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Tunable bandstop filter as reflective mixer?

Georgy.Moshkin

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Does it make any sense to use tunable bandstop filter as a mixer? For example, RFID antenna connected to 50 Ohm resistive termination with tunable bandstop in-between. By varying bandstop center frequency we can absorb or reflect interrogating signal. For example, bandstop using coupled microstrip ring resonator or similar structure, with a varactor to tune central frequency. My idea is that for some devices this configuration may provide larger impedance variation over smaller tuning voltage. As some energy is stored in this resonator, how it would affect mixing behavior?
 

BigBoss

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Mixing occurs in Nonlinear Behavior.Passive circuits are mostly Linear Devices and Mixing never occurs.
Some ferrite based passive circuits exhibit a bit nonlinearity but this is not enought to them as mixer.
 

Georgy.Moshkin

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Please check figures 4.1, 4.9
or
where structure of switchable load impedance modulated scatter is presented.

Can we replace SPDT switch with a bandstop filter?

p.s. I probably should call it modulator, although difference is still unclear to me:
"A mixer is a modulator optimized for frequency-translation."
 

mtwieg

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Sure that can be used as a mixer. In particular I'd refer to it as a parametric mixer or parametric amplifier, since you are are affecting the parameter of the circuit.
 

vfone

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I am a bit confused. So what exactly is the parameter that actually is affected in this circuit?
From what I know, an RF mixer is a frequency translation device, where an RF signal with a specific frequency is converted to an identical RF signal (hopefully) but having a different frequency.
 

mtwieg

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I am a bit confused. So what exactly is the parameter that actually is affected in this circuit?
The center frequency of the bandstop filter.
From what I know, an RF mixer is a frequency translation device, where an RF signal with a specific frequency is converted to an identical RF signal (hopefully) but having a different frequency.
Sure, and what the OP describes fits that definition. Consider if one port of the BPF is excited with a constant tone (analogous to a mixer's LO port), and its center frequency is controlled by a varactor (whose bias voltage is analogous to a mixers IF port). The signal applied to the IF port will modulate the phase/amplitude of the tone emerging at the other port of the BPF (analogous to a mixer's RF port).
 

vfone

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You mentioned that the center frequency of a Band Stop Filter is a parameter that will be changed.
But later you said that the central frequency of a Band Pass Filter (BPF) will be controlled by a varactor which in turn is controlled by a voltage similarly to a mixer IF port.
I think that in any RF mixer type, (by definition) the IF signal is a result of what happens at the RF and LO ports, and not the other way around.

Whatever filter type is about in this story (bandpass or bandstop) my question is: If this mixer will work as a frequency translation device, what are the signal levels that can handle, what is the dynamic range, noise figure, and ports isolation?
 

mtwieg

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You mentioned that the center frequency of a Band Stop Filter is a parameter that will be changed.
But later you said that the central frequency of a Band Pass Filter (BPF) will be controlled by a varactor which in turn is controlled by a voltage similarly to a mixer IF port.
Right, I mixed up BPF and BSF. Either can implement a parametric mixer.
I think that in any RF mixer type, (by definition) the IF signal is a result of what happens at the RF and LO ports, and not the other way around.
For passive mixers, the RF and IF ports usually can be outputs or inputs. Like in a normal double balanced diode ring mixer. The IF port is generally the one more suited for lower frequency operation.

Whatever filter type is about in this story (bandpass or bandstop) my question is: If this mixer will work as a frequency translation device, what are the signal levels that can handle, what is the dynamic range, noise figure, and ports isolation?
I could only speculate, since no specific implementation has been defined. I'm just saying that such a circuit does perform the function of a mixer. If you just want to do frequency translation in a signal chain, then there's probably no advantage to such an approach vs conventional mixers. I see parametric mixers/amplifiers used mostly in wireless sensors or optics.
 

vfone

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Yes, a conventional balanced diode mixer can swap the RF and IF ports. In fact, a diode DBM can swap (somehow) all the ports.
But your statement about this mixer: " ..its center frequency is controlled by a varactor (whose bias voltage is analogous to a mixers IF port).. ", makes me believe that this particular mixer can control through the IF port the mixer behavior.
What I do not understand (and here is not about swapping ports, which is just a lucky benefit of the diode DBM) is about that ONLY the IF port could change a parameter in the mixer, when this is not a valid statement in any other mixer type, where the IF port is only an output.
Anyway, I've seen many parametric mixers, but all of them use non-linear devices as diodes or transistors, or even non-linear capacitors.
I don't say this "filter type mixer" cannot work, but I was trying to understand it, and to find that was not invented just to appear in a triple E publication..
 

mtwieg

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What I do not understand (and here is not about swapping ports, which is just a lucky benefit of the diode DBM) is about that ONLY the IF port could change a parameter in the mixer, when this is not a valid statement in any other mixer type, where the IF port is only an output.
The IF port usually is the port that has the lower frequency signal. In an upconverting mixer, the IF is usually an input, and in a downconverting mixer it's usually an output.
 
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