how to impedance match a diode double balanced mixer?

[ddt694]

double balanced diode mixer

hi, all friends

I am confused!

A diode double balanced mixer (for example: mini-circuits or synergy microwave's double balanced mixers) has three ports: IF, RF and Lo.
Are all these ports's input/output impedance 50ohm or others?

In practical use, how the other circuits to do impedance match with the
mixer?

All double balanced mixer's datasheets have no the parameter of
input/output impedance? why? Are they 50ohm by default?

regards

 

[flatulent]

diode mixer

The LO drives the diodes into hard forward or reverse conditions. This connects the RF and IF ports nearly directly. The transformers on these ports can adjust the impedance levels.

The LO port has a nonlinear load on it (the diodes) and "impedance" in this case has no real meaning. The closest thing is that it will absorb approximately the same power as an xxx ohm resistive load when driven by the same amplifier source.

 

[DJ]

double balanced mixer

Practically LO port is less sebsitive to the Source match because any ripple of the LO power will not change to much the mixer performance.
Mixer impedance matching is needed as other devices. There is 50 ohm (or match) definition for the 3 ports and when one design a mixer he might look also at the 3 inputs matching.
Normaly when one design a mixer he gives up on the LO input VSWR in order to get beter performance of other parameters. The matching difinition is the same as any other device, ehen you inject a signal to any port you prefere no reflection. Any mismatch in the RF and/or IF ports degrades the conversion loss and NF of the mixer.
But not only the mixer. For upconverter the IF chain is loaded by the IF port of the mixer and the RF stage (Amplifiers or filter etc) is the RF port load.

D.J

 

[ddt694]

mixer impedance matching

Following is what i do, is it right?

RF ---> 50ohm amp ----> mixer ----> 50ohm amp ----> filter ---->IF
-------------------------------/|\
--------------------------------|
--------------------------------|
----------------------------50ohm amp
-------------------------------/|\
------------------------------- |
--------------------------------|
------------------------------- Lo

Is it necessary to use filter/diplexer in each port?

 

[toonafishy]

difination of double balanced mixer

To get the best performance, the IF port should see a very broadband match. The IF port wants to see 50 ohms even out of the band of interest. This is done with a diplexer. The RF port just needs a good match over the RF band, its not important out of band. The LO port is the least critical. You just have to have a good enough match to transfer the LO power into the mixer.

h**p://www.qrp.pops.net/diplexer.htm

RF -> 50ohm amp ->filter-> mixer -> diplexer ->50ohm amp -> filter ->IF
-------------------------------------/|\
--------------------------------------|
--------------------------------------|
--------------------------------50ohm amp
-------------------------------------/|\
--------------------------------------|
--------------------------------------|
-------------------------------------Lo

If you need image rejection, then you need to put a filter before the mixer that passes the band of interest but rejects the image band. If the LO power is not high enough, then you would need the amp between the LO and the mixer. If you need adjacent channel selectivity, then you need the filter after the mixer that passes the wanted channel but rejects the unwanted channel.

 

[ddt694]

mixer impedance mismatch

Hi,toonafishy

Thanks.

Is my method using amp to realize broadband impedance match right or not? (The amp can isolate the mixer from load change. )

 

[toonafishy]

diode as a mixer

You can do it that way. It might be easier to match the amp broadband than the mixer. This doesn't replace the need for the diplexer on the IF port.