Hallo
it is not so easy, but I will try it.
The PLL-system has a free-running oscillator, which is influenced by
several factors, as temperature, voltage and others. It changes its
frequency by "chance" or a bunch of factors.
Second part of the pll-system is a controll unit, self controlled by a
quartz-oszillator. This controlls the oscillator and measures its
frequency, knows the right frequency and gives a capacity-diode a
bias-signal to correct the frequency of the oscillator.
In a certain time depending from the circuit starts again, to measure
the frequency of the oscillator and gives a correction to the
oscillator , which is following the advice.
If You are looking to the proceeding, You see an oscillator,which all
the time is changing its frequency by the advice of the controll-unit.
There are relativly big changes in frequencies and at other modells of
oscillators relativly minor changes of frequecies.
In Reality You see not one and only oscillator frequency but maybe a
peak with a smaller or bigger band of the frequncies, the PLL managed.
You have such as a "band" of the oscillatorfrequecy.
The problem of a lot of receivers is, that they have not so clean
oscillator signals and this effect reduces the possible quality of the
receiver. That the status with receivers before DDS-generators and
especially very fast DDS-generators came. By a tact-rate of about 360
MHz !! the DDS-generator constructs digital the sinusoidal VFO-signal
and it is very clean (with fast DDS-frequency).
I use a DRT1 by www.Sat-Schneider.de which to a certain extent is an
optimum for DRM-demodulating by software. AND: It gives much more
quality even at AM, SSB.
The PLL and the DDS differ by the height of the peak of the siganl -
let me say till to 50 dB and more - in comparison to the band besides
a pll-driven oscillator-signal.
Because of that the DRM-signal is decoded at a very high level of SNR.
Much more than elder adapted receivers or weaker designs.
Phase noise is the right name. Now, relating to old receivers: They
could do the work and You have to experiment,wether it is sufficient
or not. You could test the situation and give the audio-signal of Your
receiver in the left side channel and the by decoding programm decoded
audio in the right channel of an amplifier. You will hear the
difference by the balance-knob.
All other oscillators should be stable too. The converter from 455 kHz
to 12 kHz ist not a problem because of the low frequency. In this
region it is very simple to build a stable oscillator. Especially, if
one uses a ceramic filter for the oscillator. Take 470 kHz and the
frequency will be reduced by the condensators used. You could get an
oscillator frequency of abaout 467 kHz giving the result of 12 kHz for
output. DREAM software is very tolerant and takes even frequencies of
11 kHz or 13 kHz or a little bit less or more.
My favourite receiver has a DDS-generator and works with 360 MHz, it
has even a thermical stabilized oszillator in the range of about 45
MHz (exactly 45,012 kHz) and it takes this digital reference for the
oscillator signal for the last converter for the output. It converts
from 45 MHz to 12 kHz!!!
There is a range for SNR-values for receiving:
More than 30 dB SNR excellent (Maximum is ca. 40)
Between 20 and 30 dB good.
Under 20 could be reliable.
Under 10 the software does not decode.
For proper DRM look to: attack time, decay time of the agc and an
agc-amplifier with more than 50 dB amplifying.
For now
Wolfgang
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