RF Broadband Noise Generator: Using wrong Zener diode? - Page 1
Forum section: electronics / projects

author: abitofhelp
author_rank: Contributor
timestamp: April 06, 2025, 07:33:12 am
content: I found an interesting circuit (image attached), which was created by LUKASZ SLIWCZYNSKI and published on edn.com in 1999:SETUP:The author switched-boosts a 12VDC supply to 34VDC and smoothes it with a linear regulator. The 7805 is configured to adjust the current within 8mA to 40mA. This current aligns well with the breakdown characteristics for a zener diode with Vz of 24V. The zener diode will produce the white noise that is sent for amplification.I've built the power conditioning part of this circuit because I cannot figure out how the author drives the zener diode into avalanche mode when Vz is 24V and the measured voltage from the 7805 linear regulator is 5.05VDC. If the measured voltage was close to 24VDC, then the zener would produce avalanche noise, which I could tune a little with the potentiometer adjusting current. The circuit uses a current rather than a voltage source for the zener diode, so maybe I am missing something here.QUESTIONS:1) Why did the author switched-boost the 12VDC supply to 34VDC and then reduce it to 5.05VDC with the linear regulator? I understand that the linear regulator will smooth out the ripples from the switched regulator, but the conversion of the voltages for a 24Vz zener diode doesn't seem to make sense.2) Is there an error in this circuit's design for generating zener diode avalanche breakdown noise? If not, can someone please explain how this configuration would work? I've tried driving a 24Vz zener diode with the circuit in my lab, and it is not going into avalanche breakdown, which I expect with only 5.05VDC driving it.Thank you for your help and time,Mike

author: ch_scr
author_rank: Frequent Contributor
timestamp: April 06, 2025, 07:51:34 am
content: Hello Mike,the 7805 is wired up as a constant current regulator, not as a voltage regulator. Look at theLM317 datasheet(Page 13) for a "more typical" implementation.Kind regards, Christian

author: Kurets
author_rank: Regular Contributor
timestamp: April 06, 2025, 07:54:22 am
content: The circuit is a bit bad in that it boosts to 34 but uses a filter capacitor rated to 35. 50V would be more appropriate there.However concerning the 7805 part of the circuit. The 7805 regulates the voltage between pins out and gnd to 5V. In your circuit these are connected with a resistor which will draw a current of 5/R. This will then be added to the quiescent current of the regulator and sent "down" through the zener.The three terminal regulator is not an absolute "5V referenced to circuit ground", it just regulates referenced to its own gnd pin. I hope this helps.

author: abitofhelp
author_rank: Contributor
timestamp: April 06, 2025, 08:05:28 am
content: Thanks for your explanations! My understanding of avalanche breakdown is that Vr is applied until about Vz@Iz for the zener diode. The avalanche breakdown commences with a surge of current. Can you help dispel this fog in my head?How is this circuit driving the zener diode into avalanche breakdown using the current rather than Vz? All the zener diode noise generators I find online are based on voltage sources, so I am a bit lost with how things work in this design. I've implemented the circuit up to amplification. Perhaps you can suggest a testing procedure to see the breakdown by varying the current using this circuit. Perhaps, I am not testing it properly. My power supply is set to 12VDC@500mA and I have an ammeter attached to view the potentiometer changes.Thank you again for your help.Mike

author: abitofhelp
author_rank: Contributor
timestamp: April 06, 2025, 08:11:46 am
content: @ch_scr Christian, It seems that "9.3.3 Precision Current-Limiter Circuit" is similar to this design. Thanks! I am still grappling with how the current triggers the avalanche rather than reaching and exceeding Vz.

