Re: [NTLK] Mathematics

From: Marco Mailand (Newton2k1_at_mac.com)
Date: Thu Jul 18 2002 - 07:06:24 EDT


> change, thus also a factor of 1.67. In other words, the Newton would
> be 1.67 times as fast as before or 67% faster. A factor of 44 would be an
> increase by 4400 percent?
> Frank (puzzled)
(/puzzled)
3.686 MHz * 44 = 162.184 MHz - 44 is the factor of the PLL multiplier,
(teacher_mode)
The output of a free running voltage controlled oscillator (VCO) is divided
by 44 and produces a signal of around 3.686 MHz. This divided VCO output is
compared with the frequency of the x-tal (the one which Stephanie replaced
with a 5MHz type). As a result of the comparison a voltage is produced. This
voltage is connected to the control input of the VCO and will change the VCO
output frequency accordingly. That is high voltage causes high frequency and
low voltage causes low frequency, but all in a limited range of lets say
150...240MHz. The comparator has now only to increase its output voltage
(aka VCO control voltage) if the output frequency is to low and decrease if
it is to high. But it does actually more: The comparator compares the phase
angle of the two 3.686MHz signals as well and small changes in the phase of
the VCO signal reflect immediately in control voltage changes. Therefore the
name PLL = Phase locked loop. The reason for doing so is simply the
frequency limits of x-tals. As soon as one needs something below 1MHz or
above 100MHz the x-tal becomes to big or the internal structure to
complicated beside degraded stability and/or increased phase noise and the
like problems.
(/teacher_mode)
I've done a PLL to create 400...600MHz with a 16.6MHz base frequency. On
searching x-tals I came across an interesting thing: a tuneable (voltage
controlled) x-tal.
Provided we could replace the 3.686MHz x-tal with a tuneable part which can
do e.g. 3.5 ...6MHz one could use a simple potentiometer to speed-up the
Newton and wouldn't need a switch at all. I should make a phone call to the
supplier...

--
With best regards / Viele Gruesse

Marco Mailand http://slsbd.psi.ch/timing http://slsbd.psi.ch/~mmailand https://www.paypal.com/refer/pal=YYD3VUXUVD2YG

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