| Prescaler |
Frequency
Range |
Input
Sensitivity |
| None |
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. |
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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