Re: [Discuss-gnuradio] Active Radar Hardware

[John Ackermann N8UR]

Lee, I'm not sure what power levels you're running, but if you need more 
isolation than a single circulator can provide, you can always stack 
'em.  Then, you could also put a pair of clamping diodes at the receiver 
input to protect it from any system failure.

But also realize that in addition to preventing physical damage to the 
receiver, you also need to deal with "desense" (as we old repeater 
builders call it) that results when the RX front end is subjected to a 
strong input.  It takes a finite amount of time for the front end 
operating conditions to return to normal after being exposed to a big 
signal, so that may impact your turn-around time.

Remember that I'm in the neighborhood, and between me and my local ham 
gang there's not much RF test equipment we don't have.  We'll be happy 
to help out.  In fact, we'd love to see a demo of your radar at one of 
our meetings... :-)

John
----

Lee Patton wrote:
> Jason and Daniel -
>
> Thanks for the feedback.  I'll look into PIN diodes.  The reason I
> wanted to use a circulator was because it doesn't require a switching
> signal to be generated from the USRP.  And, while generating such a
> signal is possible, it is difficult (if not impossible?) to ensure it is
> timed properly with the transmit waveform -- unless I do some FPGA
> coding, which may be the necessary next step. (Again, something with
> which I have no experience! But learning new things is fun, right?) I
> think I will look into using the actual Tx waveform as the switching
> signal. 
>
> Also, Jason, thanks for pointing the need for a variable attenuator to
> condition the signal for ADC.  Although I've been aware of this in the
> past, I forgot about it in my application.
>
> Thanks again fellas, 
>  -Lee
>
>
> On Fri, 2006-09-29 at 18:15 +1000, Jason Hecker wrote:
> > > We make MF & VHF systems and for some of our VHF systems we use a single
> > > set of antennas and a T/R switch (passive and active). However since our
> > > frequency of operation is 2-3 orders of magnitude lower it's probably all
> > > different...
> > The radar I worked on was a ~2.7GHz job.  Since the receiver was spammed for 
> > the first several hundred metres in range due to large amounts of echo from 
> > ground clutter any such early returns were gated out.  After that the circuit 
> > controlling the attenuator (PIN diodes) ramped the voltage so that the 
> > overall gain of the system increased in time.  This was to maximise or 
> > normalise the S/N ratio at the ADC.  When you are looking for metallic 
> > bogie's many 10's of KM away you need all the return you can get even with 
> > the gain achieved from integrating multiple returns.
> >
> > Anyway, the author of the grandfather post doesn't sound like he has too much 
> > RF experience (who does?)  Perhaps a circulator will do or even better, an 
> > RX/TX switch module (diplexers?) - though this might involve a hardware hack 
> > to get the switching pulse out in time.  The thought did occur to me to get 
> > something like a Furuno boating radar head with an integrated separate 
> > transmit and receive antenna.  You'd get a nice narrow beam from one of 
> > those.
> >
> >
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