80m Sniffer: FAQ, tips and ideas page
Sniffer oscillates or takes off at
high gain settings.
The main reason for this is because the IF amplifier breaks into
oscillation at around 900MHz. This occurs because the lead inductance
of C19 along with track inductance on the second gate of Q2 generates a
resonant tank at this frequency. Also the lead incuctance on Q2's drain
formed by C13 and C12 doesn't help matters either.
Originally there was a mod to change R13 to 330K. Its effect is to drop
the drain current and therefore reduce the higher frequency gain. While
this is still recommended, it does not always work. A more robust
solution is to short out either the gate2 or the drain at the
The following changes work well:
1) Remove C13 and put a 22pf
chip capacitor (either 0805 or 0603
package) across pins 2 and 1 (drain and source) of Q2. (See
2) Add a 100pf chip capacitor between pin 3 of Q2 and the ground pad
Method "1" is preferred. From November 2004 onwards, all kits will be
provided with the 22p SMD
capacitor already fitted.
Sniffer seems to lack sensitivity.
appears to work "Backwards".
is too fast or covers far too much range (nearly 400KHz)
This is a little involved but is basically caused by incorrect local
oscillator frequency. Take the following example for receiving at
3.5MHz. If the local oscillator is tuned to 6.75MHz instead of 6.5MHz,
the second harmonic of the local oscillator (13.5MHz) hetrodynes with
the 3.5MHz input frequency to generate an IF signal at 10MHz, but with
much lower conversion gain. This accounts for the "lack of
sensitivity". Also the second harmonic tunes at twice the rate of the
fundamental. this accounts for the 400KHz tuning range. Tuning occurs
"backwards" because the injection sense is "high side" and the tuning
is therefore inverted.
Increase number of turns on L3 from 32 to 34 turns.
Tone mode is erratic, impossible to adjust or goes from no tone to
This is caused by use of a 74HC4046 rather than the standard CMOS
CD4046. After a lot of frustration I have come to the conclusion that
the 74HC4046 just doesn't work as well.
Replace 74HC4046 with a CD4046. If you have been supplied with a
74HC4046 in your kit, please contact the author for a replacement part.