DFD4 / DFD4A assembly instructions.
(DFD4A has no offset capability and is for test
- The mode input is actually an analog voltage dividing the 5V supply
into four parts. Each part represents a mode. The analog voltage is developed by a
trimpot unless the unit is configured as a bench top frequency counter. When configured as
a bench top counter it is developed by a two bit D/A converter from panel switch contacts.
- The unit displays the mode momentarily whenever it is changed. When
using a trimpot, adjust it until the desired mode is displayed and then continue to turn
it one additional revolution for thermal stability.
- The offset is input via three 15 turn trimpots which represent a 21
bit number. FINE is bits 0-6, MEDIUM is bits 7-13 and COARSE is bits 14-20. This 21 bit
number is multiplied by 1KHz to produce a maximum offset in excess of 2GHz.
- Offset should be setup with the HF input shorted to ground to prevent
noise from being counted.
- Jumper between HF input and prescaler output must be removed first.
- Optionally, remove the 74HC4046.
- Start Fully counterclockwise on all trimpots which should be zero
offset. Adjust each trimpot for minimum displayed frequency.
- Set COARSE to display the value just below the desired value.
- Set MEDIUM to display the value just below the desired value but
closer to it than COARSE could provide.
- Set FINE to display the desired value.
- Set the ADD/SUBTRACT jumper as required by your application.
- This jumper does nothing if the offset is zero for a bench top
frequency counter. Adding or subtracting zero does not change the answer.
- There are many ways to calibrate a frequency counter depending on
available test equipment.
- The simplest is to zero beat a signal generator agains WWV (receiver
bfo off) while measuring the generators frequency with the counter.
- Adjust the 25 turn trimpot on the back of the PCB to obtain the WWV
frequency (20 MHz) on the counters display.
- Or listen to TCXO on receiver while zero beating it to WWV on 20MHz.
- Any other KNOWN frequency can also be used.
$59.95 (backlit display option add $10)
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