Re: PLL Lock to an Offset Frequency

Jim Thompson wrote: 

OK.  Now I follow you.  Somehow I missed the "offset loop".

I like it!

I've seen similar things done using a D-Flop as a harmonic mixer.

Yes, I've done that back in the palmy days--about 1982, in my first engineering job (me with my new astronomy B. Sc.), I had to design and prototype a pilot tone generator at 70 +- (10/22) MHz--i.e. 70.45454545... and 69.54545454... MHz, don't ask me why, I didn't design the frequency plan. IIRC I used a 74LS293 as a divide-by-11 on the 10 MHz reference signal (jam reset on 12-count) followed by a divide-by-two to get 50% duty cycle, and clocked both halves of a 74S74 with the same 10 MHz. Those were the harmonic mixers for two offset 1:1 PLLs at 454.5... kHz, using Mini-Circuits RPD-1 diode bridge phase detectors, rather than PFDs. I really liked RPD-1s, because you got about a volt out of them, and they were really quiet.

The S74 output was very noisy due to metastability--the trailing edge jitter was especially non-pretty, and once in awhile it missed a pulse. I got round this by using a homemade VCXO on each side, and keeping the loop BW small. This was okay for this board, since the difference between the two pilot tones was going to be frequency-multiplied and used as the frequency reference for a 14 GHz direct-broadcast satellite system, and the PTG only had to be in the base station, not the remotes. (I think this was the first commercial DBS ever--it went on sale in early 1984.)

The spectrum analyzer trace of the signal from that S74 harmonic mixer was not a pretty sight--it jiggled around like a water fountain. I certainly wouldn't use that technique again--XORs work as harmonic mixers too, though you don't get the same signal amplitude of course.

It may be that using more recent logic parts and choosing the locking phase very carefully would keep the metastability under control better than I was able to do. As an 10:1 harmonic mixer at 70 MHz, the edges were just too close together for STTL.

Cheers,

Phil Hobbs
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