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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.repair
Subject: Re: Signal Generator Source Impedance
Date: Mon, 29 Apr 2024 18:26:11 +0100
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On Sun, 28 Apr 2024 11:06:55 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

>On Sun, 28 Apr 2024 18:23:13 +0100, Cursitor Doom <cd@notformail.com>
>wrote:
>
>>Gentlemen,
>
>Nobody is gentle on Usenet.
>
>>The service manual for this radio I'm re-aligning states to use an RF
>>signal generator with a source impedance of 60 ohms.  I have never
>>heard of such a beast. I have a 50 ohm RF sig gen and a 70 ohm one. I
>>thought that would be all I'd ever need, but now this spanner gets
>>thrown into the works. I'm not having a good day so far!
>>Do I need to make up a suitable impedance transformer, or can I
>>proceed with the 50 or the 70 ohm ones I have, whilst making some
>>allowance for that (and if so, how should that allowance be made?)
>>
>>CD.
>
>There actually is such a generator:
><https://fairradio.com/product/sg-376/>
>"1-1.25 V into 60 ohms (IF) with attenuation to 100 db in 1 or 10 db
>steps."
>
>Such generators appeared when the manufacturer wanted to produce a
>generator that worked reasonably well with both 50 and 75 ohms.
>Hint:  (50 + 75) / 2 = 62.5 ohms
>My guess(tm) is that users were losing (or stealing) the minimum loss
>pads required to go from 50 to 75 ohms with one front panel connector:
><https://www.analog.com/en/resources/technical-articles/catv-minimum-loss-pad-for-75937-measurements.html>
>The mismatch loss from 50 to 75 ohms is only 0.4dB, so you're not
>likely to see much of an effect from using the "wrong" impedance
>unless you're working with microwave frequencies.
>
>A much bigger problem is that 50 and 75 ohm BNC connectors use
>different center pin sizes.  
><https://www.vintage-radio.net/forum/attachment.php?attachmentid=30120&d=1258015846>
>I wrecked the 75 ohm receptacle on a CATV sweep generator by force
>fitting a 50 ohm BNC plug.  Color coding my cables have largely
>prevent a recurrence.  Green tape or nail polish for 50 ohms and
>violet for 75 ohms.

Aha. Saw what you did there, Jeff: resistor colour code; green = 5 But
Violet Gives Willingly. ;-)