Experiment on Super VXO
Cryptic letters are codes for Japanese/Chinese
characters. English readers may skip them.
$BIaDL$N(BVXO$B!"$*$h$S(B$B%9!<%Q!<(BVXO$B$N<~GH(B
$B?t2DJQHO0O$rD4$Y$k
$B?^(B(a)$B$,=H/?6;R$G!"(BX1[email protected]$1$N>l9g!JIaDL$N(BVXO$B!K!"(BX1$B$H(BX2[email protected]$b$N(B
$B!J%9!<%Q!<(BVXO$B!KN>J}$N>l9g$K$D$$$FB,Dj$7$^$7$?!#(BL$B$KJBNs$KF~$l$F$"$k(B22$B%-(B
$B%m%*!<%`$NDq93$O<~GH?t$,[email protected]$jMzNr8=>]$,5/$3$i$J$$$h$&$K$9$k$?$a$N$b(B
$B$N$G$9!#%P%$%"%9Dq93(BRB$B$O(B100$B%-(B
$B%m%*!<%`$K$7$^$7$?!#$3$N;~(BQ1$B$N%(%_%C%?EEN.$O(B9mA$B$G$7$?!#(BC1$B$H(BC2$B$O6&$K(B
150pF$B$K$7$^$7$?!#(BRB$B$H(BC1$B!"(BC2$B$K$D$$$F$O.CM$O(B
[email protected]$G$9$,$*$=$i$/?t(BpF$B$G$7$g$&!#(B
$B?^(B(b)$B$,(BL$B$rJQ$($?;~$N2DJQHO0O!J%H%j%^%3%s%G%s%5$r:G>.$H:GBg$K$7$F(B
$B5a$a$k!KB,Dj$N7k2L$G$9!#IaDL$N(BVXO$B$H%9!<%Q!<(BVXO$B$N0c$$$ONrA3$H$7$F$$$^$9!#(B
$B%9!<%Q!<(BVXO$B$G$O2DJQHO0O$O2?$H(B300kHz$B$K$b5Z$S$^$9!#(B
$B$7$+$7!"NI$$$3$H$P$+$j$G$O$"$j$^$;$s!#(B
$B$$$m$$$m$J(BL$B$NCM$N;~!"<~GH?t$N;~4VJQ2=$rD4$Y$?7k2L$,?^(B(c)$B$K<($7$F$"$j$^$9!#(B
$BH>ED$E$1$N8e(B25$BJ,BT$C$FEE8;$rF~$l$FB,Dj$r;O$a$^$9!#(BL=10uH $B$O$^$"$^$"5v(B
$B$;$k$b$N$N!"(BL=10.39uH$B$d(B11uH$B$N;~$OJQF0$,Bg$-$/$^$k$G(BVFO$B$N$h$&$G$9!#(B
$B$*$=$i$/!"(BL[email protected]$,1F6A$7$F$$$k$H;W$$$^$9!#0x$_$K!"$3$NFs$D$N(BL$B$N(B
$BCM$K$D$$$F$O$3$NCM$N$b$N$,$J$+$C$?$N$G!"?^(B(c)$B$K<($7$?$h$&$K$U$?$D$N(BL$B$r(B
$BD>[email protected]\B3$7$FMQ$$$F$$$^$9!#(B
$B2DJQHO0O$H0BDjEY$r$O$+$j$K$+$1$F(BL=10uH $B$"$?$j$G2fK}$9$k$N$,$$$$(B
$B$N$G$O$J$$$+$H;W$$$^$9!#$7$+$7$b$C$HBg$-$J(BL$B$G$b!"$b$&>/$7BT$F$P0BDj$9(B
$B$k$+$bCN$l$^$;$s$7!"[email protected]$N>/$J$$(BL$B$r;H$&$3$H$b9M$($i$l$^$9!#(B
$B?^(B(d)$B$O!"(BC1$B$H(BC2$B$rF1;~$K$$$m$$$mJQ$($?;~$K2DJQHO0O$NJQ$o$kMM;R$r<($7$F(B
$B$$$^$9!#(BC1$B$H(BC2$B$rBg$-$/$9$k$H2DJQHO0O$bBg$-$/$J$j$^$9$,!"?^(B(e)$B$K<($9$h(B
$B$&$KH/?6=PNOEE05(BVout$B$b>.$5$/$J$j$^$9!#(B C1=C2=330pF$B$G$O>.$5$/$J$j2a$.$F(B
$BB,Dj$G$-$^$;$s$G$7$?!#(B100$B!A(B150pF$B$H$$$&$N$ONI$$A*[email protected]$H;W$$$^$9!#(B
$B:G8e$K!"%P%$%"%9Dq93(B RB $B$rJQ$($F$_$^$7$?!#Ev=i$N(B100$B%-%m%*!<%`$+$i(B 200
$B%-%m%*!<%`$K$9$k$H%(%_%C%?EEN.$O(B9mA$B$+$i(B5.2mA$B$K8:>/$7$^$7$?$,!"2DJQHO0O(B
$B$K$O1F6A$"$j$^$;$s$G$7$?!#C"$7!"H/?6=PNOEE05(BVout$B$,(B0.7Vp-p$B$+$i(B0.3Vp-p$B$K(B
$B8:>/$7$^$7$?!#(B
RB $B$r$5$i$KBg$-$/$7$F(B470$B%-%m%*!<%`$K$7$?$i!"%(%_%C%?EEN.$,(B2.4mA$B$K$J$j!"(B
Vout$B$,>.$5$/$J$j2a$.$FB,DjITG=$H$J$C$F$7$^$$$^$7$?!#(B
[email protected]$H$7$F!"%9!<%Q!<(BVXO$B$G$O(BL=10uH$B$G2DJQHO0OLs(B100 kHz $B$,[email protected]$i$l$^$7$?!#(B
$B<~GH?t$N0BDjEY$O$=$3$=$3$G$9$,!"(BL$B$rBg$-$/$9$k$H0BDjEY$O0-$/$J$j$^$9!#(B
C1$B$H(BC2$B$rBg$-$/$9$k$H2DJQHO0O$OBg$-$/$J$j$^$9!#$7$+$7!"(BQ1$B$N%P%$%"%9$rJQ(B
$B$($F$b2DJQHO0O$OJQ$o$j$^$;$s$G$7$?!#(B
$BCm0U$7$J$/$F$O$J$i$J$$$3$H$O!"[email protected]$i$l$?7k2L$O;H$C$??e>=H/?6;R$KFC(B
$BM-$N$b$N$G$"$k$3$H$G$9!#0c$&%Q%i%a!<%?$N?e>=H/?6;R$d0c$&<~GH?t$N?e>=H/(B
$B?6;R$r;H$($P!":GE,$J(BL$B$d(BC1$B!"(BC2$B$NCM$b0[$J$C$F$-$^$9!#(B
$B=H/?6;R$G
An experiment on VXO is done to investigate frequency coverage of
an ordinary VXO and a super VXO.
