4
GENERAL OPERATION
NOISE LIMITING CIRCUITS:
The SSB noise limiter is a peak clipper circuit which follows the average peak
audio level but clips peaks above the average level. The input signal must be at
least 0.7 volt peak for proper noise limiting.
The CW noise limit is a trough limiter which removes the low level background
noise between the signals. Experimentally, set the audio or RF gain so that the
signal is passed but the noise is blocked. If the audio level into the filter is set too
low the signals will not get through. If the gain is set too high the noise will also
get through. Setting the audio too high could overload the filter and distort the
desired signals.
NOTE:
When the noise limiter is used (SSB or CW) the audio level will decrease due to
the clipping action of circuits removing the noise.
IMPORTANT NOTES
1.
The auxiliary filter is in series with the primary filter. If the auxiliary filter is not
needed, set it to PEAK and turn the SELECTIVITY and FREQUENCY controls to
minimum (counter- clockwise).
2.
With high selectivity, the filter may have a tendency to ring in peak and lowpass
modes. If this occurs, decrease the selectivity.
3.
Due to the very high internal filter gains in the LOW PASS mode with maximum
selectivity and frequency, the filter may OSCILLATE. If this occurs reduce
SELECTIVITY and FREQUENCY controls until it stops.
4.
Strong signals can mask a weak signal by causing your receiver AGC (automatic gain
control) to reduce gain. This may cause a weak signal to almost disappear. To
minimize this problem the AGC might be turned off and the RF gain reduced.
Increase audio gain to compensate for the loss of audio.
ALIGNMENT
The MFJ-752D primary filter is a two-stage filter which required proper aligned at the
factory. If for any reason a realignment is necessary use the procedure below.
UNDER
NO OTHER CIRCUMSTANCES SHOULD THE INTERNAL TRIMMERS (R319 and
R5320) BE ADJUSTED
. The filter will not function properly once either trimmer is
moved without proper realignment.
An audio signal generator and an OSCILLOSCOPE or an AC VTVM are needed for the
alignment.