Does a dual conversion RX produce dual images?

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

Reply to
billcalley
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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 =^.^=

Reply to
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

Reply to
billcalley

No, it isn't a problem if the IF frequency is carefully selected, because 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.

Reply to
Don Bowey

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 =^.^=

Reply to
colin

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The image frequency is only a problem in the first mixer since it
translates to the first IF, being the carrier frequency plus twice
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Reply to
John Fields

For a guy who's been away from radio for awhile, John, you produced a nice clear explanation. Having been bitten by double-conversion spurs, I'd just want to add that double conversion is usually the wrong answer nowadays...high frequency gain is pretty cheap, so a single highish IF is usually best. I've seen systems where the IF is much higher in frequency than the RF, to avoid image problems.

Spurs due to Nth mixer LO harmonics die off much more slowly with N than RF harmonics do, so it's easily possible for the seventh harmonic of the second LO to mix with the fifth harmonic (say) of the first LO and produce a spur that sweeps across the IF passband as you tune.

The net is that unless you have a really careful frequency plan, double conversion produces a forest of spurs that are really hard to get rid of.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Thanks John -- great stuff indeed. I appreciate the clear explanation!

Best Regards,

-Bill

Reply to
billcalley

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Thanks! :)
Reply to
John Fields

from

Somewhere at one of my clients, Garmin I think, an engineer had some kind of slide-rule thingy that could determine spur locations.

A good start is to use an image-reject LO/mixer combination.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

yes !each conversion stage has an image frequency. The dual IF les you pick where there are and produce a more selctive receive thasnas ingle conversion receiver.

Marc

Reply to
LVMarc

On Jul 1, 2:48 pm, Jim Thompson wrote: ...

There's a spur search tool that's distributed as part of the free-to- use RFSim99.

Of course, an image-reject LO/mixer combination adds quite a bit of complication, especially if you aren't operating in a frequency range where the quadrature stuff is done for you in an IC (with TOI you can live with...)

Another way to avoid the harmonic spur problem is to use multiplier- type mixers instead of the vastly more usual DBM that's driven hard by the LO. Using a multiplier as a mixer is done only rarely, though.

Cheers, Tom (who works mainly with direct digitization and calls images "aliasing," and doesn't have much in the way of mixer spur problems...)

Reply to
Tom Bruhns

I never have that problem, since I'm always designing a CHIP ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Tom Bruhns wrote: ...

Caveat: actually, using multipliers as mixers and doing quadrature mixing is done very commonly -- in the digital domain. I should have qualified the earlier posting with "analog". In the analog domain, it's tough to maintain really good image rejection, tough to keep harmonics in multipliers to a really low level -- in the digital domain, it takes some careful design and enough bits, but once you have the design down, it doesn't drift and yields repeatable performance.

Cheers, Tom

(Yeah, Jim, when you're integrating things onto a chip, that helps...though I wonder if you can give me a mixer that yields +55dBm IIP3 and better than 8dB noise figure...)

Reply to
Tom Bruhns

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