N3ZI Digital Dial
Prescaler Examples
This first one is binary prescaler, using 4 bit counter chip, 74HC(or
AC)161
(or 163) . For
lower frequency applications (<30MHz) an 'HC' part can should be
used. The AC parts are fast but do not react well to slow sign
wave inputs, so they should only be used above 20MHz.. You
should use the least amount of division you need
since the response time of the counter is slower with higher division
factors. The "timebase factor" in the Digital Dial set up should
be set to the same as the division factor used in your prescaler.
And the maximum VFO
counting frequency will be multiplied by the same factor. For
example using Div /2 increases the capability from 5.5 MHz to 11MHz,
and still has pretty fast
response, it will update approx. 25 times a second, which is excellent
for use with a VFO. Div /4 makes the maximum 22MHz, with
12Hz update rate, Div/8 makes the maximum 44Mhz with an update rate of
6 times a second. And Div /16 increases the maximum measurable
frequency to 88MHz, with a rather slow 3 times per second update
rate.
The example is a Divide by
10 prescaler. Here I'm using a 74HC4017, which can only be
configured as a Div /10. Theoretically this will allow operation
up to
55MHz, but the HC4017 is only guaranteed to clock up to 25-30MHz, but
there are "typical" specifications which range from 40 to 70MHz,
depending on the manufacturer. NXP (Phillips) has the best specs,
if they are also best in the real world I don't know. Mine ran
fine up to
50MHz which is the highest my equipment would allow me to test.
You'll have to run
everything at 5volts if you are
shooting for high speed.
When using this mode, the update rate will be 5 times a second, which
is really rather slow for VFO use. But since this one uses a
Divide by 10, several interesting changes can be made by fiddling with
the decimal points. You can change to resolution
to 1 KHz and then it will update at 50 times a second. (timbase
factor=1, decimal points to the far right) But if you
are looking for precision, change the "timebase factor" in the setup to
100, and set the decimal point 2 digits from the left, and you will
have a counter with 10Hz resolution, but the update rate will be
very slow.
This is the same Div /10
prescaler, but using a 74HC390 chip. In my testing these parts
don't reliably clock up to 50MHz, but seem OK in up to the 30 to 45 MHz
area.
