On Fri, Sep 23, 2022 at 02:02 PM, John Kolb wrote:
I wonder if the 260 voltmeter circuit could be bootstrapped by lifting the 100M resistor from ground, adding another resistor to ground, and adding a cap from the junction to the636A cathode?
?In about the middle of this page, I see a bootstrapped tube. It looks easy, but, does it change gain, or frequency response? ????? Mikek
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Next step is to measure the measurement. A method is plotting many
measurements and ensure plot shape is appropriate. Added to:
is a Q-Curve of a T50-2 36T #24 DUT. Looks good to me.
John KN5L
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On 9/23/22 1:12 PM, John KN5L wrote: A Q Capacitor prototype is added to:
A rotor shaft ground wire is required. This old vintage capacitor was
too scratchy. Measurements are repeatable. Q of variable capacitor, for
a nominal Q=200 T68-2 core, is slightly greater than a 330pF C0G.
John
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On Fri, Sep 23, 2022 at 02:02 PM, John Kolb wrote:
I wonder if the 260 voltmeter circuit could be bootstrapped by lifting the 100M resistor from ground, adding another resistor to ground, and adding a cap from the junction to the636A cathode?
? Interesting question and beyond my skill level. I may ask that on an electronic design group and see if they have an answer. ?In the end though, I would much prefer to start solid state replacement circuits in the 260A, hopefully even improving it. ?????????????????????????????????????? Mikek
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I wonder if the 260 voltmeter circuit could be bootstrapped by lifting the 100M resistor from ground, adding another resistor to ground, and adding a cap from the junction to the636A cathode?
John KK6IL
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On 9/23/2022 5:49 AM, Mikek wrote: The Boonton 260A manual? has a section on 'Sources of Error' starting on Page 12.
<>
?I'm surprised that the the tube loading is 90M¦¸ at 50kHz but goes down from there until it is only 60k¦¸ at 50MHz.
? I'm sure with modern electronic components and bootstraping, there is a much better circuit than the 535A tube voltmeter used in the 260A.
?Looking at the 4342A manual they say that the correction factor at 1MHz using the 513A Q standard is 1.04, that tells me that even their voltmeter input resistance is 9,300,000¦¸
?This is back to about the same 14% error measuring a 1400Q coil at 800kHz, i mentioned before.
? Feel free to check my math and logic, my 513A Q standard is 253uh, indicated Q of 235 @1MHz. (effective Q of 251 with 7.9pf distributed capacitance)
?I'm here to learn, I've had a thing for Q and Q measuring for over 30 years, it fascinates me.
??????????????????????????????????? Thank, Mikek
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A Q Capacitor prototype is added to:
A rotor shaft ground wire is required. This old vintage capacitor was
too scratchy. Measurements are repeatable. Q of variable capacitor, for
a nominal Q=200 T68-2 core, is slightly greater than a 330pF C0G.
John
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>>>I was thinking the capacitive divider was to reduce the C loading of the voltmeter on the resonant circuit, but it's to reduce the input resistance.<<< I remember a maxim, "when you measure something, you change it". It is officially called the "Observer affect" from the wiki, regarding electronics, (I think it is incomplete and needs more work) "In , and are usually wired in series or parallel to the circuit, and so by their very presence affect the current or the voltage they are measuring by way of presenting an additional real or complex to the circuit, thus changing the and behavior of the circuit itself. Even a more passive device such as a , which measures the wire current without coming into physical contact with the wire, affects the current through the circuit being measured because the ." ?My real world experience is with High Q coils for the AM band, about 240uh with Qs over 1400 at 800khz. This is an R of 1,600,000 at resonance. putting a 10M¦¸ resistor in parallel lowers that to 1,379,310, this would cause a 14% reduction in the Q measurement.   ? The above measurements and coil are wound on 6" polystyrene pipe couplers, TPI were cut on a thread cutting setting lathe. The Boonton 260A manual? has a section on 'Sources of Error' starting on Page 12. ?I'm surprised that the the tube loading is 90M¦¸ at 50kHz but goes down from there until it is only 60k¦¸ at 50MHz. ? I'm sure with modern electronic components and bootstraping, there is a much better circuit than the 535A tube voltmeter used in the 260A. ? ?? Looking at the 4342A manual they say that the correction factor at 1MHz using the 513A Q standard is 1.04, that tells me that even their voltmeter input resistance is 9,300,000¦¸ ?This is back to about the same 14% error measuring a 1400Q coil at 800kHz, i mentioned before. ? Feel free to check my math and logic, my 513A Q standard is 253uh, indicated Q of 235 @1MHz. (effective Q of 251 with 7.9pf distributed capacitance) ?I'm here to learn, I've had a thing for Q and Q measuring for over 30 years, it fascinates me. ??????????????????????????????????? Thank, Mikek
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I was thinking the capacitive divider was to reduce the C loading of the voltmeter on the resonant circuit, but it's to reduce the input resistance. Wouldn't matter if you are measuring 20 ?H coils, but measuring a 1.2H inductor at 25 kHz. XsubL = 188 k¦¸. 10 M¦¸ input R starts to affect accuracy.
