Tag Archives: BFR93A

TWS-N15 Noise Source: checking out some design alternatives

So far, we have mainly been discussing series type noise sources, i.e., noise sources where neither anode nor cathode are connected to ground. Another common design is shown here – the shunt configuration (one port of the noise generation element grounded).

noise source bfr93a shunt

The assembly, more or less just a little blob of solder with a few tiny parts inside… mostly, 0603 SMD format. The output attenuator (not shown) is a 14.5 dB(!), 18 GHz coaxial attenuator.

noise source bfr93a shunt assy

Some quick measurements, at bias currents of 2.5, 5 and 7 mA…. still, there seems to be a lot of 1/f noise (increase of noise power at lower frequencies). This is model #1, with a 22 nF capacitor (see schematic)

noise bfr93a shunt configuration 1

Don’t really see any advantage over the series variant of the noise source. But will test further.

…Progress on another front, ordered a set of PCBs – they can be used for various noise source configurations. Not yet a “prototype”, but need to see what kind of GHz performance is available from such design, and how reproducible it is. No current source yet on this PCB – will add later, or on a separate board – to limit shielding to the RF section.

noise source pcb 150827-2

TWS-N15 Noise Source: some RF transistors as noise generating devices

After testing some Zener diodes and regular transistors (see earlier posts), some attempts with high frequency transistors, to generate white noise (noise power constant with frequency).

So far we have found that Zener diodes generate high noise power, and are rather flat out to 1.5+ GHz (if proper package and mounting is chosen). However, there is appreciable 1/f noise (increase of noise power) below 100 MHz, and this is difficult the equilize with just plain R-C networks.

Another attempt, with regular tansistors – they don’t have enough noise power at high frequencies, past a few 100 MHz.

Now, finally, I have received some 6 GHz BFR93A and 22 GHz BFG410W transistors, from my stock of parts back home in Germany, and have put these to the test. Same circuit is used like before, with positive current fed into the emitter, and the base grounded via some resistors (transistor is run in emitter-base breakdown condition to generate noise).

These are the parts concerned, some general notes – the BRF93A is a very useful part for all kinds of RF applications, and available at low cost.

noise bfr93a

The BFG410W, it is also quite remarkable and I use it a lot for LNA (low-noise amplifier) designs – hard to beat at their cost, delivering considerable gain, at low power. Unbelievable what the semiconductor folks have been able to achieve, a 22 GHz transistor, for a few cents each!

noise bfg410w

Here, the ENR results, vs. bias current, in mA.

noise enr vs bias bfr93a
-note that the ENR increases at low bias current!

noise enr vs bias bfg410w

As can be seen, and don’t ask me why, the BFG410W generates much less noise. Some quick change of the attenuator pad – 4 dB less attenuation. Just to check if this has any effect (besides increasing output power) – all seems well behaved and power is increased without changing any of the general characteristics.

BFG410W – lower 3 traces are 390 ohms parallel, upper 3 traces are 130 ohms parallel output attentuator (390 ohm 0603 pad resistor, paralleled with 390 or 130 ohm 0603 resistor)
noise enr vs bias bfg410w 130 ohm pad

The BFG410W appears to have the best white noise characteristics so far, note that the measurements are still not too accurate, mainly for screening of parts. With proper bias current selection, flatness, 100 to 1000 MHz, <0.2 dB should be possible. Will do some more experimentation, and fine-tuning of the filter/equilization components; ideally, the noise power should be a bit higher, to be able to use a larger, well-matched attenuator, giving good output SWR. Also, I think it is now about time to fabricate some better HF boards (still using FR4, but precision made), to get a reproducible assembly, and to have several TWS-15N prototypes made and characterized.