mixer input impedance
LNA? Does the baseband amplifier give a large loading effect to LNA?
Please see the attachment, give me a hint abot this.
Ignoring imperfections in the mixer, ie ideal switches with zero switching time etc. Zin2 will be Zin amp as measured at the mixer input frequency.
Switching time should be very small compared with the input frequency if the mixer is any good so can be ignored for most practical applications. If the switch resistance is significant then it just adds to the input impedance.
To visualise what is happening assume you are at the source looking into the mixer. If it is ideal then you would not be able to tell which amplifier you are connected to.
This highlights the need to have a mixer correctly terminated at all frequencies of interest and even those that may not be at first glance .
Peter
Very Interesting,
This Passive Mixer has a nice impedance Translation Property. I just give you the direction. If you have a baseband load of Zin(jw) and your passive mixer has a switching frequency of flo, then the input impedance of passive mixer is ( to the first order) Zin(j(w-wlo))+Zin(j*(w+wlo)). Amazing!
A good passive mixer (for instance, a balanced mixer) is well matched at its RF port over a specified bandwidth. It may be difficult to achieve the same degree of matching at its LO port.
With a nominal LO pump power, the RF port is matched, so the mixer behaves as a good load to the LNA connected to it.
For instance, Spacek Labs. Inc. makes a series of waveguide mixers with a good match (RL>10 dB)over waveguide nominal bandwidth. Also available are ultra-wideband balanced mixers using WRD-180 RF input waveguide; those operate over 18 to 40 GHz.