As long as the input C is stable, it just adds to the minimum C that can be used for resonance.
I see that Mikek's amplifier is marked 0.25 pF and 100M, in spite of having a 10M FET bias resistor. I guess that bootstrapping increases the R as well as decrease the C.
John KK6IL
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On 9/22/2022 11:24 AM, Mikek wrote: OK, so you are unloading the resonant LC improving (getting closer to) the actual Q.
I thought you were making some improvement to the injection transformer.
?Be aware the HP 400E has, as the manual says, "an input capacitance of 25pf or less on the 1V range."
The 3V range has about 12pf. No info on other ranges.
So that changes your capacitive divider from a 12 to 1 to a 14.5 to 1. Assuming I have any clue, and I might not.
Now we are back to a high input impedance amplifier.
I need to build this, to find the frequency response, but I have two HP 3400A RF meters I need to get working before I start another project.
?It might need a 50 driver on the output.
??????????????????????????????? Mikek
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On Thu, Sep 22, 2022 at 01:29 PM, John KN5L wrote:
An oscilloscope can be used as a RF Voltmeter when using a fixture with
capacitive divider.
That's exactly how I used the Kleijer amp, driving my Oscilloscope and doing 3db BW measurements. ?????????????????????????????????????????????????????????????? Mikek
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An oscilloscope can be used as a RF Voltmeter when using a fixture with
capacitive divider.
Added to section "Injection Transformer, With Capacitive Divider,
Fixture" is a photo using an oscilloscope. Image is at resonance with
Generator set for Oscilloscope 1Vrms. Measuring 0.708Vrms frequencies,
3dB BW, results with expected Q.
John KN5L
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>>>A capacitive divider seems to be a requirement. Additional information here: 10/120 pF divider work well.<<< Yes and nothing wrong with using a capacitive divider, I'm just adding, there are high input impedance amps with bootstrapping that will not add very little capacitance to your tuned circuit and have a higher parallel resistance. I built a Kleijer high input impedance amp several year ago, it uses a 17 to 1 capacitive divider. http://www.crystal-radio.eu/fetamp/enfetamp.htm
The input signal enters the amplifier via a 0.3 pF input capacitor, together with the input capacitance of the FET (T1) this forms a voltage divider, the input signal is attenuated 17 times by a capacitive divider, and then amplifier 17 times before a 50 ohm driver.
?.
If the write up can be believed, it was 0.3pf input and towards 5G¦¸ resistance.
? I've used it on high Q circuits, I can't see any change on my Q meter (after adjusting for the 0.3pf) when I add this in parallel with the resonating C.
?
"The 0.3 pF input capacitor is self-made of two copper plates of 1 square cm at a distance of 3 mm.
By changing the distance between the plates we can adjust the gain of the amplifier.
The plates must have at least 1 cm distance from the surrounding grounded box.
The input signal enters the box via a 1mm copper wire, through a 10 mm hole in the box.
The wire is supported by a piece of polyethylene, which is fixed with nylon screws.
The input amplifier (T1) is screened from the rest of the circuit.
Between the gate (input) of T1 and ground there is a 20 M.Ohm resistor.
But the input resistance of the amplifier is much higher then 20 M.Ohm, in theory even 17? times higher? (so, 5780 M.Ohm), this is because over the 20 M.Ohm resistor is only 1/17th part of the input voltage.
In practice the input resistance will be lower then 5780 M.Ohm because of dielectric losses e.g. in the gate of the FET."
?????????????????????????????????????? It's all fun!? Mikek
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A capacitive divider seems to be a requirement. Additional information
here:
10/120 pF divider work well.
"To RF Vmeter" is connected to HP400E. Using only F-F BNC adapter.
Perform 3dB BW to evaluate Q.
The Injection Transformer, with C divider, fixture can be calibrated for
performing voltage ratio Q measurements.
John KN5L
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On 9/22/22 2:25 PM, Mikek wrote: You have a label, "To RF Vmeter" at the output of your capacitive
divider, is that the HP410E or your RF Voltmeter Prototype?
