On Monday, April 6, 2020 at 9:43:33 PM UTC-4, jla...@highlandsniptechnology.com wrote:> On Mon, 6 Apr 2020 14:50:46 -0700 (PDT), mpm wrote: > > >While not a component... > > > >Early in my career, it was demonstrated to me that you could make a reasonably accurate RF millivoltmeter out of an analog Simpson 260 voltmeter. > > > >(Without modifying it, of course.) > > Mo found this at some neighborhood junk sale and bought it for me. $3 > or something. > > https://www.dropbox.com/s/tz0panr4f1nlqer/RF_Ammeter.JPG?raw=1 > > It's probably a thermocouple. > > How did that Simpson thing work? > > I have some Spice models of an RF detector using an SMS7621 > low-barrier schottly. It works around 50 mV RMS. >Wow John, that is a nice looking vintage analog meter!! Nice find. From an old, faded, memory... The trick with the Simpson 260 was you connect the red lead to the uAmp input, and put the black lead in the Common (-). But, you don't connect the black lead to your circuit. What you do instead is loosely drape is across the unit under test, and then probe with just the red lead. Essentially, the meter is reading rectified RF. The above description about just leaving one lead disconnected but loosely coiled-up in close proximity to the circuit under test is correct. But I might have the meter connections wrong. At this point, I would probably have to have a Simpson 260 in front of me to figure it out -- but it's basically along those lines. The meter's not designed to do it, but it will. And it more accurate than you would think, for such a weird, oddball approach. But hey, if you need a relative RF millivoltmeter and don't have one handy..... :) You can also fix a Quintron QBT-250 paging transmitter with a 9-iron golf club, and certain old Ford pickup trucks with garlic bread. (Maybe I shared those stories here before?, but they're a little off-topic in any case.) :) You got a use in mind for that meter? (Just wondering)
Favourite parts with off-label uses?
Started by ●April 4, 2020
Reply by ●April 7, 20202020-04-07
Reply by ●April 7, 20202020-04-07
On Tue, 7 Apr 2020 15:16:32 -0700 (PDT), mpm <mpmillard@aol.com> wrote:>On Monday, April 6, 2020 at 9:43:33 PM UTC-4, jla...@highlandsniptechnology.com wrote: >> On Mon, 6 Apr 2020 14:50:46 -0700 (PDT), mpm wrote: >> >> >While not a component... >> > >> >Early in my career, it was demonstrated to me that you could make a reasonably accurate RF millivoltmeter out of an analog Simpson 260 voltmeter. >> > >> >(Without modifying it, of course.) >> >> Mo found this at some neighborhood junk sale and bought it for me. $3 >> or something. >> >> https://www.dropbox.com/s/tz0panr4f1nlqer/RF_Ammeter.JPG?raw=1 >> >> It's probably a thermocouple. >> >> How did that Simpson thing work? >> >> I have some Spice models of an RF detector using an SMS7621 >> low-barrier schottly. It works around 50 mV RMS. >> > >Wow John, that is a nice looking vintage analog meter!! >Nice find. > >From an old, faded, memory... >The trick with the Simpson 260 was you connect the red lead to the uAmp input, and put the black lead in the Common (-). But, you don't connect the black lead to your circuit. What you do instead is loosely drape is across the unit under test, and then probe with just the red lead. Essentially, the meter is reading rectified RF.The old VOMs used copper oxide rectifiers! Before silicon was discovered.> >The above description about just leaving one lead disconnected but loosely coiled-up in close proximity to the circuit under test is correct. But I might have the meter connections wrong. At this point, I would probably have to have a Simpson 260 in front of me to figure it out -- but it's basically along those lines. The meter's not designed to do it, but it will. And it more accurate than you would think, for such a weird, oddball approach. But hey, if you need a relative RF millivoltmeter and don't have one handy..... :) > >You can also fix a Quintron QBT-250 paging transmitter with a 9-iron golf club, and certain old Ford pickup trucks with garlic bread. (Maybe I shared those stories here before?, but they're a little off-topic in any case.) :)It's a sin to waste garlic bread.> >You got a use in mind for that meter? (Just wondering)No, it's just on a shelf for admiration. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●April 7, 20202020-04-07
