Re: Signal Generator Source Impedance

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. ;-)