[Repeater-Builder] Re: GE Mastr II PLL vs. Multiplier (crystal) exciterin duplex service.

[N5IUF]

I don't want to beat a dead horse, or be critical of anyone, or act like a "know it all" but I may be able to shed a little more light on this subject for those who are interested.  Those of you who aren't, just hit the delete key now.   I have designed many different PLL circuits in transmitters and receivers, many of which are still in production today.  I think some people are assuming that because GE developed a VHF PLL exciter that had better phase noise performance over the old high band "multiplier" exciter, that PLL, by nature, has better noise performance.  This is not really the case.  GE's old high band exciter had some inherent "issues" that were the root cause of the poor performance, and a lot of the noise comes from the phase modulator, and not the crystal oscillator circuit is not the entire culprit.     In most cases, for this discussion, a PLL is a closed loop circuit that uses a crystal oscillator to Phase Lock another oscillator.  Because a PLL is a closed loop circuit, any noise transcribed inside the loop is carried throughout.  This means that the phase noise of the reference crystal oscillator directly effects the total output phase noise, just like it does in a "multiplier" design.  In fact, a PLL is like a chain, and is only as good as its weakest link.   When it comes to Phase noise of an oscillator, the higher the Q of the resonant circuit, the better the phase noise.  An LC circuit generally will have a Q of around 100 where a crystal can have a Q of 10,000 to 500,000, thus a crystal oscillator generally yields superior phase noise performance over LC circuits, such as the VCO in the GE PLL exciter.  I have seen instances where engineers have use conventional "multiplier" circuits (fundamentally similar to the old GE highband exciter, without the "issues") to achieve superior phase noise performance over a PLL circuit, because the phase noise of the VCO was the weakest link in the PLL circuit.  The dividers, phase detectors and loop bandwidth also can factor in additional phase noise.   I think GE realized that the old multiplier highband exciter needed a design revisit, and chose a PLL version over a conventional multiplier for more reasons than just enhanced noise performance.  The PLL version requires very little tuning, thus reducing labor and overall cost.  I'm certain that they could have developed a "multiplier" version that exceeds the noise performance of the PLL unit, had they chosen that route.   It is quite possible that the reason GE never came out with a UHF PLL version for that era, may have been due to not being able to beat the performance/cost of the UHF multiplier unit.   The rest of you guys who continued reading...you can hit the delete key now..   Take care,   Chris Hudgins - N5IUF   In a Mastr II PLL exciter you have two oscillators, one is the > crystal oscillator described earlier, and, in addition, a free running > L-C tuned oscillator capable of producing frequencies directly on the > desired operating frequency.  You see, we now have an oscillator that > operates directly on the high-band, at its fundamental frequency; NOT > one that has been 'multiplied' up to it.  The quartz derived oscillator > is used to set the desired operating frequency by holding it still. > This is done by 'locking' the frequency of the free running oscillator > to the quartz derived oscillator.  The advantage here is the same > frequency stability is achieved by the use of the quartz reference, but, > the output signal is much cleaner because it was not 'multiplied' up. > > Although there is more to it than this generalization, you now see why > the PLL exciter is spectrally cleaner than its multiplier counterpart.     Yahoo! Groups Links To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/   To unsubscribe from this group, send an email to: [EMAIL PROTECTED]   Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.