Path: ...!feeds.phibee-telecom.net!weretis.net!feeder8.news.weretis.net!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail From: Jeff Liebermann Newsgroups: sci.electronics.repair Subject: Re: Signal Generator Source Impedance Date: Sun, 28 Apr 2024 11:06:55 -0700 Lines: 50 Message-ID: <4k2t2jdaq5jotm57mvss543qmsk060stnq@4ax.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Trace: individual.net SSBe2bEDB29o1EwdtIBOGggEazeTnjjOfla6DTZk5kQBWjSrUN Cancel-Lock: sha1:xUZ4iUVBiZDLAjgtMXF9xmtw08I= sha256:scndeqPQ+3t0nYYJjaN5WKlIRni69JUT+gb7CjeL17w= User-Agent: ForteAgent/8.00.32.1272 Bytes: 2811 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. -- Jeff Liebermann jeffl@cruzio.com PO Box 272 http://www.LearnByDestroying.com Ben Lomond CA 95005-0272 Skype: JeffLiebermann AE6KS 831-336-2558