Saturday, June 11, 2022

RF Noise Source NF-1000 SZ evaluation


I have been working on an automated step calibrator project in recent months, so I was excited when I saw the device shown above on eBay.  The descriptions calls it the NF-1000 SZ, but you won't find that model number anywhere on the device.  The NF-1000 SZ contains a broadband radio frequency noise generator in conjunction with a 31.5 dB step attenuator.  The rated output is -10 dBm to -40 dBm  however my units maximum output was only -47 dBm!  Obviously, this is way out of spec! I could just send it back, but I just had to look inside to see if there might be some simple problem.

Noise generator block diagram. 

The general design of the NF-1000 SZ follows that of other noise sources.  RF noise is extracted from a Zener diode junction and sent through several stages of amplification.  A number of eBay noise generator modules follow this scheme.  The neat thing about the NF-1000 SZ is that it also includes a step attenuator at the output of its noise source. There are a number of step attenuators and noise sources out there, but no one is putting them together.

Shielded rf sections in the RF-1000.

After opening the case, I was pleasantly surprised to see the rf sections were shielded.  Luckily, the shields snap on and off without de-soldering. Under those lids however, things weren't so pretty. The first thing you notice is that the unit is hand soldered, and not so neatly done. The amplifiers are probably MMIC amplifiers, but the identifier has been ground off. (silly).  

The signal flow in the rf sections above runs from the Zener diode noise source on the far right and through the three MMIC amplifiers before exiting to the attenuator to the upper left.  The noise signal is coupled through a capacitor to the input pin of the first amplifier. The amplifier IC takes its B+ at the RF output pin, here about 4.7V after passing through L1 and its series dropping resistor. This arrangement is repeated with each amplifier stage.  

Now here is the odd thing. There are places for the inclusion of pi style attenuators after each stage.  You will see this in most designs.  It is good practice because it presents a stable impedance to the outputs and inputs of each amplifier.  MMICs generally have good gain into the microwave region and require near 50 ohms impedance to work into or they can be unstable.  In the NF-1000 SZ each attenuator "pad" should be populated with three resistors as represented by the green ones that I drew in above.  Where the series resistor of the pi network should be, the manufacturer has placed a 000 ohm resistor, a short.

The attenuator chip also has it's markings scratched off but I believe it is the BDA4710 device shown below..  All of the pins match.  Oddly though, they don't take advantage of its ability to control the chip in 0.25 dB steps. They spec at 0.5 dB steps, though the display allows any hundredths precision value to be input.  Also the upper frequency for the NF-1000 SZ is 1.5 GHz though the attenuator specs to 8 GHz. Of course, other things limit the upper frequency like the mystery amplifier ICs.   They could also have greatly increased the usefulness of their design by incorporating 2 of these attenuator chips and allowed for 63 dB of range.  

The serial interface is simple enough and seems to work OK.  Also, in its favor, the noise output across the HF bands is pretty darn flat, close to 1 dB flatness. 

Note: In an earlier edition of this post I failed to take into account bandwidth as it relates to the power rating of the generator.  

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RF Noise Source NF-1000 SZ evaluation

  I have been working on an automated step calibrator project in recent months, so I was excited when I saw the device shown above on eBay. ...