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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