On Tuesday, April 7, 2020 at 6:09:48 PM UTC-4, Carl wrote:> "Phil Hobbs" wrote in message > news:bc1dc43e-b9a1-7e91-15dc-5f851bc46c7b@electrooptical.net... > > > >On 2020-04-07 09:15, George Herold wrote: > >> On Monday, April 6, 2020 at 5:43:54 PM UTC-4, Phil Hobbs wrote: > >>> On 2020-04-05 13:39, jlarkin wrote: > >>>> On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs > >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: > >>>> > >>>>> On 2020-04-05 11:42, jlarkin wrote: > >>>>>> On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs > >>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> <big snip> > >> > >>>>> We've started putting little Sensirion T/H sensors in a lot of things. > >>>>> When using TECs, it's awfully nice to be able to compute the dew > >>>>> point, > >>>>> for instance, and in outdoor applications (e.g. our fire sensors for > >>>>> harvesters) it's good to be able to predict when the window is liable > >>>>> to > >>>>> fog up on the inside. > >>>>> > >>>>> We use IP67+ enclosures with bags of 5A molecular sieve inside, which > >>>>> is > >>>>> super cheap and will absorb 50% of its own mass in water. Simon has > >>>>> had > >>>>> to learn a whole lot about enclosures and mechanical design generally. > >>>>> Turns out that you have to put an air vent on the enclosure to prevent > >>>>> pumping water inside due to atmospheric pressure differences. That > >>>>> leads to working of the O-ring seals, which wears them out. > >>>>> > >>>>> We considered using a bellows, but atmospheric pressure varies +-7% or > >>>>> so, which makes for a pretty big, floppy bellows. Hermetic > >>>>> construction > >>>>> is possible but very expensive, and relies on glass or ceramic > >>>>> insulated > >>>>> connectors. The glass would have had to be brazed or indium-soldered > >>>>> to > >>>>> the lid, which leads to CTE mismatch problems. > >>>> > >>>> A balloon inside? > >>> > >>> Doesn't help--you need something to reduce the pressure drop across the > >>> seals. An aluminized rubber diaphragm inside the box, with one side > >>> vented, might work. > >>> > >>> I made a calculation that 50 grams of 5A molecular sieve would keep the > >>> inside dry for about 10 years. > >> > >> Phil, I'm not sure I understand the problem. I wonder if you could say > >> a bit more? > >> (I do dream of someday maybe using a sealed box...) > >> You've got a sealed box. > >> But pressure differences can cause the seals to fail. > >> So you put a vent in it. > >> but now that will pump water into your box. > >> (Because you have molecular sieve in there and it will > >> reduce the vapor pressure of water in the box.) > >> So you need a long vent such that the diffusion time for > >> H2O is long.... > >> > >> Is that close to stating the problem correctly? > >> > >> Some sort of long thin tube? > >> > > > >Air pressure varies by about +-7% (~1 psi) over time scales of a day or > >two. Our enclosures have a volume of about 300 ml, and will be mounted on > >top of a harvester. It will be painted a famous dark green colour, so its > >albedo will be low. Thus on sunny days, it will experience temperature > >fluctuations of as much as 70C over time scales of hours. That will lead to > >fluctuations on the order of +-2 psi in a sealed enclosure. > > > >This isn't a whole lot of force, but there are all sorts of white papers > >from Parker and other companies that say that pumping liquid water past > >O-rings is a significant problem in outdoor equipment. One reason is that > >the pressure variations basically centre on zero, and O-ring seals really > >really like a single sign of pressure difference, which causes the O-rings > >to stay seated. > > > >In a vented 300 ml enclosure with 50g of 5A molecular sieve, we can get > >about 24000 cycles of 20% air exchange at a dew point of 20C before the > >sieve saturates, so that's many years. > > > >Liquid getting in is much worse. > > > >Cheers > > > >Phil Hobbs > > Rube Goldberg (or someone bored and tired of staring at the walls :-)) would > suggest a spring-loaded piston with a solenoid release. Compress spring and > "arm" solenoid release, seal in box with oring seals and hermetic wire > feedthroughs, trigger solenoid so piston releases and raises the pressure in > the box, and voila, happy orings :-). If you have to open the box for > service, just rearm and fire the piston again once the box is resealed.Something like this? <https://www.ebay.com/c/838420942> I had a piece of test equipment that I couldn't open the shipping container, I had to wait for a really hot day to put it outside, until the pressure equalized. I bought one of these, so it was no longer temperature sensitive storage.
