Noise Source Reverse Engineering and Improvements

I ordered a noise source from ebay for about $20USD, It came in good shape and working order just 2 weeks later. I say “[in] working order” rather loosely as the thing got hot, very very hot, easily over 70*c after just 10 minutes around the LNAs. Also it leaked quite a bit, properly terminating it and placing my cellphone on top of it caused me to lose reception almost immediately.

So, curious how it worked I got my flashlight and some reading glasses and started looking up part numbers. Soon after I was able to reverse engineer the unit (thanks in no small part by the mostly one layer design)

Not wanting to do a destructive tear-down I was unable to pull values of the caps but I think one can work them out.

The LNAs were interesting, I thought they were 2 different types but after looking up numbers it appears as if they are all the same SBB5089Z as U2 and U4 being manufactured by RFMD and U3 being manufactured by Sirenza Microdevices, Not sure why this is but both data sheets appear to be copy and pastes of each other

Another interesting aspect is they are driven directly from the 12v unregulated input and they appear to be relying solely on a 100R resistor to drop the voltage down to the 5v limit, not something I would have done with the cost of 5v regulators or even a zener diode but it appears to have worked.

Now how does this work? Well it appears to be a fairly common reverse biased diode noise source using a 24v zener diode supplied in what looks like a standard voltage regulator arrangement supplied with 38.5v, This is than AC coupled, put threw a 3dB attenuator then amplified by 60dB or so with 3 LNAs, put threw another attenuator than AC coupled to the output.

The first attenuator appear to be used simply to help maintain 50 ohms from the zener, and the last attenuator appears to simply not let the LNAs experience a high impedance if disconnected, This is a fairly common practice.

The 38.5v is created with a fairly standard and cheap MC34063AC DC-DC controller setup in a boost configuration. Its output is not as well filtered as I would like appearing to just have a few uF cap on the output.
Now let’s have a look at how we can improve this, What would make this unit more usable would be to pull away all this excess heat, we do this with an old hefty heatsink I have used on many projects I cut up. Next we must deal with the leakage, to do this I soldered some leads on to the ground leads of the LNAs (this will also help with some cooling) along with the ground lead of the DC-DC output cap, I than poked some holes in a piece of copper sheeting and soldered the leads to that copper piece as shown below

Step 2 was to place the module inside of an Altoids tin, this won’t give the best shielding but it will be enough to prevent any issues. After drilling out 2 holes for the new input jack and cutting a slit for the heatsink I than glued the heatsink to the bottom of the Altoids tin with a few springs to maintain a low impedance to ground, then glued the module to the top of that with some epoxy heatsink compound. Next was soldering the tin to the ground of the module wherever I could to prevent the creation of any sort of cavity filter, resonator, or oscillator, along with tacking down the copper shield.


One last little detail was poking a hole in the case to let the LED shine through, I than made a rudimentary light-pipe using a glob of hot glue and some copper tape to use as a reflector. It’s not great but the already bright and over driven green LED shined threw wonderfully!

Lets now have a look at how well it works from 25-1800Mhz

 

Noise Source Turned OFF

 

Noise source turned ON

NOTE: The dips at about 225Mhz and 450Mhz are due to a glitch in the software and are not actually present

As you can see it generates a good -50-60dB of noise, This eliminates any fear of radiating too much noise from your antenna under test while keeping it well above the -75dB noise floor of my reciever

 

Whats next? Well we will use this to characterize some antennas!

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