in receiver design? Must it, too, be filtered out >>> just before the second mixer stage, or can it be ignored? >>
>>> Thanks!
>>
>>> -Bill
>>
>> any mixing produces high and low images, >> The problem of the first stage is that the unwanted image can be the result >> of an adjacent chanel the oposite side of the LO frequency to the wanted >> signal. this unwwanted image is ofc on top of the wanted image and so must >> be removed by rejection before the mixer. >> >> it isnt usualy a problem at all to do this in the 2nd stage, >> as there are no adjacent channels as such after the IF filter. >> >> If your first IF frequency is much higher than the input frequency range it >> becomes much easier to reject the unwanted image, as it is further away from >> the unwanted image and may not need a tuneable BPF on the input. >> this is probaly why 2 stage is done on low freq rf upto 100mhz. >> >> however im not a specialist in rf design. >> >> Colin =^.^= >
No, it isn't a problem if the IF frequency is carefully selected, because
> Thanks Colin. I wonder, too, whether the image noise would be a > problem at the 2nd IF, as it is at the 1st IF. I'll have to see if I > can find any info on that somewhere! > > Best Regards, > > -Bill > the 1st IF filters present a narrowed band of frequencies the the 2nd mixer that is fairly clean. And speaking of "carefully selected," but not images, I once had a mobile receiver (PMR 6 if I recall right) that had a 2nd IF of 1600 kHz. We had a local broadcast station at 1600 kHz that wiped me out around town. Putting a BCB filter at the receiver input cleaned it up as long as I was a half-mile or so from the station. | |||||||||||||
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Posted by colin on July 1, 2007, 1:22 pm
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>>
>> >> > Does a dual conversion receiver produce dual image frequencies? In
>> > other words, after the 1st IF image is removed by the frontend BPF >> > filter, won't the second conversion create a second image? If so, is >> > it ever an issue in receiver design? Must it, too, be filtered out >> > just before the second mixer stage, or can it be ignored? >>
>> > Thanks!
>>
>> > -Bill
>>
>> any mixing produces high and low images, >> The problem of the first stage is that the unwanted image can be the >> result >> of an adjacent chanel the oposite side of the LO frequency to the wanted >> signal. this unwwanted image is ofc on top of the wanted image and so >> must >> be removed by rejection before the mixer. >> >> it isnt usualy a problem at all to do this in the 2nd stage, >> as there are no adjacent channels as such after the IF filter. >> >> If your first IF frequency is much higher than the input frequency range >> it >> becomes much easier to reject the unwanted image, as it is further away >> from >> the unwanted image and may not need a tuneable BPF on the input. >> this is probaly why 2 stage is done on low freq rf upto 100mhz. >> >> however im not a specialist in rf design. >> >> Colin =^.^= >
> Thanks Colin. I wonder, too, whether the image noise would be a > problem at the 2nd IF, as it is at the 1st IF. I'll have to see if I > can find any info on that somewhere! the thing is the 1st IF filter takes care of this for you. so you have a narowband signal to the 2nd mixer, unlike the first stage where you need to add a tuneable filter to track the input signal. Colin =^.^= | |||||||||||||
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> other words, after the 1st IF image is removed by the frontend BPF
> filter, won't the second conversion create a second image? If so, is
> it ever an issue in receiver design? Must it, too, be filtered out
> just before the second mixer stage, or can it be ignored?
>
> Thanks!
>
> -Bill
>