This is a review of the BG7TBL noise source available on eBay for about $20 incl post. I have seen this recommended in various online forums and thought it worthy of review.
A quick mention of Excess Noise Ratio (ENR), it is a commonly used measure of the characteristic of noise sources. A noise source for testing low noise RF amplifiers needs to be less than 10dB, 5dB is common; for other receiver testing around 15dB is common, and for massive output for filter alignment etc the noise needs to be well above a spectrum analyser noise floor so an ENR of 50dB might be appropriate (but such high noise output makes it useless for LNA noise figure measurement),
Above is the device. The layout is pretty simple, it is a Zener noise source at the left followed by three MMIC amplifier stages. The circuitry at mid left is a DC-DC converter to supply 25V to the Zener.
There are a host of aspects so far that are concerning:
- there is no need to operate the Zener at such high voltage;
- lack of regulation of MMIC power supply;
- the noise output of the Zener source should be quite high; and
- three stages of MMIC will give rise to huge output, notwithstanding the on-board attenuators at Zener output and final MMIC output.
Above, a sweep with 40dB attenuation between the noise source and spectrum analyser at 12.0V supply. The noise power density is hardly flat with frequency, there is a huge slope.
In fact the output is quite sensitive to supply voltage, so there would be no point in calibrating it.
Lets convert the measured power in 100kHz bandwidth to ENR.
Above is a plot of the ENR of the bare noise source.
The board does have a 3dB pad on the output, it does little for the output match so you would probably add an external 10dB pad and in that case you would subtract 10db from the chart above. That still leaves ENR ranging from 110dB down to 90dB across 1-30MHz, HUGE output.
The measured broadband power output is 10dBm, so the final MMIC is operated at or close to saturation. Such high output may saturate many receivers, so if you had in mind using it with an SDR dongle for filter alignment, better add 30dB or more additional attenuation.
This was sold with no specifications, and for good reason… it is simply Chinese junk. Branded BG7TBL, I will be very wary of buying anything of that brand.