N3ZI Digital Dial
Input Sensitivity table

Frequency Range
Input Sensitivity
1 MHz to 5.5 MHz
300mV RMS.
74HC161/3 (any)
1 MHz to 35 MHz
35MHz to 55MHz
150mV RMS.
300mV RMS
M74HC163 (S/T)
1MHz to 35MHz
35 MHz to 55 MHz
55 MHz to 80 MHz
150mV RMS
250mV RMS
350mV RMS
74AC163 (T.I.) 10 MHz to 100 MHz 750mV RMS.


Values in this table are RMS not peak to peak. (150mV RMS equals approx 420mV peak to peak.)

Use T.I. 74AC163 chips only, Fairchild 74AC161 parts operate very erratically at low frequencies (below 20MHz)

For maximum sensitivity use a 1000pF input capacitor (C3)  Normally I recommend a 27pF input capacitor because this minimizes the loading on you Radio's VFO, but for best sensitivity a larger value can be used, especially at lower frequencies.

If you need more sensitivity, you can try this preamp kit from W8DIZ It's only $8,  I  built mine with the 200 ohm output impedance option, which works fine, but it does have low input impedance (50ohm) which may or may not be OK depending on were you are picking off your signal.   If you want to build your own there are plenty of examples in the ARRL Handbook

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