TWS-N15 Noise Source 10 MHz-2 GHz: a few more sets

Coming back to an earlier post, Noise source design, I wanted to post the final results, and the looks of these noise sources.

The case is an aluminum extrusion design, and the lids are milled to accomodate the BNC and SMA connectors. The SMA is a really high quality connector. No point in using a noise source with a cheap connector – you are normally going to connect and disconnect this often.

The constant current supply is optimized for the maximum noise output, normally, about 8 mA. The design is a current mirror, with a TL431 precision reference.

The noise section is soldered with 0603 SMD mostly, on a FR4 board.

Foam and copper tape to avoid any foreign signals getting into it. Spurious signals can mean big trouble with noise measurements.

Return loss, I think it is pretty good.


After some optimization of the circuit, the ENR output is now pretty flat, even with no specially expensive noise diode.

After all, pretty happy with the device, and others are happy two, as I give them away at low cost. If you need one, let me know.

HP 8561B Spectrum Analyzer: a smoking power supply

Recently, busy days with all kinds of business trips and vacation in between, but finally time to go back to the workshop and enjoy some repairs in free time.
I got this analyzer for cheap, but it is not working unfortunately. When I plugged it in, smoke came out. Not a good sign, but let’s first find the source of the smoke. Easier said than done, because this “compact” unit has many fragile boards, and many screws, but I managed to get down to the innermost part, the power supply. Still wondering how HP designed this unit, it must have been a mere engineering nightmare, but these units are surprisingly reliable, 30 years old, or older.

Well, it didn’t take long to find the culprits, some old RIFA caps!

One blown, the others not looking much better. So I decided to replace them all, including the 2n2 Y-rated caps. The 100 nF X2 caps have a 20 mm raster, not a common size nowadays. We may as well replace them with original RIFA parts. Not cheap, at about 3 EUR per piece, but the unit is definitely worth it.

The power supply compartment is specially shielded, and I used the opportunity to clean out the dust.

A few days later, the new caps arrived (Y caps were still in my stock, WIMA brand).

The old board is looking marvelous with the new caps mounted.

A moment of truth – furtunately, the caps were the only issue, all working fine!