Reply by ●April 7, 20202020-04-07
On 7/4/20 11:03 pm, George Herold wrote:> On Monday, April 6, 2020 at 7:32:00 PM UTC-4, Clifford Heath wrote: >> George, I'm trying to understand this. What do you need 20 zeners for? >> >> FWIW, I'm currently making a broadband noise source for testing filters >> up to 1.5GHz. Still scratching around for the best source to put before >> a string of ERA-3 MMIC amplifiers. >> >> The BFR93A data sheet says abs max Vbe is 2V, but it doesn't zener at >> 5V. Although avalanche zeners produce much more noise, I'd rather not >> boost my 5V supply (though I might need to). I wonder how much reverse >> current a microwave Shottky diodes (say HSMS-286) would survive. Abs max >> peak reverse voltage is 4V, so it might withstand 5V anyway... >> >> Any better suggestions for a device I might have in the drawer already? >> >> Clifford Heath. > > Arghh! typo. sorry a 20V zener! Run near the knee you get these big > avalanche spikes, with ~1us rise/ fall times. > > 1 GHz noise sounds hard.It doesn't look hard, a lot of people have done it. I've built the amplifier, I just need a noise diode that will give me me 20db ENR or better to feed into it. The only zeners I can easily get are 1W and above, so have big capacitance, only good for audio. I need a physically small device that will have low capacitance. That's why I started with a GHz transistor. I've thought about trying an HSMS286 series microwave Schottky, which break down at about 4V (if the data sheet is to be believed). The literature says that devices with a sharp breakdown curve last longer - a soft curve indicates partial breakdown at the edge of the die, which leads to early failure. So I'll need to evaluate the sharpness of the breakdown to see if it's likely to last a while. But first I'm going to crank the Vbe on this BFR93A to see where it really breaks down. If I can get it to zener on w 12v supply, that'll probably do. I'd prefer not to need a higher supply voltage, but I might have to compromise on that. Pure zener noise (low voltage) is much lower amplitude than higher-voltage devices which have avalanche multiplication.> What about a spectrum analyzer and tracking generator?The only generators I have are several home made ones, and I don't have my 100-4000MHz generator complete yet. The closest thing I have to a spectrum analyser is a couple of SDRs. Hmm, I should finish that generator :) Clifford Heath
Reply by ●April 7, 20202020-04-07
Am 08.04.20 um 03:05 schrieb Clifford Heath:> On 7/4/20 11:03 pm, George Herold wrote: >> On Monday, April 6, 2020 at 7:32:00 PM UTC-4, Clifford Heath wrote: >>> George, I'm trying to understand this. What do you need 20 zeners for? >>> >>> FWIW, I'm currently making a broadband noise source for testing filters >>> up to 1.5GHz. Still scratching around for the best source to put before >>> a string of ERA-3 MMIC amplifiers. >>> >>> The BFR93A data sheet says abs max Vbe is 2V, but it doesn't zener at >>> 5V. Although avalanche zeners produce much more noise, I'd rather not >>> boost my 5V supply (though I might need to). I wonder how much reverse >>> current a microwave Shottky diodes (say HSMS-286) would survive. Abs max >>> peak reverse voltage is 4V, so it might withstand 5V anyway... >>> >>> Any better suggestions for a device I might have in the drawer already? >>> >>> Clifford Heath. >> >> Arghh! typo. sorry a 20V zener! Run near the knee you get these big >> avalanche spikes, with ~1us rise/ fall times. >> >> 1 GHz noise sounds hard. > > It doesn't look hard, a lot of people have done it. I've built the > amplifier, I just need a noise diode that will give me me 20db ENR or > better to feed into it. The only zeners I can easily get are 1W and > above, so have big capacitance, only good for audio. I need a physically > small device that will have low capacitance. That's why I started with a > GHz transistor. > > I've thought about trying an HSMS286 series microwave Schottky, which > break down at about 4V (if the data sheet is to be believed). The > literature says that devices with a sharp breakdown curve last longer - > a soft curve indicates partial breakdown at the edge of the die, which > leads to early failure. So I'll need to evaluate the sharpness of the > breakdown to see if it's likely to last a while. > > But first I'm going to crank the Vbe on this BFR93A to see where it > really breaks down. If I can get it to zener on w 12v supply, that'll > probably do. > > I'd prefer not to need a higher supply voltage, but I might have to > compromise on that. Pure zener noise (low voltage) is much lower > amplitude than higher-voltage devices which have avalanche multiplication.I have done that thing with the BFR93A many years ago and at the Weinheim VHF ham meeting there was someone who measured these homebrew things. I got 22 dB noise or sth. like that and it was flat enough for tuning filters. Not good enough to measure noise figure, both for amplitude and source impedance. The breakdown voltage of the BFR93A was 5 or 6V IIRC. I think it went to ~ 2 GHz. I used a CCS just like the charger of my HP-35 :-) Today I would not use a "Zener" source. If you have them, spend 2 more ERAs and amplify the noise of a 50 Ohm resistor. Yes, they have a noise figure of a few dB but you know it and it's flat. More flat than a BFR93A breakdown, and much better than any Z-diode with its huge capacitance. 1K may produce more voltage noise, but @ 50 Ohms you know that the ERAs will behave. The problem with noise filter measurements is that the bandwidth of the noise is so large and if you want to see the stop band you need lots of power. Thus, you will have large signal conditions at the output of your noise generator. Getting closer to saturation than 20 dB will damage the crest ratio / noise statistics. The ERAs may also need more than 5V if you include the bias resistor?>> What about a spectrum analyzer and tracking generator? > > The only generators I have are several home made ones, and I don't have > my 100-4000MHz generator complete yet. The closest thing I have to a > spectrum analyser is a couple of SDRs. > > Hmm, I should finish that generator :)good idea! cheers, Gerhard
Reply by ●April 8, 20202020-04-08
On 8/4/20 12:39 pm, Gerhard Hoffmann wrote:> Am 08.04.20 um 03:05 schrieb Clifford Heath: > I have done that thing with the BFR93A many years agoOh, thanks, nice to know I'm on a right track. I basically just guessed that the BFR93A should work, not that I saw it elsewhere.> flat enough for tuning filters. Not good enough to measure > noise figure, both for amplitude and source impedance. > The breakdown voltage of the BFR93A was 5 or 6V IIRC. > I think it went to ~ 2 GHz.That would be good enough for my first try!> I used a CCS just like the charger > of my HP-35 :-)I just thought I'd use a resistor, since I have a known supply voltage. Not a good idea?> Today I would not use a "Zener" source. If you have them, spend > 2 more ERAs and amplify the noise of a 50 Ohm resistor.Well now it's interesting, but I have exactly the right amount of room left on this proto-PCB for two more, and I have 20 ERA-3's. It's on a grid-punched board with assorted top-side copper strips. I used sticky-backed copper foil to make a ground plane on the back. It doesn't seem such a bad way to prototype RF circuits, given that my toner-transfer process is in recess (the new printer creates porous blacks that don't etch well).> Yes, they > have a noise figure of a few dB but you know it and it's flat.Assuming the ERA-3 or the layout doesn't filter it, it should stay flat.> 1K may produce more voltage noise, > but @ 50 Ohms you know that the ERAs will behave.Well I could try 220R and just check it's not oscillating I guess. Otherwise 50.> The problem with noise filter measurements is that the bandwidth > of the noise is so large and if you want to see the stop band > you need lots of power. Thus, you will have large signal conditions > at the output of your noise generator. Getting closer to saturation > than 20 dB will damage the crest ratio / noise statistics.Right. Not sure what that will do to the spectrum though?> The ERAs may also need more than 5V if you include the bias resistor?I figure they want 40mA, and based on data sheet values for other voltages (from 7V up) it looks like they settle around 3.3V, so I calculated a bias resistor for 5V and was pretty close. I guess it will be more temperature sensitive than it would be with a higher supply voltage. I'm using 0805 resistors but not series inductors, for now. Will that affect flatness (rather than just lose some gain)? Anyhow, I'll raise it to 12V and try that before adding more ERA-3s and switching to a resistor. Many thanks for your advice, Clifford Heath.
