transceiver noise floor

[marco110]

does anyone has any recommendation on reducing the noise floor in the receiving path of an transceiver, which uses the same antenna for tx and rx, having a high power transmit path. the signal direction / isolation for the front end is done using a circulator.

 

[VSWR]

Reducing the transmitter phase noise will lower the receiver noise floor.

 

[lguancho]

This is a typical radar system design consideration. Consider a high power traveling wave tube amplifier connected to the a circulator. The receiver is also connected to the antenna through the circulator.

During the receiving window, the TWTA supposingly should be shut down, cutting off any high power transmission. However, due to finite beam off isolation, some transmitter noise will be coupled to the receiver input (via circulator finite isolation or get reflected from antenna due VSWR mismatch, and then leak to the RX input). Detailed system study should consider this fact when specifying the TWTA specification.

During receive operation, the isolation between RX and TX path could be enhanced by shutting of the local oscillators in the TX path, using high switching speed RF switches.

In the front-end design, care should be taken to minimise RF losses in the path connecting the circulator to the RX.

For the local oscillators used in the receiver, low phase noise is essential also to reduce the receiver input noise.

 

[marco110]

thanks lguancho for the advice.
Does using a circulator with better isolation help?
I've a pretty high path loss from the circulator to the Rx. Does adding a LNA help? I'm afraid it will also amplify the coupled or reflected Tx power.

 

[lguancho]

Of course, circulator with higher isolation will help to improve the TX noise leakage. You must also ensure reasonable antenna VSWR to minimise reflected TX power which will leak directly to the RX through the circulator.

A frontend LNA is almost necessary in every radar receiver. Usually, you need some limiter-attenuator module before the LNA to clamp down the TX leakage power to a acceptable power level that the LNA could handle. Look at Eyal Microwave website for some limiter information.

A low noise LNA at microwave frequency range could be very expensive. I have used LNA of 1.5dB NF at Ku-band from Miteq.

 

[lijulia]

how to converter transmitter phase noise to the receiver noise floor, is it important if it's TDD mode? Thanks!

 

[lguancho]

Hi, all. Most radar systems are TDD (pulsed) in nature. There is a small TX window, followed by a RX window.

For radar TWTA, there is a "Pulse off Noise Power Density" specification. I include a Pulse TWTA application note from Amplifier Research. Inside, it quotes a pulse off noise power spectral density of -140dBm/Hz (Note for phase noise is usually in dBc/Hz). It is this pulse off noise that will leak to the RX input, via the circulator.

Assuming a circulator with 20 dB isolation, the TX noise (TX off state) leaking to the RX will be -140dBm -20dB, i.e. -160dBm in 1 Hz bandwidth.

This may seem trivial, but with modern radar signal processing bandwidth down to 1 Hz, the amount of RX input noise floor tolerable may be in similar range. So care must be exercised to ensure the pulse -off noise spectral density ( other important TWTA spec includes NF, pulse off isolation etc) to ensure the TX leakage signal to RX is acceptable.

Hope this will clarify the air.

 

[lguancho]

Sorry, pals. Here is the app note I am talking about. 8)

 

[marco110]

sorry i don't quite get it.
if there's a limiter-attenuator module at the front end before the LNA, will it also reduce my received signal?

 

[lguancho]

Well, think it in this way. Which is more critical: protect your receiver from burn-out by high power transmission leakage during the TX window or to achieve better overall system noise figure:

Consider this situation:

You have a 2KW transmitter connected to an antenna with input VSWR of say 1.5:1 What is the amount of TX power that will get reflected from the antenna? Probably greater than 100W. Even though the TX window is in the range of 10- 40usec, do you think your LNA can survive this leakage power without any protection?

This is the purpose of the Limiter attenuator. It has two functions: [1] to protect the LNA during TX window [2] improve dynamic range at near distance when radar input is strong

OK? All these could be found in Radar Handbook I uploaded just 1 week ago.

Also, there are many design of limiter-attenuator. Waveguide type of implementation will have very good insertion loss and power handling

 

[marco110]

i'll look into the radar handbook.
the design from Eyal seems to use PIN diode for the limiter design. is there other means to implement it for board level?