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a mixer made by lange coupler

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buyan

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Hi all,

I have simulated a single balanced mixer, which makes use of a lange coupler and two diodes. One port of the lange coupler is RF input port and another port is LO input port. The rest two ports of the Lange coupler are connected to two diodes in series.

Now I would like to simulate the RF/LO and LO/RF isolation in ADS, which is supposed to be quite high. However, the simulation result showed very small.

The simulation setup of LO/RF isolation is:
RF and IF ports are both connected to 50 ohm.
LO is connected to 50 ohm source taken from Source_Frequency.

May I know whether there is anything wrong with the simulation setup?

Thanks so much. ^___^
 

The diodes connect one to each of the 2 ports, between the port and ground. Not in series.

You need to model an IF connection, especially the DC return to ground.

You need enought LO power to turn on the diodes.
 

Hi biff44, the collection is as followed.

RF port 1-------------------2
1 1
1 1
1 1
1 1
LO port 3-------------------4

The two diodes are in series and the same orientation, just like

port 2 -----!>-----!>---------port 4
diode 1 diode 2

The IF is extracted out from the center of the two diodes and is followed by a low pass filter.

for LO/RF isolation simulation, I connect port 1(RF port) and IF to 50 ohm load. The LO is connected to a source selected from Source_Freq.

How can I provide a source to turn on the diode in a frequency range, instead of a single frequency?

Sorry, I cannot get your point about the ground of diodes.

Thanks for your help.
 

Well, the LO energy has to switch the diodes on and off. It has to switch both diodes equally on or off to act in a balanced way. If you have the two diodes in series between ports 2 and 4, it is not clear to me how the LO energy can turn the diodes on or off. Current has to flow thru both diodes to turn on, but the voltages at port 2 and port 4 are only 90 degrees out of phase with each other--not an efficient way to apply LO energy to the diodes.

A better way would be to place an RF shunt capacitor in between the two series diodes. One lead of the capacitor is connected to the series connection of the diodes, and the other lead of the capacitor is grounded. That way, LO energy appearing at port 2 causes current to flow in the diode connected to port 2, independent of the other diode.

Similarly, LO energy appearing at port 4 causes current to flow in the diode connected to port 4, independent of the other diode. In this case, the capacitor has to be large enough to be an RF and LO AC ground, but be high impedance at the IF frequency. You can take the IF signal off of the capacitor.

A different circuit would actually connect each of the diodes independently to ground, breaking the "series connection". The IF is taken off of either the RF or LO port thru a lowpass filter. The LO and RF port use a series capacitor that is large in impedance so that you do not lose IF signal out of either of those ports.

I am not a mixer expert, but what I am trying to say is that unless you get enough LO energy into each diode to switch on and off, neither circuit will work. You non-linear simulation program is smart enough to know that, and will give you poor results.
 

Hi,

Link for an IEEE article, about designing SSB mixers and Lange couplers:

hxxp://www.eecs.umich.edu/rebeiz/Current%20Res.%20Papers/Microm_Lange_Coup_SSB_Mix.pdf


rgds,

Al
 

Thanks a lot for both of you.
 

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