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| 型號： | SP5055S |
| 廠商： | Mitel Networks Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件頁數(shù)： | 2/10頁 |
| 文件大?。?/td> | 99K |
| 代理商： | SP5055S |
AN168
2
Phase Detector Gain (See Figure 7)
The phase detector outputs pulses of current I
μA with a
pulse frequency equal to the comparison frequency and
width proportional to the phase error. These pulses are
averaged by the loop filter so that the phase detector gain is
given by:-
I
2
Kd = μA/radiation
....(1)
Loop filter analysis (See Figure 8)
The loop filter converts the current pulses from the charge
pump into a voltage proportional to the phase error. The filter
recommended for normal applications is shown in fig 8(c). The
transfer characteristic is :
F (S) =
where
T
1
= C
1
T
2
= (C
1
+C
2
) R
2
T
3
= C
2
, R
2
Procedure for design of filter
Fig.8b shows an exact equivalent of the filter in figure 8(a).
It is not possible to implement this configuration since the only
points which are accessible are the input and output of the op-
amp, but it serves as a useful design model. If C
and R
are
incorporated as a current "pulse integrator" into the phase
detector then the remaining components consisting of the op-
amp, resistor [1 +
C
2
/C
1
] R
1
and capacitor C
1
can be regarded
as the loop filter.
This procedure allows us to treat the filter as a 2nd order
loop rather than the more complex 3rd order loop of figure 8(a).
This loop will have a natural frequency of
ω
and damping
factor
ζ
which we can select based on the application. The cut
off frequency of the "pulse integrator" would normally be set to
5
ω
or more. By manipulation of the transfer function (see
appendix) we can derive simple approximate design formulae
for C
1
, R
2
and C
2
. These are:-
KdKo
PN
ω
O
C
1
=
C
2
= C
1
/5
R
2
=
Choice of Natural Frequency and Damping Factor
When the synthesiser is reprogrammed, the application will
usually require the VCO to settle to the new frequency within
a specified time to a specified accuracy. Appendix 3 (Time
Domain Response) shows that the time domain response to a
frequency step is an exponentially decaying sinusoid. From
this the natural frequency
ω
can be calculated if we specify the
settling time t
and the accuracy
ω
e
/
ω
, provided we already
know the damping factor
ζ
.
The damping factor must be chosen so that the system
remains stable. For this the phase margin should be
reasonably high say 0, > 45° or so. See Appendix 3 (Phase
margin). The amount of ‘overshoot’ might also be used to
estimate a value for
ζ
. See Figure 11.
2
ζ
ω
O
C
1
VARACTOR LINE FILTERING
Special care should be taken with the varactor line. A low
pass filter may be placed in the varactor line to prevent ripple
being fed along the line and mistuning the oscillator. A typical
application is shown in Fig.4.
Fig.4 Varactor line filtering
The NPN transistor TR1, connected to the drive output,
should be placed as close to the drive output pin as possible.
The input to this transistor presents a very high impedance.
Any length of track between the drive output of the synthesiser
and the base of TR1 can act as an antenna which will feed
unwanted signals into the transistor. To minimise this effect,
a low value capacitor of, say 39pF may be connected between
the base and collector of TR1 (as shown in Fig.5) without
modifying the dominant loop characteristics.
TO
OSCILLATOR
DRIVE
OUTPUT
2N3904
10k
47k
10n
Fig.5 Varactor drive transistor modification
It is important that no other RF signals which may be
present in the tuner, for example IF outputs, are routed
anywhere near the synthesiser as they can also couple into
the device.
All of the above suggestions are made in an attempt to
achieve the best possible phase noise and sideband
performance for the synthesised oscillator. Whilst a good
synthesiser application does not guarantee good phase noise
performance, a bad synthesiser application will almost
certainly limit the overall performance of the tuner and
degrade phase noise compared to that of a free-running
oscillator.
CALCULATION OF LOOP COMPONENT VALUES
Applications Circuit (See Figure 6)
A typical synthesiser application circuit is shown in Figure
6. The optional additional filtering (referred to by Note 1 on this
diagram) rolls off at a frequency well above the main loop filter.
Its main purpose is to reduce any noise picked up on the
varactor control line. Consequently its effect is ignored in this
analysis. The following is a summary of the derivation of the
basic design equations used to calculate the loop filter
components.
VARACTOR LINE
DRIVE
OUTPUT
39p
in
(1+s
T
2
)
s
T
1
(1+s
T
3
)
2
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
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