Re: Optimal full scale deflection on analog meters
As mentioned above, it kind of depends on what the meter is expected to be measuring. The 1, 2 and 5 used by Tek works well for scopes because you are looking at a graticule and, at least in the past, always having to do a conversion in your head for the voltage or time represented by the graticule marks. Multiplying by 1, 2 or 5 is pretty simple.? I don't think this reasoning fits analog meters. Tek did make a Type 130 L-C meter and the scales are 1 and 3 lined up at the far right. Looking at my bench, I have a Heathkit M-25 and an HP 410C, they both use the 1.5 and 5 scales. These are general purpose bench VOMs so not focused on any specific type of measurements. The scales make sense for general purposes. The HP has the 1.5 and 5 lined up and the Heathkit has the 1.5 and 5 offset. I think this is just due to the divider design chosen but could have been the lined up if more precision resistors were used.? Also note the center scale zero on the heathkit. This allowed the user to move the needle to the center for zero and read + and - voltages without reversing leads. Most analog meters cannot do this. The real big meter and the ability to read + and - volts on the heathkit make it more useful on the bench.  ???  My Simpson 260 VOM has 1, 2.5 and 5 scales. I think these made sense for the era they were produced, mostly tubes.  My Triplet 630 VOM has a general purpose set of scales; 1.2, 1.5, 3 and 6. Judging by the scales, it is not focused on the tube era. Note filaments were typically 6.3VAC and 12.6VAC which would be off scale on the closest range, but putting them mid scale might be a good compromise. This would have been better if the meter was a little extended to 13V and 7V full scale for tube era measurements on the 12 and 6 scales.  I have several HP AC/DB meters; 3400A, 400A, 400F and 412A AC millivoltmeter. The scales are setup for AC and DB measurements. These meters are 1 and 3.16 full scale. These ranges make sense for AC or DB measurements. In this case you are often thinking about values in dB. In that case you want to the meter to increment scales in 10db step for simplicity in calculations.? My HP 410A has 1 and 3 (though lined up for 1 and 3.16 like other AC meters) scales. While this is a bench VTVM, it was focused on both audio (down to 20hz) and high frequency AC (up to 700Mhz) as a main selling point.  I also have a GW Instek 4278 dual needle/channel AC millivoltmeter. It think it has very useful meter scales. 1.1 and 3.5.? The problem with the 1 and 3.16 scales is that there is no room above to get a measurement that is just beyond the 1 or 3.16. Having a little more scale when you are adjusting something around these numbers means you don't need to move up to the next scale, putting the needle back to the left side of the meter and in a harder to read section of the scale. Also having two channels with dual needles is really useful in stereo design/repair and when designing amplifier stages. I wish the meter was about an inch to two inches wider, like my Heathkit, for a little easier to estimate between the minor divisions.?  Hopefully this shows some options and reasons for picking certain scales.? If I was designing a new "precision" bench analog meter, I would also consider adding range expansion. For example if the indication was near the center of the meter such as 5 on a 10 scale for example, I would like the option to amplify and shift the value to expand the full scale of the needle to indicate something like 2.5V and 7.5V, or 4V and 6V full scale. Could also be a percentage of the indicated value based on where the needle is sitting when the expansion is engaged.? I have seen some meters like this in the past for specialized instrumentation. If the meter was microprocessor driven, this would be an easy option to implement, verses the ways it was done in the past using offset voltages.
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Re: Rise of Boonton's BR-535 tube (valve) for the 260 Q Meter
Yes, that's a good, high impedance buffer. The bootstrapping
suppresses the JFET's Cgd, and the inherent bootstrapping of a
source follower takes care of Cgs. The external bootstrapping also
suppresses the loading by the input bias string.
The main thing to watch out for is dynamic range. As long as the
circuit has enough headroom to handle the largest signals that will
be applied (in the sense of neither saturating nor slew limiting),
you should be in good shape.
--Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
On 8/19/2022 16:30, Mikek wrote:
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I ran across this as a possible start for a circuit to replace the
1659 tube. It's been a while so I can't remember what else in
needs, but I don't think much.
I was looking for a DIY high input impedance probe, I would need
to add a driver to this for 50¦¸ output to drive a cable.