Reply by ●April 8, 20202020-04-08
On 8/4/20 1:51 pm, Clifford Heath wrote:> On 8/4/20 12:39 pm, Gerhard Hoffmann wrote: >> Am 08.04.20 um 03:05 schrieb Clifford Heath: >> I have done that thing with the BFR93A many years ago > > Oh, thanks, nice to know I'm on a right track. I basically just guessed > that the BFR93A should work, not that I saw it elsewhere. > >> flat enough for tuning filters. Not good enough to measure >> noise figure, both for amplitude and source impedance. >> The breakdown voltage of the BFR93A was 5 or 6V IIRC. >> I think it went to ~ 2 GHz. > > That would be good enough for my first try! > >> I used a CCS just like the charger >> of my HP-35 :-) > > I just thought I'd use a resistor, since I have a known supply voltage. > Not a good idea? > >> Today I would not use a "Zener" source. If you have them, spend >> 2 more ERAs and amplify the noise of a 50 Ohm resistor. > > Well now it's interesting, but I have exactly the right amount of room > left on this proto-PCB for two more, and I have 20 ERA-3's. > > It's on a grid-punched board with assorted top-side copper strips. I > used sticky-backed copper foil to make a ground plane on the back. It > doesn't seem such a bad way to prototype RF circuits, given that my > toner-transfer process is in recess (the new printer creates porous > blacks that don't etch well). > >> Yes, they >> have a noise figure of a few dB but you know it and it's flat. > > Assuming the ERA-3 or the layout doesn't filter it, it should stay flat. > >> 1K may produce more voltage noise, >> but @ 50 Ohms you know that the ERAs will behave. > > Well I could try 220R and just check it's not oscillating I guess. > Otherwise 50. > >> The problem with noise filter measurements is that the bandwidth >> of the noise is so large and if you want to see the stop band >> you need lots of power. Thus, you will have large signal conditions >> at the output of your noise generator. Getting closer to saturation >> than 20 dB will damage the crest ratio / noise statistics. > > Right. Not sure what that will do to the spectrum though?Ok with 12V, 2.6mA through the BFR93A's b-e, it zeners at 5.8V. Two stages of amplification is possibly too much - the noise is assymetrical around 0V, see this scope photo: <https://www.dropbox.com/s/bmjnhm988160zjc/NoiseScope%20BFR93A%202.5mA%202xERA3.jpg?dl=0> I guess I need to reduce gain somewhat? I haven't investigated the spectrum yet.>> The ERAs may also need more than 5V if you include the bias resistor?They're now biassed as recommended from 12V, with 240R. Replacing the BFR93A with 100R drops the noise to under 1mV - still visible, but will need more amplification (no oscillation visible). One more stage might be enough gain. -- Clifford Heath> I figure they want 40mA, and based on data sheet values for other > voltages (from 7V up) it looks like they settle around 3.3V, so I > calculated a bias resistor for 5V and was pretty close. I guess it will > be more temperature sensitive than it would be with a higher supply > voltage. > > I'm using 0805 resistors but not series inductors, for now. Will that > affect flatness (rather than just lose some gain)? > > Anyhow, I'll raise it to 12V and try that before adding more ERA-3s and > switching to a resistor. > > Many thanks for your advice, > > Clifford Heath. >
Reply by ●April 8, 20202020-04-08
Am 08.04.20 um 05:51 schrieb Clifford Heath:> On 8/4/20 12:39 pm, Gerhard Hoffmann wrote:> Well I could try 220R and just check it's not oscillating I guess. > Otherwise 50. > >> The problem with noise filter measurements is that the bandwidth >> of the noise is so large and if you want to see the stop band >> you need lots of power. Thus, you will have large signal conditions >> at the output of your noise generator. Getting closer to saturation >> than 20 dB will damage the crest ratio / noise statistics. > > Right. Not sure what that will do to the spectrum though?No idea. But a customer of mine had some problems with the autocorrelation of pseudo-noise for ranging purposes. The measured S-curve did not look so triangle-like as it was supposed to be.