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david.g3ryp
开云体育Jay
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Quite a lot to go at here.
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1) The most likely reason why the vfo stops
oscillating as it approaches full mesh is that the fixed and moving plates are
coming into contact thus shorting the capacitance out. You can test this with an
ohmmeter. Hold it up to the light and see if you can see which plates are
shorting - then a bit of judicious bending!? There are other reasons but
check that first.
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2) The range should be 4000 to 4350 as you say (to
cover the whole band). You may want only the ssb segment.
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Now a little simple maths which helps in vfo design
:
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The resonant frequency of? an inductance (L)
and capacitance (C) combination is :
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F squared = 25330.3 / LC where F is in mhz, L is in
uh and C is in pf.
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Transposing that we can work out the LC factor for
any given frequency. Obviously there's an infinite number of combinations, some
more sensible than others.
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So the LC factor for 4000khz = 25330.3 / (4*4) =
1583 and
??????????????????????????
for 4350khz = 25330.3 / (4.35*4.35) = 1339
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So to get from 4 to 4.35 the LC factor must change
from 1583 to 1339 (a difference of 244).
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We can do this by changing the capacitance or the
inductance (or both) usually it's the capacitance.
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So if the capacitance in the circuit changes by
50pf we can work out the inductance and the fixed capacitance in the circuit
:
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244/ 50 = 4.88 (uh) and the fixed capacitance 1339
/ 4.88 = 274pf.
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If you're using a simple Hartley oscillator it's
very straightforward from there but in the BITX20 circuit it's a bit more
difficult? as you need to work out the capacitance change which involves a
few sums to work out the series and parallel capacitance combinations! For this
reason I almost always use a simple Hartley oscillator.
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However nothing ventured, nothing gained - so in
the BITX20 :
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Let's just assume the 22p trimmer is set at 20pf
and the 22pf/zener is 10pf
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At minimum capacitance of the 365pf variable - say
15pf the capacitance across the coil is :
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560pf in series with 560pf = 280pf in parallel with
15pf = 295pf this is in series with
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56||20||10 = 86pf which means the total capacitance
in circuit is 66.6pf.
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With the varaible at 365pf the total capacitance is
75.9pf. Thus the 365pf is only changing the total circuit capacitance by 9.3 pf!
You could work out how much your 500pf is changing it.
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244/9.3 = 26.2uh and 1339/26.2 = 51pf.
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That isn't too far out but there will be circuit
strays including?the self capacitance of the coil to allow for as
well.
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As a general rule if you want to increase the range
with a given variable capacitor you need to increase the inductance and reduce
the capacitance. Make the whole thing as solid mechanically as you can and check
the drift. It will probably shift quite a lot in the first minute or so after
switch on but then settle down.
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Now someone's going to tell me I've got all the
sums wrong!
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David |