The super VXO was invented and named by JA0AS(I. Shimizu, Silent Key)
and JH1FCZ(T. Okubo).
Figure(a) shows the schematic diagram of the experiment. X1 and X2 are
HC18/U X'tals of nominal frequency 14.218 MHz. In the experiment,
only X1 is connected in the place shown in the diagram for
measurements of ordinary VXO. For measurements of super VXO, X2 is
added in parallel to X1. Inductance L is the main parameter of
this experiment.
A 22-kilo ohm resistor is added
in parallel to L in order to avoid a frequency skip or
hysteresis.
Q1 is a small signal general purpose transistor with fT =
80MHz.
Q2 is a small signal cascode MOS FET with gm = 10 mS.
The bias resistor RB was chosen to be 100 kilo ohms. With this value,
emitter current of 9 mA flowed in Q1. C1 and C2 were both chosen to be
150pF.
RB, C1 and C2 of different values were also tried at the end of this
experiment.
TC is a plastic trimmer capacitor with maximum capacitance of 100pF. Minimum
capacitance is not known, supposedly several pF.
Results are shown in figure(b) on the frequency coverage as measured by
varying TC from its minimum value to 100pF.
The measurements were done with L of 5.6, 8.2, 10, 10.39, 11 and 12 uH.
For 10.39uH, two inductors 10uH and 0.39uH are used in seires. For
11uH, 10uH and 1uH in series.
A difference is prominent between ordinary VXO and super VXO. As L is
increased, the coverage of ordinary VXO did not increase so much,
while the coverage of super VXO increased rapidly up to about 300
kHz(!) with L=12uH.
However, we have to be careful about that wide coverage.
See figure(c). Drift of the frequency in time is measured for L=10, 10.39, and
11uH. In this measurement, X1 and X2 are used in parallel, and TC is
set at 100pF. A batch of measurements with a certain value of
L was started 25 minutes after
the soldering, and power supply was switched on just before the start of
the batch.
While the frequency with L=10uH is acceptably stable in time, the
frequency is jumping or drifting considerably with L=10.39 and 11uH.
The drift of the frequency is most probablly due to the
temperature coefficient of the inductors, and due to the VFO-like
nature of the oscillator.
I would stop at L=10uH for the stable operation although
you might eventually get better stability with larger L's if you wait
longer time or if you use inductors with small temperature
coefficients.
Now, I play a little with the capacitance of C1 and C2. All other
parameters are fixed at RB=100kilo ohms, L=10uH, and X1, X2 in
parallel. Figure (d) shows the frequency coverage as a function of
capacitance of C1 and C2. C1 and C2 are varied simultaneously. You
see that the coverage gets wider as the capacitance is increased. The
measurement with C1=C2=330pF was not possible because Vout(output
voltage to the frequency counter) got too small as is shown in
figure(e). 100 to 150pF may be a good choice.
Finally, the bias resistor RB was varied. Other conditions,
C1=C2=150pF, L=10uH, and X1, X2 in parallel were kept. Originally, RB
was set at 100kilo ohms and emitter current was 9mA. This time,
RB was cahnged to 200kilo ohms, then emitter current decreased to
5.2mA. The frequency coverage did not change although Vout decreased
from 0.7Vp-p to 0.3Vp-p. RB was further increased to 470kilo ohms,
then emitter current decreased to 2.4mA. However, Vout got too low,
less than 0.1Vp-p with this low current. Therefore, frequency
measurement was not possible.
In conclusion, frequency coverage of about 100 kHz was obtained with
L=10uH, C1=C2=150pF, X1 and X2 put together in parallel and RB=100kilo
ohms. The stability was acceptable, but got worse with larger
L's. The coverage got wider as C1 and C2 were set at larger values but
did not depend on the bias of Q1.
It should be emphasized that these results are more or less specific
to the X'tals used in the present experiment. Optimum values for L,
C1 and C2 may differ from the values obtained in this experiment if
X'tals with different parameters or different frequencies are used.
You need to do an experiment to optimize the parameters for your X'tals.
MINOWA, Makoto
7N3WVM
Last revised 1997-09-30.