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Re: Rise of Boonton's BR-535 tube (valve) for the 260 Q Meter
I ran across this as a possible start for a circuit to replace the 1659 tube. It's been a while so I can't remember what else in needs, but I don't think much.
I was looking for a DIY high input impedance probe, I would need to add a driver to this for 50¦¸ output to drive a cable.
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Re: Optimal full scale deflection on analog meters
My 630-NA has the range switch positions 0.6, 3, 12, 60, 300, 1200/6000. In addition, it has a switch V/2 or A/2 which doubles the sensitivity, 0.3, 1.5, 6, etc. There are also 120 or 240 mV ranges by placing the range switch in the 1.2 mA position. Thus twice as many ranges by adding a range doubler switch and two extra scales on the meter.
John
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On 8/19/2022 9:40 AM, Dave AA6RE wrote: Meter FS Ranges
It depends on the measurement being done and certainly the may be a change over time as to what is best. Each era has different needs.
A couple examples:
1. Early on operational amplifiers, Op Amps, were mostly dual rail +/-15V. Eventually low voltage dual and single supply options were introduced: +15 V, +/- 5 V, 3 V etc
2. Tube radios often need ranges to measure filament voltage, 6 V/ 12V. Plate voltages were 80 V to 200 V as I recall. Ham transmitter voltages could be higher.
3. System voltage levels in PCs, instrumentation, especially portable, have been on a downward trend as integrated circuit technology has used finer lithography and thus lower voltage capability. PC CPUs operate at lower voltages and so do ASICs.
I imagine the solar power industry and EV batter markets have different needs than 50 years ago too.
Digital DVMs and DPMs originally were based on dual slope integrating ADCs. Resolutions were: 3 1/2 ( 1/2000) digits; 3 3/4 digits? (1/4000); 4 1/2 (1/2000) digits, etc.
Later the DVM/DPM were made with auto-ranging capability so the user just had to connect the test leads,? and the ADC selectred the correct tap on the input divider network to take a measurement from. This made measurements "hands free". It has its pros and cons.
There many be no universal answer. Just the one that meets the objectives of the current in terms of accuracy, cost and user needs.
A lot of hobbyist arduino and RasPi work is done with 5V, 3/3.3V supplies and of course USB. Again anothjer set of needs.
--
Dave, AA6RE
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Re: Optimal full scale deflection on analog meters
Perhaps I missed something but I've seen no mention of 1, 2, 5. This happens to be the scale factors that Tektronix and others have settled on and they certainly seems reasonable to me for a general purpose instrument.
One factor that I would take into consideration is the set of minor scale divisions that result from the choice of
full scale readings. Multiplying fractional or oddball readings in ones head could introduce errors (at least at
my age).
Stephen Menasian
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Re: Optimal full scale deflection on analog meters
On Fri, 19 Aug 2022 at 11:28, ray_g4lua via <raymond.gathergood= [email protected]> wrote: Having used all kinds of multi-meters over a 50 year career, I have concluded that no matter where the range limits are set, you always come across a measurement that? falls around the overlap!
AVO are perhaps the best known manufacturer of analogue meters, back in the day. I have personally owned an AVO Multi-minor, an AVO Model 7, an AVO model 8 and a much rarer "AVO Test Set Multi Range No1" - the Mil Spec version of the AVO 8?(NATO part number 6625-99-105-7050).
The multi-minor, model 7 and Model 8 were all ranged as: 1.0, 2.5,10, 25... etc., up to?2500V FSD
The Test Set Multi Range No1 was different; it's ranges were 1.0, 3.0, 10, 30, 100... up to 3000V FSD
However, my thoughts are 1.0, 2.5, 5.0, 10, 25, 50... would be better; based on the fact the the best accuracy for an analogue meter is above 50% FSD.
Obviously if one has specific uses, then different scales would be useful.? But otherwise, I think if one wants N different full--scales between each decade, then they should be differ by a factor of 10^(1/(N-1)). So optimal values would theoretically be
1, 2.15, 4.64, 10.
To get above 50% of full scale, one would need to go insert another range
1, 1.78, 3.16, 5.62, 10
However, I think that lot would be pretty dam confusing. Could a human brain cope with
1, 1.8, 3.2, 5.6 and 10?
