single antenna doppler iq sensor without circulator

Hello all! Modern doppler sensors usually use separate antennas for tx and rx signal, so quadrature receiver is easy to implement. Older one use single horn antenna with circulator. Is there a way to make single antenna sensor with quadrature output, all in microstrip technology, without circulator?

Some designs use a branchline coupler, with the receiver at the isolated port. Not as efficient as a circulator (you loose 3dB in transmit and another 3dB in receive path), but simple and low cost.

Terminator3 said:

Hello all! Modern doppler sensors usually use separate antennas for tx and rx signal, so quadrature receiver is easy to implement. Older one use single horn antenna with circulator. Is there a way to make single antenna sensor with quadrature output, all in microstrip technology, without circulator?

Click to expand...

Circulators are used to decrease the DC component from the mixer. For a short-range Doppler radar only one antenna can be used, with a directional coupler to inject the TX power into the antenna, with the mixer RF port directly connected to the antenna.
Most classical "door openers" used this concept without problems. Airplane height meters used two antennas only to keep accuracy at close to ground operation.
The only result of "leakage" of TX signal into RX input only causes the DC output which is mostly unused.

Thank for the replies, both ideas are very interesting, and with your help i finally found what i am interested in in google.

(you loose 3dB in transmit and another 3dB in receive path)

Click to expand...

volker@muehlhaus, using 4-port branchline coupler picture from here
**broken link removed**
If i understand right, Port 1 must be connected to LO, port 2 connected to antenna, isolated port 4 connected to mixer. Can i use port 3 output as LO for mixer?

Circulators are used to decrease the DC component from the mixer. For a short-range Doppler radar only one antenna can be used, with a directional coupler to inject the TX power into the antenna, with the mixer RF port directly connected to the antenna.

Click to expand...

jiripolivka, unfortunately i did not found such sensor (with directional coupler). One year ago i bought and disassembled three different door openers, two 10GHz and one 24GHz, all use similar topology with Wilkinson power divider and two antennas for TX and RX. The only difference was that 10GHz ones used DRO with ceramic resonator, and 24GHz used simple microstrip line resonator.

Terminator3 said:

volker@muehlhaus, using 4-port branchline coupler picture from here
**broken link removed**
If i understand right, Port 1 must be connected to LO, port 2 connected to antenna, isolated port 4 connected to mixer.

Click to expand...

Correct. Port 1 is transmitted signal, port 2 is antenna, port 3 is terminated into 50 ohm and port 4 is the receiver.

Terminator3 said:

Can i use port 3 output as LO for mixer?

Click to expand...

Not sure about the circuit that you have in mind. In any case, for good isolation between ports 1 and 4 you need very good matching at port 3.

Terminator3 said:

Thank for the replies, both ideas are very interesting, and with your help i finally found what i am interested in in google.

volker@muehlhaus, using 4-port branchline coupler picture from here
**broken link removed**
If i understand right, Port 1 must be connected to LO, port 2 connected to antenna, isolated port 4 connected to mixer. Can i use port 3 output as LO for mixer?


jiripolivka, unfortunately i did not found such sensor (with directional coupler). One year ago i bought and disassembled three different door openers, two 10GHz and one 24GHz, all use similar topology with Wilkinson power divider and two antennas for TX and RX. The only difference was that 10GHz ones used DRO with ceramic resonator, and 24GHz used simple microstrip line resonator.

Click to expand...

Many years ago, MITEQ and Plessey made simple good door openers and Doppler sensors with a waveguide horn in which a Gunn oscillator was built next to a mixer diode. There were no explicit couplers nor a circulator, and the device worked well. Once I connected a larger hon to it and observed persons over 100 meters, cars even farther.

A Wilkinson coupler can be used in microstrip, to connect TX and mixer to one antenna. If the antenna is well matched, the TX-RX isolation can be some 20 dB which is just fine for a Doppler sensor. I repeat that the leakage power only results in the DC component at mixer output which is not used.

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For microstrip one can use a rat-race coupler which can offer a better isolation. There are many versions of Doppler sensors, all work well without expensive circulators.

Terminator3 said:

all in microstrip technology, without circulator?

Click to expand...

Well, well, well. I`m already made that. Isolation more than -60dB , one antenna, one polarization and two ports.There are also some another tricks. But it is commerce sensitive. Cheap devise, works well for a long distance ( 100 m ? ). Circulators , typically gives -20dB isolation, if all ports has same matching.
Performance blue S12 isolation, and gain 16dBi each section :razz: very nice

Bob60, so your design isolation is better than circulator? nice...

Much better but with some limitations of course. That staff is in narrow band. Circulators has limitations as well, I have a lot experiences, but they are more wide with not so big rejection with depends from matched
ports. No matching no isolation . 8-O No pain, no gain.

jiripolivka, when u said about injecting into tx, did you mean U-shaped microstrip line section with one port terminated to 50Ohm, or branchline coupler with 4 ports?

When using 4-port branchline coupler it is unclear what we use as LO for mixer (if port 3 is terminated into 50 ohm).
For example
port1 - connected to LO
port2 - antenna
port3 - terminated to 50Ohm
port4 - connected RX mixer
as port4 is isolated from port1, where whould we get LO for mixer? Or it is assumed that there is some LO reflected from antenna and goes from port 2 to port 4 with RX?
Also it is unclear, should we terminate port 3 to 50 Ohm, or use it's output as LO for mixer, so port3 and port 4 connected to mixer.

With directional coupler (lambda/4 + 50Ohm termination at proper side)it is also a little unclear, where we get LO from. Again, reflected from antenna? Or should we use two directional couplers with different directions - one for LO and one for RX.

In both cases using LO that is not reflected from antenna leads to leakage problem. So it seems that we must use LO reflected from antenna, or it will become leaked signal if we use some other LO source (for example from Wilkinson power divider before coupler).

But the problem is, if we using reflected LO, it is impossible to make quadrature mixer. Because LO and received RF will go through one microstrip, so it is impossible to make 90deg shift for IQ mixer.

So i conclude, that good isolation can be achieved only if we use simple mixer (no quadrature signal).
And if we need quadrature signal, we must provide separate ways for LO and RF, and doing so gives some problems about leaked LO cancellation...

I'm not sure about your circuit and why you want to connect LO to the antenna coupler.

Let's take this circuit diagram:
https://en.wikipedia.org/wiki/File:Bsp2_CW-Radar.EN.png

Now the coupler would be connected where this circuit diagram has the TX and RX antennas.

volker@muehlhaus, i see your point.