Waveguide usage in Radar Receiver Front End

[kommanche]

Hi Friends,

I want to ask an issue that I am curious about. I understand the need of waveguides in a CW Radar transmitter,for example in CWAR high power staff... But I do not understand the use of waveguides, big klystron oscillators, big switches at the receive part. I think that microstrip structures, simple PLLs, microstrip filters will do the same work. Do you have any ideas about that ? Are extremely stable and extremely low noise oscillators are needed in a CW Radar system?

Thanks,

 

[biff44]

Parts of the front end that share both high TX power and low Receive power need to be in waveguide. These would potentially include a high power circulator, rotary joint, Limiter/active switch, bandpass filter, etc. After the high power limiter, the rest of the components can be in microstrip/stripline, etc. IF the receiver electronics were a far distance from the antenna....you might keep stuff in waveguide for lower transmission line loss.

 

[kommanche]

And do you know any advantage of klystron oscillators over TCXO referenced PLL which uses standard VCO in terms of stability and phase noise?

 

[davenn]

And do you know any advantage of klystron oscillators over TCXO referenced PLL which uses standard VCO in terms of stability and phase noise?

Klystrons are likely to be less stable than a TXCO ref'ed PLL. But you have to remember that the TCXO's and PLL's we see these days are a relatively new invention. Klystrons and magnetrons have been around for the best part of 100 years. They are the best way of producing high power directly at the microwave freq of interest without the need for long and more expensive multiplier/amplifier chains.
And THAT is their major advantage

Dave

 

[jiripolivka]

Just try to compare the insertion loss of a 0.085" coaxial line against the same length of a waveguide: at Ka-band (3 GHz), the loss ratio is 3...6 dB or more. If you generate a costly power to be transmitted fro a TX to the antenna, it makes not much sense to utilize only one quarter of it.
Similarly, in radar systems one needs a low line loss from antenna to the receiver, to lower its noise figure. The same principle governs the microwave part design.
Klystrons and magnetrons and other vacuum tubes are still more efficient at high power than solid state devices. Also their life is good; if used in satellite transponders, vacuum tubes are less vulnerable to radiation and overheating.

Modern MMICs offer nice parameters, like power and noise figure AT THEIR PORTS. In a system, transmission lines must be used, and nominally in radar, waveguides are much better than coax or microstrip if the distance is ~ 1 ft or more. High-power ferrite switches, ATRs, etc., have been designed for waveguides for the best results.

 

[kommanche]

Thanks, One last question, I saw some radar system that does not use an LNA and directly feed received signal to the mixer. Do you know the reason? I think there should be an LNA at the front.

 

[jiripolivka]

Thanks, One last question, I saw some radar system that does not use an LNA and directly feed received signal to the mixer. Do you know the reason? I think there should be an LNA at the front.

The reason for "missing" LNA is historical. Until recently, in microwave bands <9 GHz, only mixer front-end receivers were available. Microwave LNAs are still expensive, and their noise figures are not much better than those of mixer front-end receivers.
LNAs are also ,more sensitive to pulse overload while mixers survive better.
A typical SSB NF of a mixer-front-end receiver is ~ 7 dB while a "protected" :LNA has ~5-6 dB NF. The cost of a LNA is >3 x than of a mixer.

 

[kommanche]

Thanks for the helpful info.