It might make the meter a bit messy, but would allow any reading to be over 50% of the scale.
73 de
Ray G4LUA
_._,_._,_
Dave?
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Re: Optimal full scale deflection on analog meters
?
Meter FS Ranges
?
It depends on the measurement being done and certainly the may be a change over time as to what is best. Each era has different needs.
?
A couple examples:
?
1. Early on operational amplifiers, Op Amps, were mostly dual rail +/-15V. Eventually low voltage dual and single supply options were introduced: +15 V, +/- 5 V, 3 V etc
?
2. Tube radios often need ranges to measure filament voltage, 6 V/ 12V. Plate voltages were 80 V to 200 V as I recall. Ham transmitter voltages could be higher.
?
3. System voltage levels in PCs, instrumentation, especially portable, have been on a downward trend as integrated circuit technology has used finer lithography and thus lower voltage capability. PC CPUs operate at lower voltages and so do ASICs.
I imagine the solar power industry and EV batter markets have different needs than 50 years ago too.
Digital DVMs and DPMs originally were based on dual slope integrating ADCs. Resolutions were: 3 1/2 ( 1/2000) digits; 3 3/4 digits? (1/4000);? 4 1/2 (1/2000) digits, etc.?
Later the DVM/DPM were made with auto-ranging capability so the user just had to connect the test leads,? and the ADC selectred the correct tap on the input divider network to take a measurement from. This made measurements "hands free". It has its pros and cons.
There many be no universal answer. Just the one that meets the objectives of the current in terms of accuracy, cost and user needs.?
A lot of hobbyist arduino and RasPi work is done with 5V, 3/3.3V supplies and of course USB. Again anothjer set of needs.
-- Dave, AA6RE
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Re: Rise of Boonton's BR-535 tube (valve) for the 260 Q Meter
I zapped the input jfet on my HP4342A some years back, due to
static discharge--now I always touch the ground post before
connecting anything to the Q meter posts. I replaced the jfet with
a 2N4416A and the meter recalibrated just fine and works same as
it did before. You can of course look up the datasheet for the
2N4416A, if desired.
73,
Steve AA7U
On 8/19/2022 4:19 AM, Mikek wrote:
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On Thu, Aug 18, 2022 at 10:49 PM, John Kolb wrote:
Following the 2 pF cap C3 is a 56 pF feedthru cap, C4,
to ground, so the capacitive divider is 2 pF/(56 pF + Q1 input C
+ C1.3 pF, in series with 2.87K).
?Assuming a 1970s build, what would be a reasonable guess at the
gate capacitance of that vintage FET? I think HP would pick a low
gate capacitance.
I'll try to do an excel spread sheet over the frequency range of
the instrument. Anything else that I should add into the equation?
Other than strays.
????????????????????????????????????????????????????? Thanks,
Mikek
??????????????????????????????????????????????????????????????????????????
Thanks, Mikek
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Re: Optimal full scale deflection on analog meters: What would HP DO??
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Re: Rise of Boonton's BR-535 tube (valve) for the 260 Q Meter
TYPO:
Original quote should have read
2 pF/(56 pF + Q1 input C + C1, 3 pF, in series with 2.87K).
Why the 3 pF/2.87K RC network? 18.5 MHz corner freq. Looks like it lowers the output of the capacitive divider at high freqs where one might expect the following amplifier to be falling off at igher freqs and need the input boosted, not cut.
John
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On 8/19/2022 4:43 AM, Mikek wrote: On Fri, Aug 19, 2022 at 04:19 AM, Mikek wrote:
Following the 2 pF cap C3 is a 56 pF feedthru cap, C4, to ground, so
the capacitive divider is 2 pF/(56 pF + Q1 input C + C1.3 pF, in
series with 2.87K).
I'm not getting it, maybe math failure (on my part). I calculated at 1MHz. Reactance of 56pf = 2,844¦¸, 1.3pf = 122,489¦¸, (FET guess) 5.2pf = 30,622 and the R = 2870¦¸.
?Total = 158,825 / 2pf reactance of 79,618 = 0.5¦¸.? That does not seem right!
Any ideas?
Just noticed a small error, 1.3pf reactance and 2,870¦¸ do not add directly, I'll ignore it as it's small.