author: Kurets
author_rank: Regular Contributor
timestamp: April 06, 2025, 08:29:58 am
content: Think of it like this: if no current is flowing, the 7805 would see zero volts across out and gnd. This would drive the control loop to maximize the output voltage. If the zener did not conduct the output of the 7805 would reach close to 32V which is above 24V. So there is enough voltage available in the circuit for the zener to conduct, meaning that the circuit will work as intended. (A minor detail is that as the quiescent current of the chip goes through the gnd pin, you cant really talk about the control loop being active if the gnd pin current has nowhere to go)Essentially, due to it being the threshold when the zener conducts the gnd pin of the 7805 will end up at 24V and current throjgh the zener being 5/R+Iq

author: mawyatt
author_rank: Super Contributor
timestamp: April 06, 2025, 01:38:02 pm
content: Wouldn't using a lower voltage zener, say between 7~10V and a 12VDC Power Input with a series pot/resistor to the zener to set the current and then amplify the zener noise as required work as well?Seems like a lot of added circuitry for a noise source unless the lower voltage zeners don't produce enough noise. Also would expect the switching noise to corrupt the lower end of the noise spectrum.Best

author: jonpaul
author_rank: Super Contributor
timestamp: April 06, 2025, 02:19:35 pm
content: see many refs on noise diodes and noise gens.Instead of TRNG, try a PRNG Pseudo random likehttps://www.sound-au.com/project182.htmReadly made but rare:National Semiconductor MM5837 - an 8-pin PMOS IC that was a complete digital noise source.https://www.alldatasheet.com/datasheet-pdf/pdf/9284/NSC/MM5837.htmlJonSee

author: daqq
author_rank: Super Contributor
timestamp: April 06, 2025, 07:33:37 pm
content: Quote from: jonpaul on April 06, 2025, 02:19:35 pmAt a quick glance these aren't RF. The technique could be used for RF, but you'd need a pretty fast DAC with a lot of other good stuff to achieve some decent broadband RF performance.

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2025, 07:45:53 pm
content: I understand the use of a three-terminal regulator as a current source, but the "78L05" is connected between a -34 V supply and a diode that will see -24 V when conducting.Should that have been a 79L05 negative regulator with those voltages?

author: abitofhelp
author_rank: Contributor
timestamp: April 06, 2025, 11:02:44 pm
content: @Kurets Thank you for the information about how the 7805 drives the zener to avalanche. I found additional information for the lm317a, which discusses three-terminal regulation, so I've learned a lot. Thanks! Armed with my new knowledge, I think I found a simple error in my implementation of my circuit, which would account for the zener not going into avalanche. I am going to make a tweak and will report the results.

author: abitofhelp
author_rank: Contributor
timestamp: April 06, 2025, 11:06:30 pm
content: @TimFox I measured Vout on the tl497 in my test circuit and found +32.9V relative to ground. The input voltage is +12VDC@200mA. The tl497 is configured as a step-up, positive regulator, so I don't know why the schematic shows "-34V".

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2025, 11:13:34 pm
content: I misread the voltage in your posted diagram as a negative due to a badly-resolved "tilde" symbol in the image.Of course, the Zener, as drawn, requires a positive voltage.I believe the higher-voltage diode is used since it is more of an "avalanche" device than lower-voltage units.The usual transition between "true" Zeners and avalanche diodes is between 5 and 8 volts.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 06, 2025, 11:25:12 pm
content: Quote from: abitofhelp on April 06, 2025, 08:11:46 amYou don't. There is no magic trigger, or early mode. That's why you cannot measure anything unusual.The prose is a bit creative, claiming some special effect.See:https://en.wikipedia.org/wiki/Zener_effectIn general, diode junction breakdowns occurring below 5 volts are caused by the Zener effect,whereas breakdowns occurring above 5 volts are caused by the avalanche effect. Breakdowns occurring at voltages close to 5 V are usually caused by some combination of the two effects.They are all called Zener diodes for simplicity, but >>5V are actually avalanche diodes.At very low (100uA) currents some papers talk about a break-back effect, but that is a low frequency sawtooth, and I've never seen a real diode behave that way, as the VZ would be out of spec.You can buy 50uA zeners, so that does not seem to be a fundamental issue.https://www.nexperia.com/products/diodes/family/50-UA-ZENER-DIODES/A number of vendors offer MMSZ4709, 24V tested at 50uAFor an RF noise source, you want to actively avoid any low frequency effects.When the zener is conducting, it has a quite low impedance, so I doubt it matters to the zener if it is nominally current driven, or fed from a resistor at constant voltage.A low ripple supply is desirable in both cases.A common circuit for Zener control is to have the zener 'provide its own voltage'. A an opamp has dual feedback paths for an output some ratio above Vz, and then Vz is fed from that output.See circuit #1 herehttps://electronics.stackexchange.com/questions/283651/opamp-zener-regulator-configurationThat has high PSRR, tho needs checking it starts reliably.For RF noise extraction, you may also need to watch the natural RC rolloff. egBZX58550-C24 Zener with 5mA is 70 Ohms and 55pF , Tau of 3.85nsPDZ24B at 5mA is 30 Ohms, max 80pF for Tau 2.4nsROHM UDZV24B has curves for Cz and impedance, suggests 3pF > 15V and ~ 60 ohms 1-10mA for Tau ~ 0.18nsWith very low Cz zeners, finding a current source that does not impact that would be tricky, so a simple resistor feed would have less impact on the rf noise spectrum.

