Path: ...!weretis.net!feeder9.news.weretis.net!feeder8.news.weretis.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Cursitor Doom Newsgroups: sci.electronics.repair Subject: Re: Signal Generator Source Impedance Date: Mon, 29 Apr 2024 18:26:11 +0100 Organization: A noiseless patient Spider Lines: 46 Message-ID: References: <4k2t2jdaq5jotm57mvss543qmsk060stnq@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Mon, 29 Apr 2024 19:26:12 +0200 (CEST) Injection-Info: dont-email.me; posting-host="cac28d9792094f37962d482f9c07d1b9"; logging-data="1981016"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+2/uDZhYm8Sl9J2u2PWhZPUr418Kul9ks=" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:Dzgy+4jjCMu3/dmkn6RtQuCZ2tA= Bytes: 3124 On Sun, 28 Apr 2024 11:06:55 -0700, Jeff Liebermann wrote: >On Sun, 28 Apr 2024 18:23:13 +0100, Cursitor Doom >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: > >"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: > >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. > >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. ;-)