EMRFD Message Archive 15193
Message Date From Subject 15193 2018-12-03 10:00:56 Mike Hadley H mode mixer
Is there any advantage in using a diplexer between my H mode mixer and crystal filter. This is a 9.0 MHz IF system.
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15194 2018-12-03 16:15:22 Mike Nothdurft Re: H mode mixer
I have the same question for the Tayloe mixer.
15195 2018-12-03 18:23:48 Tayloe, Dan (Noki... Re: H mode mixer
For a tayloe mixer, the mixer looks like on open circuit on frequency, and a very low impedance far off frequency (ideally a short). The purpose of a diplexer is to terminate unwanted frequencies in a normal, non-frequency discriminating mixer. However, in this case the detector output looks like a short at undesired frequencies (except for odd harmonics of the fundamental), so it seems like a diplexer is not as useful.
The unwanted frequencies still exist inside the tayloe mixer itself across the internal switch impedance (5 ohms for a ‘3253?) even when the output is a short caused by a high frequency AC signal across a detector capacitor. That is ideally (i.e., at most) a 11:1 voltage division (switch 5 ohms/(system 50 ohms + switch 5 ohms)) or a ~21 dB reduction for unwanted wide band signals inside the mixer. If you terminated the output with a diplexer so that 50 ohms was seen at all frequencies, then the detected voltage would drop 6 dB (sub optimal) and the multiplexer chip portion of the mixer would see the desired and undesired frequencies equally well, which seems suboptimal as well for spurious mixing products.
It would seems offhand like terminating a tayloe mixer with a matched impedance would lower the performance.
I think the tayloe detector would ideally be followed by a high impedance detector and the noise voltage would be set by the system impedance and the noise voltage for the first amplifier stage. Depending on how the quadrature detector was constructed (two outputs (0, 90 degrees) or four (0, 90, 180, 270) ) a 50 ohm input impedance might look like 200 ohms on each of 4 outputs or 100 ohms on each of two outputs at the “open circuit” detection frequency.