Result of AM anti-phase waveforms

Ken Jacobs · 2011-12-26T23:42:59+00:00

Can anyone please advise what the outcome would be of having twosinewaves of equal amplitude and frequency, and having one amplitudemodulate the other?I suspect it may be a uniformly rising and falling DC level, but Idon't have the test gear to confirm it, and the software sim isuseless.Thanks for any help.Ken Jacobs

Allan Herriman 14 years ago

That's a neat trick - you get swept acquisition with the addition of only a couple of resistors.

The positive feedback is always fighting the negative feedback and the loop will lock with a non-zero steady state phase error at the input of the phase detector. This may have bad (higher reference spurs) and good (avoiding dead zones) effects.

About 20 years back I saw that described as the "standard self sweeping loop filter" arrangement. It was regarded as old then, and I don't know who designed it originally. I am unaware of any relevant patents.

I think I only used it once in a design - it's only useful when the phase detector lacks a frequency discriminator mode (which would otherwise lead to long pull-in times).

Regards, Allan

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Phil Hobbs 14 years ago

I wouldn't be at all surprised to hear that it was invented long before my time. It was new to me, though--this was just about the first PLL I'd ever laid eyes on, though I'd heard of them. With my brand new physics & astronomy degree, and only a hobby electronics background, I lucked into a job at a now-defunct outfit called Microtel Pacific Research, which was part of the local phone company, BC Tel. I was solely responsible for about 2/3 of the time and frequency control electronics for the first commercial direct-broadcast satellite system, Spacetel from AEL Microtel. The FM noise spec was 5 Hz RMS in a 10-100 Hz bandwidth around a 14-GHz carrier. I had no idea how hard that was when I started, but it eventually worked fine.

(And you can use a MOSFET driven by the lock detector to ground out the positive FB if you like--that's 3 parts instead of 2, but it gets rid of the small DC offset.)

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net

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Fred Bloggs 14 years ago

It takes like five minutes to find US 4,206,420 Phase Locked Loop with Automatic Sweep, maybe a good start.

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