Metodo per la
stabilizzazione della frequenza in un sistema Eterodina
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Method
for frequency stabilization in a Heterodyne system
In a heterodyne system composed of a mixer, a local oscillator and the
intermediate frequency it’s the oscillator stability which determines the
presence of the input signal (assuming that the latter is stable at the
start) shifted in the intermediate frequency. Any change in the oscillator
frequency is reflected in the intermediate frequency. If the bandwidth of
the IF is large enough the input signal will still be present until the
drift of our LO will not be so wide as to make it disappear from the
"window" of the IF. Several systems have been designed to stabilize the
frequency of oscillators. For example in professional HF superheterodyne
receivers the local oscillator has experienced over the years, several
transformations; first conversion with quartz oscillator and with the second
controlled by VFO, Wadley loop system (Racal, Barlow-Wadley, Drake, Yaesu,
etc.), PLL with tubes (see Siemens E 311), transistor PLL (HRO 500 etc. ),
digital synthesizers, etc.. etc. The principle of the method presented here
is very simple and it is with recovery of the drift of the oscillator
frequency but with no phase comparator, reference signal and voltage
controlled oscillator, recalls in part the Wadley loop. As can be seen from
the block diagram of Fig.1 it is based in the use of two mixers and a single
local oscillator. The RF signal is sent to the first mixer and is added
to that of the oscillator to have an up conversion, the IF signal obtained
is filtered by the filter FL1 and then we will have fIN + fLO = fIF,
to fIF it must be added the possible drift of the LO frequency,
consequently fIF +/- drift fLO. The signal of the same
oscillator is sent to the second mixer to which it is subtracted the
frequency of the IF obtained at the output of the first mixer (and hence
from FL1), resulting in the output a fIN (or second IF, fIF2)
stable because the drift in frequency that was added in the first conversion
it is subtracted in the second one and therefore it is canceled, this until
the LO deviation does not exceeds the bandwidth of the FL1 filter. Not only
we will have a stable original input frequency but also filtered by FL1 and
possibly amplified. A practical example: input frequencies (or receiving
frequencies) fIN = 0--30MHz, fIF= 70 MHz as a central
frequency
fC,
therefore our LO must oscillates from 70 to 40 MHz. We establish the
bandwidth of FL1 in 30kHz, then our converted input frequency should be in a
window that goes from 69.985 to 70,015MHz. Suppose you want to receive the
15 MHz frequency, our oscillator must be tuned to 55 MHz (70-15 = 55).
Assume a drift of the oscillator of + 5kHz (55.00 + 0.005 = 55,005MHz)
accordingly we will have 55.005 + 15 = 70.005 MHz, this intermediate
frequency (filtered by FL1) is reconverted in the second mixer, from 70.005
- 55.005 = 15,000 MHz which is our input frequency or second intermediate
frequency. Actually this second IF will go from 14.980 to 15.010 MHz, where
we find our 15MHz desired signal. If the drift was -5kHz we would have
54,995 + 15 = 69,995MHz as a first IF that will be reconverted in the second
mixer in: 69.995 to 54.995 = 15MHz (which is located between 14.99 to 15.02
MHz, being the bandwidth of FL1, 30kHz ).The important thing is that the
frequency drift does not exceeds the width of FL1. Being an up conversion
the RF image (In this hypothetical case: 70 +
70 = 140 MHz, 70 + 40 = 110 MHz) is easily
removed by using a Low Pass filter. It is advisable to insert an identical
filter at the output of the second mixer to eliminate or mitigate an "image
of the second intermediate frequency" that would be created. In the specific
case;
70+70=140MHz
and 70+40=110MHz. What can be the use of such a system? The most obvious is
as a:
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