???????????????????????????????????????????????????????????? Thanks, Mikek
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Re: Rise of Boonton's BR-535 tube (valve) for the 260 Q Meter
On Fri, Aug 19, 2022 at 04:19 AM, Mikek wrote:
Following the 2 pF cap C3 is a 56 pF feedthru cap, C4, to ground, so the capacitive divider is 2 pF/(56 pF + Q1 input C + C1.3 pF, in series with 2.87K).
I'm not getting it, maybe math failure (on my part). I calculated at 1MHz. Reactance of 56pf = 2,844¦¸, 1.3pf = 122,489¦¸, (FET guess) 5.2pf = 30,622 and the R = 2870¦¸. ?Total = 158,825 / 2pf reactance of 79,618 = 0.5¦¸.? That does not seem right! Any ideas? Just noticed a small error, 1.3pf reactance and 2,870¦¸ do not add directly, I'll ignore it as it's small. ???????????????????????????????????????????????????????????? Thanks, Mikek
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Re: Optimal full scale deflection on analog meters
My Marconi SHF power meter has what
looks like sqrt(10) for the secondary scale, marked up to 3, and
that corresponds to 9.5 on the 0-10 scale, which does suggest full
scale would be 3.1623-ish. My HP435B has the same ratio, as does
the HP3400A.
For a new design, if there is enough
scale area, Ray's suggestions make better sense. Most of my
equipment has digital or graphic displays, but I still like a
pointy meter when I'm making fine manual adjustments to things
under test.
--
Neil G4DBN
On 19/08/2022 11:28, ray_g4lua via
groups.io wrote:
toggle quoted message
Show quoted text
Having used all kinds of multi-meters over a 50 year career, I
have concluded that no matter where the range limits are set,
you always come across a measurement that? falls around the
overlap!
AVO are perhaps the best known manufacturer of analogue meters,
back in the day. I have personally owned an AVO Multi-minor, an
AVO Model 7, an AVO model 8 and a much rarer "AVO Test Set Multi
Range No1" - the Mil Spec version of the AVO 8?(NATO part number
6625-99-105-7050).
The multi-minor, model 7 and Model 8 were all ranged as: 1.0,
2.5,10, 25... etc., up to?2500V FSD
The Test Set Multi Range No1 was different; it's ranges were
1.0, 3.0, 10, 30, 100... up to 3000V FSD
However, my thoughts are 1.0, 2.5, 5.0, 10, 25, 50... would be
better; based on the fact the the best accuracy for an analogue
meter is above 50% FSD.
Obviously commercially produced meters had to be built to a
price, so compromises were made; i.e. limit the number of
components and use minimal number of ways on the range switch.
If you want to produce something for your self as a one-off then
maybe those commercial pressures don't exist!
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Re: Optimal full scale deflection on analog meters
On Fri, 19 Aug 2022 at 11:28, ray_g4lua via <raymond.gathergood= [email protected]> wrote: Having used all kinds of multi-meters over a 50 year career, I have concluded that no matter where the range limits are set, you always come across a measurement that? falls around the overlap!
AVO are perhaps the best known manufacturer of analogue meters, back in the day. I have personally owned an AVO Multi-minor, an AVO Model 7, an AVO model 8 and a much rarer "AVO Test Set Multi Range No1" - the Mil Spec version of the AVO 8?(NATO part number 6625-99-105-7050).
The multi-minor, model 7 and Model 8 were all ranged as: 1.0, 2.5,10, 25... etc., up to?2500V FSD
The Test Set Multi Range No1 was different; it's ranges were 1.0, 3.0, 10, 30, 100... up to 3000V FSD
However, my thoughts are 1.0, 2.5, 5.0, 10, 25, 50... would be better; based on the fact the the best accuracy for an analogue meter is above 50% FSD.
Obviously commercially produced meters had to be built to a price, so compromises were made; i.e. limit the number of components and use minimal number of ways on the range switch. If you want to produce something for your self as a one-off then maybe those commercial pressures don't exist!
73 de
Ray G4LUA
Obviously, within reason, more ranges should give you better accuracy, as you can make more use of the scale. So I would agree that 1.0, 2.5, 5, 10, 25 ... etc is better than either 1, 3, 10, 30 or 1, 2.5, 10, 25,
But sticking to no more than one extra range covering a decade, is 2.5, 3, 4.5, 5.5 or whatever else best for the intermediate range between 1 and 10?
