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G8MNY > TECH 23.05.20 08:50l 149 Lines 7175 Bytes #999 (0) @ WW
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Subj: Simple 80m VFO
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Sent: 200523/0743Z @:GB7CIP.#32.GBR.EURO #:57198 [Caterham Surrey GBR]
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jul 18)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This is based on the requirement to calibrate a VFO as a radio practical task,
in the UK's intermediate ham course (for 50W 2E Licences). The circuit I used
is similar to the published design the RSGB course book. (but this one works
1st time with different Cap values & higher Q coil!)
Plugging in an optinal 3.5MHz crystal & tuning to its osc frequency, showns
just how stable the crystal is in comparison to the VFO.
VFO CIRCUIT ŚÄÄÄÄÄÄĀÄÄ100źÄÄo +9V
FET d ³ ³ R1
120pF 470p UC734 ĆÄÄÄŁ C7 ===
ŚÄÄÄÄÄÄÄĀÄÄ´ĆÄÄÄÄĀÄÄÄÄ´ĆÄÄÄĀÄÄÄÄÄĀÄÄÄÄÄÄÄĀÄÄÄÄ>³s 10n ³
³ ³ C3 ³ C4 | ³ ³ g ĆÄÄÄĀÄÄÄÄÄÄ)ÄÄ1kÄÄÄ(o) Output
³ ³ ³ o 3.5 ³ 1n === - Ā - ³ ³ R2 ³ BNC
³ ³ ³ ÄĮÄMHz ³ C5 ³ ³ ³ ³ ³
_³ /\³ Preset ³ Preset ²²² ³ ĆÄÄÄÄÄ)ÄÄÄ´ ³ ³
|\³150 === C2 )|/\ L1 ÄĀÄ _³_ ³ ³ ) ³ ³
=== pF ³\60pF )| 22 o \_/ D1 === 1n ³ ) 1mH ³ ³
C1³\ ³ /)| turns | ³Ge ³ C6 ³ ) L2 ³ ³
³ ³ ³ | ³ ³ ³ ³ ³ ³
ĄÄÄÄÄÄÄÄĮÄÄÄÄÄÄÄÄĮÄÄÄÄÄÄÄÄÄĮÄÄÄÄÄĮÄÄÄÄÄÄÄĮÄÄÄÄÄĮÄÄÄĮÄÄÄÄÄÄĮÄÄÄÄÄÄÄ´
_³_
HOW IT WORKS
This is a Copitts oscillator type with a split capacitor C5 & C6 across L1 via
AGC bias DC block capacitor C4. The split capacitor enables the FET amp to
drive the circuit into oscillation (2x AC voltage on FET gate than on FET
source).
As the oscillation builds up the Germanium diode D1 (1Mź//silicon diode 1N4148
can be used instead) rectifies positive voltage to ground, thus producing a
more & more negative gate voltage that reduces the FET's AC gain. This AGC
action maintains a constant low distortion RF output level, independent of
tuned circuit Q & loading. Feeding the output via R2 (1k) ensures the loading
is always very small & can be fed directly to a tranciever for an S9+30-signal.
The frequency is set by the value of L1 in parallel with all the capacitors,
approx 180pF from C1-C5 network. Capacitors C1//C2 in series with C3 form a
band spread, so that just 3.5 - 3.8MHz is covered. Capacitor C2 limits the
minimum tuning C, so it is used for setting the 3.8MHz scale position. L1 sets
the main resonance & is very course, it is used for setting the 3.5MHz scale
position. (An alternative to an adjustable L1, is to make C3 a preset instead).
CONSTRUCTION
C Trim (compression type)
_________________________V___
/ || ___ \ L1 Large 6mm type ferrite core
³ || _________ ,=[___]==³ inductor with 22 turns of thin
³ || ³ C1 ³/\ C2 ³ enamelled copper wire wound on
³ PP3 || ³ TUNING ³ ³ ³ & stuck down to the former with
³ BATT || ³ @ ³ C3 ³ heat glue. It is bolted to the tin.
³ || ³_________³ \ _____Ž³
³ || C5 |_./' ,/³_____ < Trim
³ - - - || L2 C6______C4 L1 Ž³ L Core
³ ŚÄR1ÄÄĮÄÄÄÄT- \_ ³ T __n__ 1 Case earth
³ ³ -R2ÄÄÄÄÄÄĮÄC7_ D1_ ³ UC734 /1 2\ 2 Drain
\_____________________________/ Bottom ³ ³ 3 Source
³ ³ Ś~ÄÄ~æ View \3___4/ 4 Gate
³_³BNC ³Xtal³
ĄÄÄÄÄŁ
I used a "tobacco tin" for the box, after finding I had a suitable C1 air
spaced tuning capacitor that would just fit inside. Then I drilled all the
holes for C1 mounting & its shaft. I found the lid (bottom) slightly fouled
C1's bearing, so I dented the lid a bit to miss it.
Then I drilled holes for.. C2 trimmer access, L1 mounting & adjustment access &
for the output BNC. When drilling with C1 mounted be careful.
A PCB/tin can barrier is put in to keep the 9V PP3 battery at one end.
With the "tobacco tin", the components can just be soldered in place where they
will fit. I use heat glue to hold in place a insulation tube over R2, & to
steady the connection of C3/C4/L1 on the body of L1 after testing!
TESTING
The circuit draws only 0.5mA when oscillating, but 5mA when it is not, so that
is a good guide if you have no oscilloscope etc. Using a counter or a Rx, I
found to get the required frequency bandspread, C3 had to be 120pF, you may
find it different depending on the value C1 & other stay effects. If the
frequency is too high with maximum L1 core in place, add some more turns, if
too low remove some.
STABILITY
No attempt was made to improve the stability with..
1/ a more sold box
2/ power rail regulation
3/ output buffer amp
4/ temperature compensation
Even so with this simple oscillator, it is was easy to use it as a BFO with an
AM Rx on 80m to resolve CW & SSB OK.
SPECTRUM
This is quite a clean oscillator for just 1 transistor, it's 2nd harmonic @
7MHz was about -55dBc & higher ones weaker still.
STUDENT CALIBRATION PRACTICAL
For the course I needed to be able to remove paper dial scales per student, &
have the L & C adjustment holes clearly marked up. So the tin was cleaned up
with wire wool & spray painted so the sticky paper labels would stand out.
The dials scales were marked out on paper with 0 & 180ų base line reference &
about 10ų from each end the scale was marked with the wanted 3.8 & 3.5MHz band
ends (which ends are high & low depends on C1 construction). The 3.6 & 3.7 left
off & copies then made for the students to use, a pair of sticky tabs
(removable price labels) hold each scale in place for the practical.
_.-"""""-._
./' 3.6 '\.
/'3.7 | '\
.' \ ___ `. Completed Students'
.' ./' `\. '. Paper Scale
³ / \ ³
³ | | ³
³ 3.8_ | | _3.5 ³
³ ³ Punched ³ ³
³______³ (_) ³______³
³0ų hole 180ų³
³_____________________________³
TAB TAB
The students' task is to place the scale on the unit, put the knob on, aligning
it to get the mechanical 0 & 180ų range. Then adjust the L1 & C2 (use ferrous &
non ferrous & plastic tools to show the effects) to get the oscillator matched
to the 3.5 & 3.8MHz scale, (too & throw tests with a pre-calibrated Rx via
attenuator) then once those are accurately set up, mark off 3.6 & 3.7MHz
positions on the scale.
For the students to see what was possible, I made a paper label with clear
protective cover, scale marked at each 10kHz.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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