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2025, 11:30:09 pm
content: When conducting, an avalanche diode's equivalent resistance is very low, and tends to short out the capacitance.At voltages below breakdown, the capacitance is important and a strong function of the reverse voltage (normal bias polarity for a Zener), while the shunt resistance is extremely high.At 0 V bias, the capacitance can be somewhat high.(Try to keep the Zener dark if it is in a glass package: the junction area is high and photocurrent is measurable in room light.)

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 06, 2025, 11:48:56 pm
content: Quote from: TimFox on April 06, 2025, 11:13:34 pmThe higher voltage also helps lower the diode Ceff. Note that Ceff falls with voltage, but the impedance rises again with voltage after a broad minimum 10~15VROHM gives nice curves for each diode, a couple attached for their EDZ 15V and 24V models.For RF noise a slightly lower voltage would have a higher self-roll off frequency, and give a lower power budget at a given current. Candidates would need to be tested.

author: abitofhelp
author_rank: Contributor
timestamp: April 07, 2025, 12:56:50 am
content: Oh @timfox, how funny! I've looked at that voltage declaration on the schematic at least 100 times and never noticed it was a tilda!

author: abitofhelp
author_rank: Contributor
timestamp: April 07, 2025, 07:53:08 am
content: Does anyone have an idea why the author of the circuit boosts 12V to 34V and then drops it to 5V? I cannot see a benefit when the 7805 is a current regulator…

author: ch_scr
author_rank: Frequent Contributor
timestamp: April 07, 2025, 08:16:55 am
content: Quote from: abitofhelp on April 07, 2025, 07:53:08 amYou need more than 12V to get the 24V zener to conduct. A zener (like any diode/LED) has no inherent current limit -> One could use a resistor, or a constant current source like in this case.The current limiter is a bit uncommon (and power-wasting) with the 7805, but not a problem in itself.The circuit does not drop the voltage to 5V (but rather by 5V across the current limit resistor (and potentiometer))

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 07, 2025, 09:15:26 am
content: Quote from: abitofhelp on April 07, 2025, 07:53:08 amJust because it has 7805 on the package, does not mean it outputs 5V.Here it is used as a current source, because the -ve pin is not GND, it sits at ~24V and the OP pin is ~29VThe total sourced current is the sum of 7805 Iq quiescent current ( ~5mA ballpark, varies by brand and Vdrop) plus the output resistor of 8-10mAIq varies with temperature and voltage drop, so 7805 is not a great current source.

author: jonpaul
author_rank: Super Contributor
timestamp: April 07, 2025, 11:32:40 am
content: Noise diodes were selected Zeners, as production parts varies widely in noise density and spectrumSpecially selected Zeners with specific bias can yield consistent noise density.Audio, RF and microwave band diodes are availableJ

author: ftg
author_rank: Frequent Contributor
timestamp: April 07, 2025, 02:56:26 pm
content: Quote from: mawyatt on April 06, 2025, 01:38:02 pmThe higher voltage Zeners produce more RF noise, thus they get used more often.I recall reading that the sweet spot for practical noise source use being somewhere around 20V to 28V for the Zeners.I'd have to dig up some references to figure out what other mechanisms were involved in addition to the lower Ceff mentioned by PCB.Wiz.