There seems a few things that limit our ability to make an accurate measurement
* The error at full scale.
* An error due to our estimation of reading the scale - a mirror on the scale helps there.
* Non linearity of the scale.?
I agree with Geff, G8HUL, that what is optimal for one person will not be another. But I suspect there is an optimal middle range if the inputs are randomly distributed between 0 and 1000 V. What gives the best worst case error? What gives? the best RMS error? I might write a little Monte Carlo simulation program and see what that comes up with. As a starting point, I could assume that the meter has 100 divisions, and a person can read to +/- 1 division.
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Re: Rise of Boonton's BR-535 tube (valve) for the 260 Q Meter
On Thu, Aug 18, 2022 at 10:49 PM, John Kolb wrote:
Following the 2 pF cap C3 is a 56 pF feedthru cap, C4, to ground, so the capacitive divider is 2 pF/(56 pF + Q1 input C + C1.3 pF, in series with 2.87K).
?Assuming a 1970s build, what would be a reasonable guess at the gate capacitance of that vintage FET? I think HP would pick a low gate capacitance. I'll try to do an excel spread sheet over the frequency range of the instrument. Anything else that I should add into the equation? Other than strays. ????????????????????????????????????????????????????? Thanks, Mikek ?????????????????????????????????????????????????????????????????????????? Thanks, Mikek
|
Re: Optimal full scale deflection on analog meters
Having used all kinds of multi-meters over a 50 year career, I have concluded that no matter where the range limits are set, you always come across a measurement that? falls around the overlap!
AVO are perhaps the best known manufacturer of analogue meters, back in the day. I have personally owned an AVO Multi-minor, an AVO Model 7, an AVO model 8 and a much rarer "AVO Test Set Multi Range No1" - the Mil Spec version of the AVO 8?(NATO part number 6625-99-105-7050).
The multi-minor, model 7 and Model 8 were all ranged as: 1.0, 2.5,10, 25... etc., up to?2500V FSD
The Test Set Multi Range No1 was different; it's ranges were 1.0, 3.0, 10, 30, 100... up to 3000V FSD
However, my thoughts are 1.0, 2.5, 5.0, 10, 25, 50... would be better; based on the fact the the best accuracy for an analogue meter is above 50% FSD.
Obviously commercially produced meters had to be built to a price, so compromises were made; i.e. limit the number of components and use minimal number of ways on the range switch. If you want to produce something for your self as a one-off then maybe those commercial pressures don't exist!
73 de
Ray G4LUA
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Re: Optimal full scale deflection on analog meters
What is 'optimal' to one user would be annoying to another. So it depend on what the use of the meter is and the preference of the person using it.
Your criteria for what is optimal may not fit with someone else's.
For example having a range that stopped at 4.5V might be very annoying for someone who deals with a lot of 5V supply rails, etc etc etc.
Regards
Jeff G8HUL
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-----Original Message----- From: [email protected] < [email protected]> On Behalf Of Dr. David Kirkby, Kirkby Microwave Ltd Sent: 19 August 2022 10:14 To: [email protected]Subject: [Test Equipment Design & Construction] Optimal full scale deflection on analog meters Consider you built a volt meter with an analog scale, and chose full-scale deflections of 1, 10, 100 and 1000 V. Then you decided that the gaps were too big, so would add intermediate full scale deflections, so have 1, x, 10, y, 100, z, 1000 V. What are the optimal values of x, y and z? My gut feeling is that the optimal value of x is sqrt(10) = 3.16, because the ratio between 1 and 3.16 is the same as between 3.16 and 10. One would probably round them to 1, 3, 10, 30, 100, 300 and 1000 V. However, I am not sure about this. I have seen some instruments with FSDs of 1, 2.5, 10, 25, 100, 250, 1000 V. If one assumes that all voltages between 1 and 10 are equally likely to be present, their mean would be 4.5, so scales of 1, 4.5, 10, 45, 100, 450 and 1000 would be optimal. But I don¡¯t think that¡¯s optimal. I looking at a design where values could be between 0 and 100. The full scale deflections were 20 and 100, which didn¡¯t seem right to me. Any thoughts? -- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@... <mailto:drkirkby@...> Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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Re: Optimal full scale deflection on analog meters
David,? Probably not directly addressing your design intent, but let's note for digital units typically a 2V and multiples scale is typically present (due to display configuration, which oftentimes only has an "I" first digit. Radu.?
