EMRFD Message Archive 3304

Message Date From Subject
3304 2009-07-13 11:53:53 bkopski Experiments with a Calibrated Noise Source
Recently a friend helped me to complete and calibrate a noise source based on EMRFD Fig. 7.73 but with an alternative diode. I chose to assemble it as a self-contained unit with an internal battery / regulator combination so it's a stand-alone test instrument. I've included three related photos in the K3NHI folder.

As an early exercise of my new source, I decided to see what kind of 10 MHz NF variation I might find among several 2N3904 samples. To do this I assembled the simple feedback amplifier circuit referenced via post 3239 and provisioned it with a socket so I could easily swap transistors. (This is one-half of the bidirectional circuit of Fig. 3 in the posted reference paper.) This amplifier assembly was followed up by a second similar amplifier to boost signal on its way to my SA. The latter was tuned to 10 MHz with a nominal 300 kHz RBF.

Digging deep in my junk box I was able to find seven 3904 samples all differing in mfgr. or date code i.e. they were all clearly of different vintage. Two had no mfgr. identification. One in particular is over 30 years old – an original Motorola 2N3904 with gold plated leads – ya can't hardly get them kind no more!

One by one I ran these 7 samples thru my new test setup and determined the associated NF numbers. Following is the spread I recorded:

Device NF (dB)

1. 5.46
2. 5.17
3. 4.97
4. 4.97
5. 5.17
6. 5.38
7. 4.96

As a note of interest, the highest NF is that of transistor #1 – that aged Motorola part.

Another experiment involved the wide band amplifier of Fig 6 of the reference paper but built with a 2N5179 device. The measured NF is 10.7 dB. Note these experimental results are in reasonable agreement with the values given in the reference paper.

From this one might conclude that the referenced amplifier circuit built around `most any 3904 would typically display a nominal 5 dB NF and thereby be a reasonable starting-point reference noise-wise. Further, a "cross check" or "second data point" is available with the measured wide band amplifier circuit performance. Thus, two easily constructed "test" amplifiers and the noise figure discussions / arithmetic given in EMRFD could be a reasonable starting point reference for HB'ers having no alternative.

An example application of this "pseudo transfer standard" approach might be to "back into" (determine) the ENR of a low cost noise source such as the ELECRAFT N-gen kit or an HB noise source. Admittedly the results would typically not be perfect but I see this comparing favorably with having no other easy option – at least for starters. What ever your candidate noise source, do be sure to use adequate pad on the output or otherwise assure a solid 50 ohm impedance, do regulate the power supply, and then follow the related info in "the book"!