author: mawyatt
author_rank: Super Contributor
timestamp: April 07, 2025, 04:12:17 pm
content: This thread peaked our interest in Zener based Noise Sources and we remembered the well known avalanche effect with reversed biased bipolar transistor EB junctions, this will become our "Noise Zener" to investigate. So we grabbed a 2N3904 and a couple resistors (50Ω {used two 100Ω in parallel} and 1KΩ), a ceramic 1000pF (for decoupling), and a Protoboard.The setup shown uses a SA (SSA3021X+) to measure the noise power from the reversed biased EB junction. A stable good lab PS (SDP3303X) is used for bias power for the "Noise Zener".We selected 5MHz with 1MHz BW for the SA since this was a more "Quite Zone" in out lab (we have a very noisy lab environment and did a sweep to find a lower noise frequency area).PN1 shows the Noise Floor for the SA without the "Zener" conducting, subsequent PN2-6 show increasing Zener bias from ~10.5 to 11V.PN7 shows the Noise properties as the SA is slowly swept (3Hz RB) and the bias supply stepped to show a stepped noise response across the SA display, and PN8 shows the Noise Noise ON/OFF across the SA sweep. Think of these displays PN7 and 8 as more of a Noise Level vs Time rather than the usual SA Frequency Sweep. Edit: Note the SA has a 10dB External Attenuator attached.With this crude setup not sure how the noise bandwidth would behave, a better short leaded or SMD PCB setup would be required to verify the effective BW of this type "Noise Zener". Will leave for others to experiment with if interested.Anyway, seems the reversed biased bipolar EB junction can produce a significant amount of noise when biased properly, and serve as a crude Noise Source in a pinch.Best

author: mawyatt
author_rank: Super Contributor
timestamp: April 07, 2025, 04:14:13 pm
content: Just for fun we hooked this up (with 50Ω terminator) to our SDS800HD and used the FFT set to 5MHz CF and 1MHz BW, average 16. The DSO noise floor in PN155 shows considerably more unwanted signals than the SA, we made no attempt to isolate those tho, and PN156 shows the "Noise Zener" fully biased (11V).Best

author: TimFox
author_rank: Super Contributor
timestamp: April 07, 2025, 04:26:09 pm
content: Packaged noise sources, as used with noise-figure analyzers, comprise a selected Zener diode and a constant-current source.The noise is switched on and off by switching the supply voltage to the source.A 50-ohm attenuator network at the output gives a good output VSWR, while reducing the actual noise output to a calibrated value of 6 or 15 dB ENR (noise power in excess of thermal noise in 50 ohms).Thirty years ago, when working with an RF preamplifier with an input impedance much higher than 50 ohms, we ordered a custom source with negligible output attenuation, but I don't remember how high the noise voltage was.Before Zener sources, there was the Sylvania 5722 vacuum diodehttps://frank.pocnet.net/sheets/201/5/5722.pdfhttps://radiojove.net/SUG/Pubs/UFRO%205722%20Noise%20Source%20User%27s%20Manual%202020%2004%2012,%20Typinski%20(2020).pdf.This diode was used in "saturated emission" mode, i.e. with negligible space-charge current limitation, by operating at a relatively low cathode temperature and high plate-cathode voltage so that all of the emitted electrons flowed to the plate, unimpeded by a space-charge cloud.In typical vacuum tubes, the plate current is determined by an equilibrium space-charge cloud that reduces the E field at the cathode surface: to first order, the current in such a diode depends only on the 3/2 power of the plate voltage and is relatively independent of cathode temperature.However, in saturated-emission mode, the current is relatively independent of voltage, but a strong function of cathode temperature.The advantage is that in saturated emission, the shot-noise current can be calculated directly from the DC current from first principles.