> >> The ERAs may also need more than 5V if you include the bias resistor? > > I figure they want 40mA, and based on data sheet values for other > voltages (from 7V up) it looks like they settle around 3.3V, so I > calculated a bias resistor for 5V and was pretty close. I guess it will > be more temperature sensitive than it would be with a higher supply > voltage. > > I'm using 0805 resistors but not series inductors, for now. Will that > affect flatness (rather than just lose some gain)?Pure resistance from high VCC is probably easier for the beginning. Use smaller R in series, it's a matter of C over the resistor vs. C from resistor to GND. You get predictability and spend VCC. I had quite good success with home etching, it helps during the lock-down. This here is from the same double-sided Eurocard (100 * 160 mm); the foil has been made with a Kyocera color laser on Bungard pre-sensitized material. I can etch what I can print. The A3 OKI color laser I used to have previously was clearly better, but the machine was a royal pain. The bottom side is GND and unetched. No ado with alignment. If it needs to be really precise, i.e. microwave filters, I go to a nearby print shop and have them make an offset film from my .pdf file. That increases the wall time from .pdf to soldering from 1h:30 to 3h and costs €8. < https://www.flickr.com/photos/137684711@N07/49748806273/in/dateposted-public/ > < https://www.flickr.com/photos/137684711@N07/49749272951/in/dateposted-public/ > Both boards were made on one Eurocard. Cheers, Gerhard
Reply by ●April 8, 20202020-04-08
onsdag den 8. april 2020 kl. 10.31.41 UTC+2 skrev Clifford Heath:> On 8/4/20 1:51 pm, Clifford Heath wrote: > > On 8/4/20 12:39 pm, Gerhard Hoffmann wrote: > >> Am 08.04.20 um 03:05 schrieb Clifford Heath: > >> I have done that thing with the BFR93A many years ago > > > > Oh, thanks, nice to know I'm on a right track. I basically just guessed > > that the BFR93A should work, not that I saw it elsewhere. > > > >> flat enough for tuning filters. Not good enough to measure > >> noise figure, both for amplitude and source impedance. > >> The breakdown voltage of the BFR93A was 5 or 6V IIRC. > >> I think it went to ~ 2 GHz. > > > > That would be good enough for my first try! > > > >> I used a CCS just like the charger > >> of my HP-35 :-) > > > > I just thought I'd use a resistor, since I have a known supply voltage. > > Not a good idea? > > > >> Today I would not use a "Zener" source. If you have them, spend > >> 2 more ERAs and amplify the noise of a 50 Ohm resistor. > > > > Well now it's interesting, but I have exactly the right amount of room > > left on this proto-PCB for two more, and I have 20 ERA-3's. > > > > It's on a grid-punched board with assorted top-side copper strips. I > > used sticky-backed copper foil to make a ground plane on the back. It > > doesn't seem such a bad way to prototype RF circuits, given that my > > toner-transfer process is in recess (the new printer creates porous > > blacks that don't etch well). > > > >> Yes, they > >> have a noise figure of a few dB but you know it and it's flat. > > > > Assuming the ERA-3 or the layout doesn't filter it, it should stay flat. > > > >> 1K may produce more voltage noise, > >> but @ 50 Ohms you know that the ERAs will behave. > > > > Well I could try 220R and just check it's not oscillating I guess. > > Otherwise 50. > > > >> The problem with noise filter measurements is that the bandwidth > >> of the noise is so large and if you want to see the stop band > >> you need lots of power. Thus, you will have large signal conditions > >> at the output of your noise generator. Getting closer to saturation > >> than 20 dB will damage the crest ratio / noise statistics. > > > > Right. Not sure what that will do to the spectrum though? > > > Ok with 12V, 2.6mA through the BFR93A's b-e, it zeners at 5.8V. > > Two stages of amplification is possibly too much - the noise is > assymetrical around 0V, see this scope photo: > <https://www.dropbox.com/s/bmjnhm988160zjc/NoiseScope%20BFR93A%202.5mA%202xERA3.jpg?dl=0> >I seem to remember something about using two zeners and subtracting them to get symmetric output