?I'm looking for caps to make the divider/vmeter and connect it to my
Boonton 260A and see how much it affects Q.
Hoping it is the 400E, I have one, so I could set that up.
?????????????????????????????????????? Thanks, Mikek
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You have a label, "To RF Vmeter" at the output of your capacitive divider, is that the HP410E or your RF Voltmeter Prototype?
?I'm looking for caps to make the divider/vmeter and connect it to my Boonton 260A and see how much it affects Q.
Hoping it is the 400E, I have one, so I could set that up.
?????????????????????????????????????? Thanks, Mikek
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On Thu, Sep 22, 2022 at 11:36 AM, John KN5L wrote:
Schematic of functional RF Volt Meter:
First Section with Title: "RF Voltmeter Prototype" this version has
10/120 pF divider in the schematic.
?? Yes, I saw it, I'm just adding that in can be better, as in 0.25pf and 100M¦¸ input impedance. ?Yours adds 9.3pf to the tuning capacitor, (should use high Q caps) and I have no clue what the parallel resistance would be. ? Just trying to add what I think might be useful info. ????? ? ? ? ? ? ? ? ? ? ? ? ? ? Thanks, Mikek ???????????????????????
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Hi Mikek On 9/22/22 1:24 PM, Mikek wrote: I need to build this, to find the frequency response
?It might need a 50 driver on the output. Schematic of functional RF Volt Meter: First Section with Title: "RF Voltmeter Prototype" this version has 10/120 pF divider in the schematic. John KN5L
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OK, so you are unloading the resonant LC improving (getting closer to) the actual Q. I thought you were making some improvement to the injection transformer. ?Be aware the HP 400E has, as the manual says, "an input capacitance of 25pf or less on the 1V range." The 3V range has about 12pf. No info on other ranges. So that changes your capacitive divider from a 12 to 1 to a 14.5 to 1. Assuming I have any clue, and I might not. Now we are back to a high input impedance amplifier. I need to build this, to find the frequency response, but I have two HP 3400A RF meters I need to get working before I start another project. ?It might need a 50 driver on the output. ??????????????????????????????? Mikek 
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Mikek,
Here is a graph showing the output impedance magnitude vs frequency for various transistors, including the 2N5109.
The last two are microwave transistors and they provide the most constant output Z.
Jacques, VE2AZX
ve2azx.net
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Hi Mikek,
Section "Injection Transformer, With Capacitive Divider, Fixture"
John KN5L
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On 9/22/22 11:36 AM, Mikek wrote: I need a rough schematic, are saying both a 50 to 1 transformer and a
capacitive divider?
?????????????????????????????????????????????? Thanks Mikek
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I need a rough schematic, are saying both a 50 to 1 transformer and a capacitive divider?
?????????????????????????????????????????????? Thanks Mikek
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Hi Mikek and David,
HP 4342A a single 50:1 turn ratio toroid with a range of 22kHz to 70MHz.
The simplest way to make a Q meter is a 50:1 Injection Transformer with
capacitor divider Fixture and use bench test equipment, RF signal
generator and RF Volt Meter. Accurate results can be achieved with this
method.
Driving an Injection Transformer with 50 generator will result with some
amount of Q loss. With increasing loss as DUT Q increases. A correction
chart can be made using modeling.
Options for RF Volt meter include vintage HP-400E and Leader LMV-181.
Both of which are good to 10MHz.
For ability above 10MHz will require development of a RF Volt Meter with
extended frequency range.
John KN5L
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On 9/22/22 10:07 AM, Mikek wrote: On Thu, Aug 4, 2022 at 01:19 AM, Dr. David Kirkby, Kirkby Microwave Ltd
wrote:
A step-down transformer seems the most obvious way, but that
requires a turns ratio of sqrt(50000)=224. Even with a single turn
on the secondary, there will be too many turns on the primary for
this to work at 150 MHz.
Hi David,
Have you any more thoughts on the above conundrum? The only solution I
see is to have two or three transformers to cover the frequency range.
????????????????????????????????????????????????????? Mikek
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On Thu, Aug 4, 2022 at 01:19 AM, Dr. David Kirkby, Kirkby Microwave Ltd wrote:
A step-down transformer seems the most obvious way, but that requires a turns ratio of sqrt(50000)=224. Even with a single turn on the secondary, there will be too many turns on the primary for this to work at 150 MHz.
Hi David, Have you any more thoughts on the above conundrum? The only solution I see is to have two or three transformers to cover the frequency range. ????????????????????????????????????????????????????? Mikek
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John Kolb
