Ham STAMPFL RF SHARK, Инструкция по сборке

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HF Splitter
SSB Filter
An ideally working AGC for SSB must regulate the recipient as quickly as possible and keep
the recipient insensitive during speech or character pauses. In the SSB part of RF SHARK,
this is easily achieved using a large charging capacitor.
Unfortunately, the AGC is a bit too slow on the rising fl
flank. Strong SSB signals can lead to
distortion. In this case the ATT -24 dB remedy. In other words, an ideal SSB AGC is not
possible with broadcast ICs. For true one-sided reception, the two side bands must be
separated!
This task is carried out by the SSB fi
filter CFJ455K from Murata, which was installed in many
amateur radio devices in the 80s and 90s.
The language can only be understood by adding the missing carrier. Each sideband needs
its own beat frequency of 453 and 457 kHz. These signals are obtained from freely
programmable oscillators from Cardinal. Unfortunately, the application range is between 1
and 133 MHz. This is the reason for the 4-fold working frequency, which is then divided by a
factor of 4 using double-D fl
flip-fl flops. The lower and upper sidebands must be compensated
by 2 kHz so that the frequency display is consistent with the SSB reception frequency. The
programmable oscillators from Cardinal are also suitable here.
The fi first mixer in RF SHARK is an active double balance mixer with a high dynamic range.
If the mixer is additionally supplied symmetrically by means of an RF transformer, as in this
circuit, the maximum modulation increases by 3 dB. Sum and difference signals consisting
of the antenna and local oscillator frequencies are formed at the output of the fi
first mixer.
The mixer output is divided into two equivalent channels by means of an RF splitter, thus
enabling the 1st IF to be brought out in broadband. The second signal path is followed by
an 8-pin quartz fi
filter. This only allows the difference signal to pass. The quartz fi filter is
forcibly matched using a 1.5 k resistor. If the quartz filter were operated directly with the
output resistance of the fi
first mixer, the fi filter's transmission curve would be very poor due to
strong ripples and asymmetrical edges. Thanks to the possibility of compensating the
insertion loss in the fi
first mixer, the resistance adjustment by means of 1.5 k Ω has no
negative impact.
The output of the quartz fi filter should be terminated with 1.5 k Ω . The fi filter is well
adapted due to the parallel connection of the inputs of the two A4100D and the
TDA1572. The fi
filtered IF signal, which is greatly reduced in bandwidth, is now
ready to be converted into the second and last IF of 455 kHz. RF SHARK offers two
AM bandwidths. For this, not the fi
filters are switched, but entire receiver blocks. The
A4100D is optimized for AM reception and shows very good noise behavior. The IC
is not suitable for SSB reception due to the lack of a regulated IF output.
The TDA1572 also shows very good technical data and has this output. It enables a
product detector to be connected using a small coupling capacity.
THE MIXER::
HF Transformer
Quartz fi filter
FUNCTIONAL STATEMENT - RF SHARK
When you select the sideband, not only does the matching local oscillator become active,
but also the one that mixes the 1st IF into the 2nd. The missing 2 kHz are added or
subtracted here. In the AM mode of operation, a third crystal oscillator of the same type
emits a central signal.
New in RF SHARK is a moving coil instrument for the display of the relative fi field strength.
This also serves as a coordination aid when operating the preselector. A 4 ohm
loudspeaker ensures powerful reproduction. But fi
first, diodes that work as switches must
feed the various audio outputs to the speaker amplifi fier. A 2.7nF capacitor in front of the
volume control frees the LF of HF residues.
For a better signal-to-noise ratio, the mass of the headphone jack is at the «minus» via a
100
Ω resistor. A stereo version is mandatory for headphone operation!