author: abitofhelp
author_rank: Contributor
timestamp: April 12, 2025, 06:54:04 am
content: Thank you for all of your contributions and discussions, everyone! With your information and some time in the lab, I learned a lot about constant current regulators and some common "greenhorn" mistakes, to which I fell prey. PCB.Wiz's comment about how the ground and tab are used on the LM7805 was spot-on! I assumed it was named gnd, so it needed to be attached to my ground plane on the PCB. WRONG! I switched my studying and research from the LM7805 to the LM317 because I found much more information about how it worked and how to use it as a constant current source. However, I walked into another "greenhorn" trap... The order and meaning of the pins on the LM7805 and LM317 are not the same. Further, the symbolic block used for the LM7805 shows the pins exactly as they are on the chip. However, the symbolic block for the LM317 does not align with the pins on the chip. Arggg... Ultimately, these two issues were the culprits of my implementation of the circuit not working. I need to update the schematic to match my final version, because I decided to use the LM317 rather than the LM7805 and made a few minor tweaks. I will post it here in case it can help others.Thanks again, everyone, for your help!

author: SteveThackery
author_rank: Super Contributor
timestamp: April 12, 2025, 12:52:29 pm
content: Quote from: abitofhelp on April 07, 2025, 07:53:08 amThis has been answered a couple or so times, now, but I just wanted to respectfully ask: are you absolutely clear in your understanding? The 7805 is wired as a constant current source, not as a voltage regulator. In simple terms, the 7805 will increase its output current until it sees 5V between its output terminal and its reference terminal. At that point it is "happy" and will maintain that constant current into whatever load happens to be connected to it (within limits). Please study the diagram.When I first discovered the idea of using a voltage regulator chip as a constant current source I thought it was very cool and clever.

author: PGPG
author_rank: Frequent Contributor
timestamp: April 12, 2025, 01:18:10 pm
content: I have never been using Zeners as noise source but I think, I understand this circuit.Quote from: abitofhelp on April 07, 2025, 07:53:08 amHe doesn't.Quote from: abitofhelp on April 07, 2025, 07:53:08 amCurrent source (ideal) has infinite internal resistance so no loads the noise sorce. If you use resistor to limit current in Zener diode than in small signal circuit model you get that resistor being in parallel to Zener diode so the (probably high impedance) noise sorce model is loaded with this resistor what circuit author wanted to avoid.Quote from: ch_scr on April 07, 2025, 08:16:55 amI suppose that the main circuit author goal was to get as high internal impedance current source as possible. I suppose he have tested more common solutions like transistor current source or LM317 current source and may be found that 7805 current sorce have better characteristic. Fo single transistor current source the problem is Miller effect making Ccb being multiplied by something close to beta and such 'huge' capacitance is seen as parallel to current source making this source being current at DC and relatively small frequencies but not at higher frequencies. Probably 7805 in current source configuration has smaller capacitance parallel to it. Even so at some high frequency this capacitance becomes being 'visible' so to increase the current source impedance at high frequency the ferrite beads were used.I don't know (and don't want to search it) what MAR6, MAR3 and ERA5 are but look at their loads. In each the ferrite beads were used to increase the load impedance at high frequency. Unfortunately it is not more specified at schematic. As it is old schematic and I remember at the beginning of 2000 years SMD ferrite beads were rather expensive so may be all these are simply small ferrite beads put on wire so having about 50 ohm at 100MHz. In my circuits I am typically using 1k ferrite beads so one far away from the other. May be it is clear from the reast of publication what are these ferrite beads parameters.Quote from: PCB.Wiz on April 07, 2025, 09:15:26 amI think this is the last important parameter here.

author: MathWizard
author_rank: Super Contributor
timestamp: April 13, 2025, 10:52:49 pm
content: Quote from: jonpaul on April 06, 2025, 02:19:35 pmI might try something like the project182. But I also have MCU's. So yeah I wonder what's possible with them, if I bare metal some PRNG code. I'm sure there's a bunch of eqn's for all that sort of stuff, and how to output some digital signal into some analog drive circuit, w/o DAC's.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 14, 2025, 05:56:24 pm
content: I found some 24 volt Zeners and had a go. Used a Rigol DP832A and varied the voltage across the diode in series with a 3k ohm resistor. The best current was around 80 uA (most noise). Two screen shots below. The scope was ac coupled and connected across the diode.Several diodes were tried and all about the same.The marker at 1 is an FM broadcast station.