On Fri, Aug 19, 2022, 12:14 PM Dr. David Kirkby, Kirkby Microwave Ltd < drkirkby@...> wrote: Consider you built a volt meter with an analog scale, and chose full-scale deflections of 1, 10, 100 and 1000 V. Then you decided that the gaps were too big, so would add intermediate full scale deflections, so have?
1, x, 10, y, 100, z, 1000 V.
What are the optimal values of x, y and z?
My gut feeling is that the optimal value of x is sqrt(10) = 3.16, ?because the ratio between 1 and 3.16 is the same as between 3.16 and 10. One would probably round them to?
1, 3, 10, 30, 100, 300 and 1000 V. However, I am not sure about this. I have seen some instruments with FSDs of
1, 2.5, 10, 25, 100, 250, 1000 V.
If one assumes that all voltages between 1 and 10 are equally likely to be present, their mean would be 4.5, so scales of? 1, 4.5, 10, 45, 100, 450 and 1000 would be optimal. But I don¡¯t think that¡¯s optimal.
I looking at a design where values could be between 0 and 100. The full scale deflections were 20 and 100, which didn¡¯t seem right to me.?
Any thoughts? --
Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@...Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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Optimal full scale deflection on analog meters
Consider you built a volt meter with an analog scale, and chose full-scale deflections of 1, 10, 100 and 1000 V. Then you decided that the gaps were too big, so would add intermediate full scale deflections, so have?
1, x, 10, y, 100, z, 1000 V.
What are the optimal values of x, y and z?
My gut feeling is that the optimal value of x is sqrt(10) = 3.16, ?because the ratio between 1 and 3.16 is the same as between 3.16 and 10. One would probably round them to?
1, 3, 10, 30, 100, 300 and 1000 V. However, I am not sure about this. I have seen some instruments with FSDs of
1, 2.5, 10, 25, 100, 250, 1000 V.
If one assumes that all voltages between 1 and 10 are equally likely to be present, their mean would be 4.5, so scales of? 1, 4.5, 10, 45, 100, 450 and 1000 would be optimal. But I don¡¯t think that¡¯s optimal.
I looking at a design where values could be between 0 and 100. The full scale deflections were 20 and 100, which didn¡¯t seem right to me.?
Any thoughts? -- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@...Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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Re: Rise of Boonton's BR-535 tube (valve) for the 260 Q Meter
It helps as a sanity check to remember that the grid-cathode
conductance increases quadratically with frequency, if the plate is
terminated in a low impedance. By the time you approach ft (here,
about 100MHz, roughly), the conductance becomes similar to gm in
magnitude (again, roughly).
Or, more simply: The input conductance is small only at a small
fraction of ft.
--Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
On 8/19/2022 01:45, Chuck Moore via
groups.io wrote:
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Show quoted text
Bingo Ken
Thanks for spotting and correcting that. Makes a big
difference.
Regards
Chuck WD4HXG
Hello Chuck,
I think you've misread the chart, at 50 MHz it shows
the input resistance to be 55K ohms.
Ken g8beq.
On 18/08/2022 19:34, Chuck
Moore via groups.io wrote:
The VTVM is connected across the variable capacitor
and the input of the VTVM is way
up there impedance wise, up until around 50 MHz were
Boonton designers decided the
accuracy was no longer sufficient.? The graph below
displays the input resistance of the
Q Meter VTVM as a function of resistance. At the low
end, it is nearly 100 MegOhms. By
50 MHz the input resistance drops to about 55
MegOhms. Still pretty darn respectable.
<Graph.PNG>
The circuit of the Q Meter VTVM is displayed below:
<Q%20Meter%20VTVM.PNG>
The area circled in red is where the inductor is
connected during test
and captures the resonating variable capacitor.
Obviously the inductor and variable capacitor are in
series. But initially
I thought the 0.02 Ohm precision resistor was in
series with the inductor
and variable. Actually the precision resistor is in
parallel with the series
tuned inductor and variable cap. The circuit from
the manual was
redrawn to make it easier to recognize what is
happening.