Reply by ●April 8, 20202020-04-08
On Wednesday, April 8, 2020 at 4:31:41 AM UTC-4, Clifford Heath wrote:> On 8/4/20 1:51 pm, Clifford Heath wrote: > > On 8/4/20 12:39 pm, Gerhard Hoffmann wrote: > >> Am 08.04.20 um 03:05 schrieb Clifford Heath: > >> I have done that thing with the BFR93A many years ago > > > > Oh, thanks, nice to know I'm on a right track. I basically just guessed > > that the BFR93A should work, not that I saw it elsewhere. > > > >> flat enough for tuning filters. Not good enough to measure > >> noise figure, both for amplitude and source impedance. > >> The breakdown voltage of the BFR93A was 5 or 6V IIRC. > >> I think it went to ~ 2 GHz. > > > > That would be good enough for my first try! > > > >> I used a CCS just like the charger > >> of my HP-35 :-) > > > > I just thought I'd use a resistor, since I have a known supply voltage. > > Not a good idea? > > > >> Today I would not use a "Zener" source. If you have them, spend > >> 2 more ERAs and amplify the noise of a 50 Ohm resistor. > > > > Well now it's interesting, but I have exactly the right amount of room > > left on this proto-PCB for two more, and I have 20 ERA-3's. > > > > It's on a grid-punched board with assorted top-side copper strips. I > > used sticky-backed copper foil to make a ground plane on the back. It > > doesn't seem such a bad way to prototype RF circuits, given that my > > toner-transfer process is in recess (the new printer creates porous > > blacks that don't etch well). > > > >> Yes, they > >> have a noise figure of a few dB but you know it and it's flat. > > > > Assuming the ERA-3 or the layout doesn't filter it, it should stay flat. > > > >> 1K may produce more voltage noise, > >> but @ 50 Ohms you know that the ERAs will behave. > > > > Well I could try 220R and just check it's not oscillating I guess. > > Otherwise 50. > > > >> The problem with noise filter measurements is that the bandwidth > >> of the noise is so large and if you want to see the stop band > >> you need lots of power. Thus, you will have large signal conditions > >> at the output of your noise generator. Getting closer to saturation > >> than 20 dB will damage the crest ratio / noise statistics. > > > > Right. Not sure what that will do to the spectrum though? > > > Ok with 12V, 2.6mA through the BFR93A's b-e, it zeners at 5.8V. > > Two stages of amplification is possibly too much - the noise is > assymetrical around 0V, see this scope photo: > <https://www.dropbox.com/s/bmjnhm988160zjc/NoiseScope%20BFR93A%202.5mA%202xERA3.jpg?dl=0> > > I guess I need to reduce gain somewhat?Hi Cliff, I'm not at all a HF noise guy. (So listen to Gerhard and not me .:^) But the non-symmetry is typical. You might want to look at the signal as a function of the bias current. That is your 'best' knob.> > I haven't investigated the spectrum yet.How fast a 'scope do you have? Have you tried triggering on the noise (pulses) With a DSO you can average the pulses/ noise peaks and get a pseudo-spectrum... at least a guess at the max freq. Good luck, George H.> > >> The ERAs may also need more than 5V if you include the bias resistor? > > They're now biassed as recommended from 12V, with 240R. > > Replacing the BFR93A with 100R drops the noise to under 1mV - still > visible, but will need more amplification (no oscillation visible). One > more stage might be enough gain. > > -- > Clifford Heath > > > I figure they want 40mA, and based on data sheet values for other > > voltages (from 7V up) it looks like they settle around 3.3V, so I > > calculated a bias resistor for 5V and was pretty close. I guess it will > > be more temperature sensitive than it would be with a higher supply > > voltage. > > > > I'm using 0805 resistors but not series inductors, for now. Will that > > affect flatness (rather than just lose some gain)? > > > > Anyhow, I'll raise it to 12V and try that before adding more ERA-3s and > > switching to a resistor. > > > > Many thanks for your advice, > > > > Clifford Heath. > >
