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Kevin,
I now realize the mistake
I was making in looking at this. I am used to seeing newer PLL's with
a much higher reference oscillator frequency and having a divider in the
reference side before the Phase Comparator. In that case the stability
does improve over that of the reference oscillator, but that obviously does not
apply here.
Sorry, I will shut up now.
Wade - KR7K
----- Original Message -----
Sent: Sunday, December 19, 2004 4:16 PM
Subject: Re: [Repeater-Builder] GE Mastr II PLL vs. Multiplier
(crystal) exciter in duplex service.
Wade,
If the PLL reference from the crystal is X3, and the
VCO sample has been divided by 4, what is the product of 3 times 4? The
answer is 12. If the transmitter was any more stable in frequency than the
reference, shouldn't one think the stated frequency stability would be better
than 2 PPM or 5 PPM, which is the stability of the ICOM itself? Many times
the manual states the VCO is locked to the 12th multiple of the ICOM. This
means the output of the GE PLL exciter will have the stability of the ICOM,
times 12, period.
Kevin Custer
Wade Lake wrote:
I stand corrected,
in part anyway. In this GE radio the deviation is indeed at a divide by
12 from the output. This is why I said "usually", I am not familiar with
the intricate details of all radios. Especially GE's, I was a
Motorola tech for quite a few years. I will leave the GE's to
others like you who are more familiar with their inner
workings.
However, even in this
particular radio, I noticed the PLL circuit uses a X3 from the original ICOM
freq as the PLL reference. This is made obvious by the divide by 4
fed back from the output of the VCO. So even though the PLL circuit here
is not more stable by a factor of 12, as I initially stated, it theoretically
should be more stable by a factor of 4. This does not appy to deviation
in this case but it will most definately apply to frequency
drift.
My 2 and a half cents worth.
Wade - KR7K
-----
Original Message -----
Sent: Sunday, December 19, 2004 11:04 AM
Subject: Re: [Repeater-Builder] GE Mastr II PLL vs. Multiplier
(crystal) exciter in duplex service.
Hi Wade,
I'm sorry, but I have to disagree with your
stability theory on the GE Mastr II PLL high-band exciter. If you refer
to the PDF manual for the PLL exciter:
<http://www.repeater-builder.com/ge/lbi-library/lbi-30398n.pdf>
You will see under the "Description" the exciter utilizes the 12th multiple of
the FM ICOM to lock the VCO on frequency. It goes into more detail about
this in the "Circuit Analysis" section of the same manual. So, the FM
ICOM's multiplication certainly does factor into the stability of the PLL
exciter, and one can generalize it has the same frequency stability as its
multiplier counterpart. In addition, the modulation of the PLL exciter
is produced in the crystal reference (FM ICOM) as well, and is also multiplied
up to the desired deviation. Since the time constant of the Lead/Lag
filter allows for near instantaneous correction of the VCO, changes in
frequency at the audio rate are superimposed onto the output
frequency.
Hope this helps...
Kevin Custer
Wade Lake
wrote:
Kevin Custer wrote:
The advantage here is the same frequency stability is achieved by the use of the quartz reference
Actually, a PLL oscillator is much more stable than a multiplied crystal oscillator. because with a multiplied quartz oscillator, frequency drift and frequency error (usually deviation as well) is multiplied by 12, at least in the case of the High band GE MASTR II.
Not to dissagree with you Kevin, your answer is good, I think you nailed it. I just wanted to point out that stability is a very strong point of the PLL. Since it operates on the desired frequency, no frequency error/drift is multiplied.
Wade - KR7K
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