<Q%20Meter%20lashup.PNG>
The meter measuring the voltage across the variable
capacitor is scaled to
read Q despite it being a single range voltmeter.
The scale on the meter
is a fixed slide rule of sorts in that the scaling
is based on the ratio of the
measured voltage to the 20 milliVolts across the
0.02 Ohm resistor. The
premise of the measurement is that the numeric value
of the ratio of either
the voltage across the inductor or the capacitor at
resonance, is the Q. If
the voltage appearing across the variable capacitor
is 2 Volts then:
??????? 2 volts/0.02 volts =100.
Professional draftsmen generally drew the formal
published schematics
using hand drawings/sketches provided by engineers.
The draftsman's
focus when drawing the schematic was to get all the
pieces on a sheet
of paper in compact of a space as possible. Not
being trained to organize
the pieces in an order that logically lumps the
individual pieces according
to function, the draftsman would readily place an
audio amp adjacent to
and RF Amp on a schematic if it allowed a smaller
space to be used.
Unless the engineer hovered over the draftsman's
shoulder, the draftsman
filled the page with symbols and connected them with
lines that matched
the connections provided in the rough hand sketch
from the engineer.
The assumption was the service tech would be savvy
enough to deduce
the circuit topology.
Regards
Chuck WD4HXG
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Re: Rise of Boonton's BR-535 tube (valve) for the 260 Q Meter
Bingo Ken
Thanks for spotting and correcting that. Makes a big difference.
Regards
Chuck WD4HXG
On Aug 19, 2022, at 3:56 AM, Kenneth Greenough via groups.io <g8beqglossop@...> wrote:
Hello Chuck,
I think you've misread the chart, at 50 MHz it shows the input
resistance to be 55K ohms.
Ken g8beq.
On 18/08/2022 19:34, Chuck Moore via
groups.io wrote:
The VTVM is connected across the variable capacitor and the
input of the VTVM is way
up there impedance wise, up until around 50 MHz were Boonton
designers decided the
accuracy was no longer sufficient.? The graph below displays the
input resistance of the
Q Meter VTVM as a function of resistance. At the low end, it is
nearly 100 MegOhms. By
50 MHz the input resistance drops to about 55 MegOhms. Still
pretty darn respectable.
<Graph.PNG>
The circuit of the Q Meter VTVM is displayed below:
<Q%20Meter%20VTVM.PNG>
The area circled in red is where the inductor is connected
during test
and captures the resonating variable capacitor.
Obviously the inductor and variable capacitor are in series. But
initially
I thought the 0.02 Ohm precision resistor was in series with the
inductor
and variable. Actually the precision resistor is in parallel
with the series
tuned inductor and variable cap. The circuit from the manual was
redrawn to make it easier to recognize what is happening.
<Q%20Meter%20lashup.PNG>
The meter measuring the voltage across the variable capacitor is
scaled to
read Q despite it being a single range voltmeter. The scale on
the meter
is a fixed slide rule of sorts in that the scaling is based on
the ratio of the
measured voltage to the 20 milliVolts across the 0.02 Ohm
resistor. The
premise of the measurement is that the numeric value of the
ratio of either
the voltage across the inductor or the capacitor at resonance,
is the Q. If
the voltage appearing across the variable capacitor is 2 Volts
then:
??????? 2 volts/0.02 volts =100.
Professional draftsmen generally drew the formal published
schematics
using hand drawings/sketches provided by engineers. The
draftsman's
focus when drawing the schematic was to get all the pieces on a
sheet
of paper in compact of a space as possible. Not being trained to
organize
the pieces in an order that logically lumps the individual
pieces according
to function, the draftsman would readily place an audio amp
adjacent to
and RF Amp on a schematic if it allowed a smaller space to be
used.
Unless the engineer hovered over the draftsman's shoulder, the
draftsman
filled the page with symbols and connected them with lines that
matched
the connections provided in the rough hand sketch from the
engineer.
The assumption was the service tech would be savvy enough to
deduce
the circuit topology.
Regards
Chuck WD4HXG
|
John Kolb
Dr. David Kirkby, Kirkby Microwave Ltd
Terry King