Gift to

Joe wesrennaver W4FE:. C
155 Wood fiefd Drive
A ub urn AL 36830
Undertones
- a fractional frequency oscillator
Last t i me I w rote a
ham ma gaz in e arti cle,
Wayne Green and com pany
m isspell ed my name (April ,
1957, CQJ so I've been hesita
nt about try ing agai n. Oh
well, after 22 yea rs he deserves
ano ther chanc e.
Now, all of you have
heard of cry stal oscillators.
Almost all of yo u have
heard of overtone osc illators,
where t he ci rcuit
oscillates on an approxima
te w hol e integer, odd
harm onic of the crystal fundam
en tal frequency. But
how m any have heard of
t he " Undertone O sci ll a­
tor"? (My name; gotta call it
somet hing!) Thi s one osc il-
0 ,
DiVIS.ON F"'CTORS
cz
7 -4 ~ p F
Fig. 1.
56 73 Maga zine • Octo ber, 1980
rate s on som e fracti on of
t he c ry stal fundam en t al
suc h as V2 , Vi , lf4 ... 1/100,
ete. W ell , read on, my
friend, it is ve ry sim ple.
Thi s circu it is a by-prod ­
uct of work o n a cryst alc
o nt ro l le d vfo with a
1000 -kH z ra nge . (Works
very well , thank you.) Fig. 1
shows th e very simple circu
it. Ul c is an op ti onal bu f­
fe r to isolate th e osci llator
fr om hid capacita nces. Ul
is any TTL NA ND gate,
7400, etc . If the NAND gate
has m ore than tw o inputs,
e.g., a 7410, tie all unused
inputs to + 5 V de. 74Sxx
ICs. have been tried but
aren't quite as stable and
Tab le 1.
draw too much current.
74LSxx ICs m ight wo rk we ll.
CM O S NAN D gates, e.g.,
4011, mi ght work OK at
lower freq ue nc ies. The ci r­
cuit has been te sted fro m
fundamental oscillation s
thro ugh 1/350th of f und
a m e nta l . A l most any
c rystal w ill work except
som e very low fr equ ency
roc ks suc h as a 20G-kH z one
that I tried .
The osci llation fr equ ency
is set by Cl and Rl /R2.
C3 is optional for trimm
ing th e osc illator to an exact
fr equ ency. C2 is op ­
tio nal fo r help in locking
the frequency to a function
of th e crystal fr equ ency.
Rl /R2 and R3 bias Ul a into
its linear region. The output
is a TTL-compat ibl e square
wa ve. Larger d ivi sion f actors
(lower fr equenci es) are
lim ited because it is too difficu
lt to select t he desired
di vision facto r. 1/100 is
about th e m aximum I have
fo und pract ical.
R1 gi ves a w ide range, as
Tab le 1 shows. Di fferent
undert on es ca n be selected
by sma ll Rl changes, so a
10-turn trim me r pot helps in
picking the desired one . At
small di vi sion fa ctors (higher
frequenc ies), Rl ad justm
ent is no t so crit ical , so
Rl /R2 and Cl can be f ixed
com ponents selec te d experimenta
lly. A lso, at small
di vision f actors, better operat
ion is obta ined with C1
selected so that Rl is near
the higher end of its range
(1000 O hms) fo r the desired
frequency. At higher fr e­
quenc ies, t he load capaci ­
ta nce affects the setup of
the ci rcuit. Therefore, it is
better to have the circuit
co nnected to the next stage
or load wh en adjusting. If
the 7400 IC is used and t he
other two gates are un committed,
one of them used as
a buffer (unused inpu t to
+ 5 V dc) el im inate s t his
prob lem.
The table shows some
ranges for d iff erent va lues
of Cl obt ained from a
11 000-MHz cry stal. A frequenc
y counte r o r a gene r­
al cover age receiver are
helpful for establishing the
osc ill at ion fr equency. The
circuit w ill oscillate even
witho ut a crysta l. Between
select ed di vi sion f ac tor s, it
osc ill at es o n random, unsta
ble f requencies.
A strange th ing is that
som e di vi sion fa ctors lock
in better than other s. For insta
nce, w ith a give n cr ysta l
and Cl va lue, 1/15, 1/1 6,
1/17 and 1/18 might lock in