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 Post subject: Step Attenuator on a IG-42
PostPosted: Nov Mon 18, 2019 3:20 pm 
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Please look at the scale below and tell me if i have this correct. And if i do please explain the question below.

Full scale meter reading obtained by using the fine attenuator

X1 = 10 uv x 1 = 10 uv = .00001 volt
X10 = 10 uv x 10 = 100 uv = .0001 volt
X100 = 10 uv x 100 = 1,000 uv = .001 volt
X1K = 10 uv x 1K = 10,000 uv = .01 volt
X10K = 10 uv x 10K = 100,000 uv = .1 volt = max output

Why does the highest attenuator setting = the highest output seems backwards to me.
Thanks

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Mon 18, 2019 3:33 pm 
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Cadmandu wrote:
Why does the highest attenuator setting = the highest output seems backwards to me.

Think of the legends as the meter scale multiplier, like on a VOM, rather than in terms of attenuation factor.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Mon 18, 2019 4:55 pm 
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Thanks Steve still sounds like a miss named control to me. It's like electron flow be current flow

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Mon 18, 2019 5:12 pm 
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One of the main applications Heath had in mind for the IG-42 was receiver sensitivity measurements. For that application you would want to know how many microvolts of signal the generator is producing, so the control is marked off in microvolts. If you look at it that way it is not backwards and makes perfect sense. It is also surprisingly accurate if you calibrate it against an RF voltmeter as suggested in the manual.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Mon 18, 2019 5:39 pm 
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Thanks Chris good info. Can a scope measure the output voltage ?
RF volt meters are very expensive I like the Bolton 91b but the probes are hard to find

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Mon 18, 2019 6:56 pm 
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On a typical scope like a Tek 465, the lowest range on the vertical amps is 5 mV/division with a straight probe (not a x10 one). That gives you 1 mV per minor tick mark. You might get away with it at low frequencies on the X1,000 and X10,000 output ranges of the generator but probably not on the three lower ranges. At higher frequencies you'd be fighting roll-off in the scope and elsewhere.

For now I'd just use the basic IG-42 calibration procedure in the manual without fancy test equipment; it should get you close enough for everyday use. One of the things I like to do is measure the sensitivity of radios before and after working on them; since that is a relative measurement, the meter on the IG-42 doesn't have to be exactly on the money. You'd be surprised how many times a theoretical "improvement" leads to worse performance than before. If you really want an RF voltmeter, hang in there; good used Boonton and Millivac units with probes do still turn up from time to time. Just be sure of your right-of-return before plunking down your hard earned cash; the probes can easily be damaged if more than a few volts is applied to them. They contain custom, matched diodes so replacement is not a simple matter if you get one that has been "gazomped."

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Tue 19, 2019 3:16 pm 
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Thanks Chris i just bought a Millivac MV-723B for 60.00 include probes and shipping. I have a Tek 453 and a Hitachi V-352F


Attachments:
Millivac MV-723B.docx [11.96 KiB]
Downloaded 4 times

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Wed 20, 2019 3:28 pm 
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Well I used my Tek 453 yesterday to monitor the output of my IG-42. I ran a BNC cable from the Ch 1 out to my Fluke 1935A counter and a bnc cable from the output of my IG-42 to Ch 1 of the 543. I setup the RF gen to put out .1 volt by setting the fine attn to 10 uv FS on meter and thr step attun to X10K which should be 100,000 uv or .1 volt. I then set the scope to .1 volts/Div and the time/div to .2 us/div.
In my understanding of what the outcome should be is as follows. I should have a sign wave of just one division high but im getting alittle over 2.2
Is my generator putting out .2 volts or is my scope wrong? Both channels displayed the same thing.

The scope still has the original filter caps and the IG-42 is untouched except for a bnc that i installed. I did some voltage readings on the generator that i would like to talk about also. Can i ask about them in this post?


Attachments:
Voltage Reading IG-42.xlsx [13.85 KiB]
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Last edited by Cadmandu on Nov Wed 20, 2019 4:50 pm, edited 1 time in total.
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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Wed 20, 2019 4:31 pm 
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Sounds like it is working exactly the way it is supposed to. The generator is designed to work into a 50 ohm load. If you do not have it terminated into 50 ohms, the output levels will be double what you expect. Attenuators are passive networks, so the load impedance always has to be known (and present) in order to get the expected power or voltage level out. You can do the node or mesh equations to demonstrate that the level will always be double if the load on an attenuator is much higher than the nominal impedance or an open circuit.

