N3ZI Digital Dial
Prescaler Examples
50.000 MHz Operation
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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. 
Divide by 2,4,8,16 prescaler



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.
Divide by 10 prescaler

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.
Div /10 using HC390


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