Cool, thanks for the reply Kevin! That makes fascinating reading, what a great design.
Cheers, Matt ----- Original Message ----- From: "Kevin Custer" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Sunday, December 19, 2004 1:46 AM Subject: Re: [Repeater-Builder] GE Mastr II PLL vs. Multiplier (crystal) exciter in duplex service. Hi Matt, Matt wrote: >Hi Kevin, > >Just a quick question regarding your earlier email - I thought crystal oscillators were cleaner (as far as noise sidebands are concerned ) than PLL types. > Actually it depends on the circuitry associated with crystal. While some better crystal controlled exciters can exhibit lower side-band noise levels than poorly designed PLL units, better PLL designs can offer a considerable improvement in signal spectral purity. >Is this not the case in the GE Mastr II you mention below, as you indicate the PLL type of exciter is better? Is this particular to this design? > Correct, please refer to this information presented by GE and Dave Karr on the subject: <http://www.ka9fur.net/geduplex/duplex.html> If you look at the curves for the highband multiplier exciter, then compare them to the PLL model, you'll see the difference. These are the second and third links under the "Mastr II" section of Dave's page. The multiplier exciter shows needing about 78 dB of noise reduction at 600 kHz, where the PLL model needs about 56 dB at that same spacing, or a difference of 22 dB less transmitter side-band noise reduction for the same performance. >Don't get me wrong here Kevin- I'm not trying to poke holes in what you are suggesting, I just thought xtals would be cleaner? > Actually, in the case of the GE Mastr II PLL exciter, the frequency determining element is still a quartz crystal located in a channel ICOM. The crystal frequency determines the operating frequency just like the multiplier exciter, however, there are elementary differences one needs to understand to see why the PLL exciter is spectrally cleaner. In a typical "Multiplier" exciter, a quartz crystal is used as a reference to determine the operating frequency. In addition to being the frequency reference, the quartz crystal is also the signal generator in which the operating frequency is the direct result of "multiplying" the crystals fundamental frequency a particular number of times. In the GE Mastr II highband exciter, this number is 12. As a result of having but one oscillator, and the math, the frequency is not the only thing multiplied; but also all of the noise on either side of it. Sure, the "Q" or bandwidth of the tuned circuitry helps with the spectral purity, but since they aren't critically sharp (like in a tuned tube PA circuit) noise on either side of the carrier is amplified and multiplied as well as the operating carrier. Since most exciters of this era would operate over several hundred kHz without retuning, one can easily see that circuit Q isn't going to help much with making the signal cleaner. In the GE PLL exciter, a quartz crystal is also used as a reference for determining the operating frequency, but that's where the similarities end. 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. Hope this helps... Kevin Custer >Kevin Custer wrote: > >If you have a GE Mastr II two meter or highband repeater that has inadequate duplexer isolation, change out the exciter to a PLL type and take advantage of the 20+ dB less transmitter side-band noise. Another thing is to consider using a tube type PA deck, like the GE 4EF5A1, with a typical 'multiplier' exciter. This could allow power levels in excess of 200 watts or more without suffering from inadequate transmitter side-band noise suppression. > >Kevin Custer > Yahoo! Groups Links 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: http://docs.yahoo.com/info/terms/
