Could some electronics guru here please clarify the following How to separate the IF frequency components at the output of, e.g., a double balanced mixer? The following is one possible scheme. Feed the IF output into a 50:50 splitte, ideally not resistive. Apply one splitter output to a narrow band bandpass filter with a band center frequency? LO - RF and the other splitter output is fed into another band pass filter with band center frequency LO + RF.
Are there any other more elegant ways ?
I think you are discussing a transmitter, in which case I think you want to look up the "phasing method" of generating SSB.
If it is for a receiver then you should look up "image reject mixers".
In either case, when this is implemented in analogue circuitry, component matching will limit the amount of suppression that you can get. For some situations, either a lot of trimming is needed (sometimes this is even adaptive, during operation), or you might need to also use some filtering at the same time, but with more relaxed filter specs than you would need with a simpler architecture.
Often people do this stuff in DSP, e.g. in the analogue part, split the rf signal and feed it to two mixers with quadrature LO, digitise both the IF signals, and then do the image suppression in DSP.
The 'phasing method' of SSB generation is the same thing as an image rejection mixer (which is a more recent term).
An analog mixer is not an ideal multiplier resulting in just two output frequency components. The mixers (balanced or not) switch the signal with the injection oscillator frequency, resulting in a multiplication with the sign of the injection. This creates plenty of other undesired mixing products in addition to the opposite sideband (image) signal.
An image rejection mixer needs two copies of the input signal at 90 degree phase difference, and two copies of the injection signal also at 90 degree phase difference. Mixing the signal pairs in two mixers and combining the results will suppress the undesired image response, provided there is good amplitude and phase balance between the channels. The rejection does not usually apply to the other undesired mixing products.
-- -TV
Another approach is to mix up to some high IF where the filtering is easier on account of the sidebands being further apart. (For a SSB transmitter, you want to use a low IF for the same reason. Filters get easier to build as their fractional bandwidth gets up near 1/4 or thereabouts. Very wide ones are usually best made using an LP and HP in cascade.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
There are some seriously good analog multipliers around.
The "phasing method" is equivalent to vector rotation frequency shifting. Imagine a little girl standing in the center of a carousel that's spinning at N rpm. If she spins herself at M rpm relative to the carousel, she's twirling at N+M relative to the kids sitting in the dirt. Or N-M if she spins the other way.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Maybe the best analog multipliers used as mixers are the Gilbert cells, but even they are usually well overdriven from the injection side to maximize the signal port linearity.
-- -TV
You can do that. I'm not sure why you need to pick off both LO+RF and LO-RF . That is unusual. Usually one is dumped. An output scheme (on the IF port) that produces a broadband termination to the mixer is often desired.
??? RF??X??LO ???
????????? ???? ?Diplexer ? ????????? ???? IF_LO IF_HI
You haven't specified separation, or much of anything, so it is hard to say much.
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