If you happen to have a half watt or one watt, 47-ohm carbon composition resistor, try throwing it across the generator output at the end of the cable. (A "T" fitting and a commercial terminator could be used if you have them). That should get you within a few percent of the expected output level.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Wed 20, 2019 4:56 pm 
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Thanks Chris that makes me very happy. I will try again with a tek 50 0hm term.
Please review my spread sheet and tell me why the grid voltage on the 12AU7 is 14 vdc and not 30 vdc. This is the modulator half of the tube.
Should i test it with the MCW on at 30%
The grid of the AF Osc pin 2 is not reading anything should be -1
Thanks for all your help.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 12:10 pm 
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Do the audio oscillator and modulator work when you turn them on? I'd expect that at least some of the tube voltages vary considerably with control settings. Sometimes (but not always) they tell you what positions the controls should be in to get those voltage readings, also what kind of meter was used.

With analog circuits, voltage readings seen in service literature are not necessarily exact unless a tolerance is stated. They depend on the tolerances of the components. So if the resistors and capacitors are 10% parts, and tubes are within 20% to 40% on their various parameters, the voltages you measure may differ from the schematic a few volts one way or the other. That does not explain a difference of 54% (14 volts instead of 30), but it could explain 0 V instead of -1 V.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 2:39 pm 
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Hi Chris
I used a Fluke 73 III DVM
The MCW works because i saw its waveform on my scope. But it does not modulate past 20%
I think my 12AU7 is weak. I will change it out and try again with the readings.
My Millivac MV-723B RF Milli Voltmeter will be here Fri can I connect the output of my IG directly to the meter?

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 4:35 pm 
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I was talking about the meter Heath used when they took the voltage reading. Most likely it was one of their own VTVMs, which would have had an 11-megohm input resistance on DC volts. I don't think a Fluke 73 comes quite as high, which might make a difference in very high impedance circuits. You should be able to turn the modulation up to 50%, so if it is not going there it could be a weak 12AU7, or perhaps a resistor or two that have drifted out of spec. Your other voltages looked pretty good so I would not suspect low B+ is the problem.

The Millivac 723B should not be bothered by connecting it to the output of the signal generator; top range is 10-V full scale and 1-mV full scale on the lowest range. This will not give you a very high degree of accuracy at 1-uV or 10-uV, but you should be able to calibrate the IG-42 meter at a higher output level, then verify that the attenuator in the generator is dropping you the proper amounts when you switch to the lower ranges.

Check your MV-723B probe to see if it has a plain unterminated cap or one with a built-in 50-ohm termination; both were available when the meter was new. If the cap is unterminated you need to provide a termination externally in order to get the correct levels. A terminated cap should not be terminated a second time for reasons that should be obvious.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 7:12 pm 
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Cadmandu wrote:
I setup the RF gen to put out .1 volt by setting the fine attn to 10 uv FS on meter and thr step attun to X10K which should be 100,000 uv or .1 volt. I then set the scope to .1 volts/Div and the time/div to .2 us/div.
In my understanding of what the outcome should be is as follows. I should have a sign wave of just one division high ...

Not quite. The output of the signal generator is calibrated in RMS volts. Assuming a sine wave output, 0.1 volts RMS is 0.1 x 1.414 x 2 ≈ 0.28 volts peak to peak. Your scope should show a sine wave that is 2.8 divisions in height from bottom to top, 1.4 divisions both above and below the 0 volt baseline.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 7:30 pm 
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Thanks is that with the 50 ohm terminal ation. Does it say that in the manual.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 7:33 pm 
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Cadmandu wrote:
Thanks is that with the 50 ohm terminal ation.

Yes.
Cadmandu wrote:
Does it say that in the manual.

I don't know. Probably not.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 7:38 pm 
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So all rf gen do rhat

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 8:09 pm 
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Cadmandu wrote:
So all rf gen do rhat

Most post-WWII calibrated RF signal generators rate their output in dBm (power into 50 ohms, relative to 1 mW). Yours happens to rate its output in milliVolts. Conventionally that would mean RMS milliVolts, unless the manual for the instrument specifies otherwise.

Now your oscilloscope shows the entire waveform, so if you measure the distance on the scope graticule from the bottom of the sine wave to the top, that's peak to peak voltage. For a sine wave, peak to peak voltage is related to RMS voltage by a factor of 2.818.

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 8:39 pm 
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Does it take special circuit to put out peak to peak

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 Post subject: Re: Step Attenuator on a IG-42
PostPosted: Nov Thu 21, 2019 10:55 pm 
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Cadmandu wrote:
Does it take special circuit to put out peak to peak

I don't understand what you mean. RMS and peak to peak are just two ways of specifying a measurement of a signal, there's nothing different about the output from the signal generator.

With regard to the AC range on meters: some DMMs measure RMS volts, and some measure average volts, and some VTVMs measure peak-to-peak and some measure the positive peak and some measure the negative peak, and some measure the average voltage. Here, it does take a special circuit to make a particular type of measurement.

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