Ten-Tec SSB-CW-AM Instruction Manual page 102

The first stage in the Model 1254 is an input bandpass filter.
Since we are not interested in signals beyond the 0.100 to 30
MHz tuning range of the receiver, this filter protects the mixer
from energy outside this range. In particular, the filter must
suppress any energy that occurs at the first IF frequency- We
would not want input signals at the IF frequency leaking-past the
first mixer and directly into the IF amplifier. The ability of a
receiver to ignore an input at its first IF frequency is called "IF
rejection". The 1254 has an IF rejection of about 40 dB which
means it takes an input 40 dB greater (a factor of ten thousand
in power) than the basic sensitivity to produce an equivalent
volume in the speaker.
This filter must also suppress what is called the "image
response" of the receiver. This image occurs because for a given
LO frequency there will always be two input frequencies that
convert to the IF. Lets take an example that would apply to this
design. Lets say that you have tuned to the WWW broadcast at
15.000 MHz. The LO generated by the receiver to tune this
frequency would be 60.000 MHz so that the difference product
from the mixer falls at the first IF frequency of 45 MHz. There is
another input frequency however, 105.000 MHz, that would
also create a difference of 45 MHz. This is the image response
that must be controlled by the input filter. Good image rejection
in this receiver is especially important since the image occurs in
the FM broadcast band where there will be some rather strong
signals. The image rejection in the 1254 is typically 70 dB.
The 1254 also has a variable attenuator ahead of the first mixer.
The purpose of this stage is to limit the peak-to-peak amplitude
of AM signals once they have been tuned in. Because the
information on an AM signal is contained in the shape of its
modulated envelope we don't want these signals to be so large
that they saturate any of the receiver stages. This would cause
clipping of the waveform and result in distorted audio.
The variable attenuator uses a couple of special 'PIN" diodes.
These devices, D13 and D14 in the schematic, are designed to
exhibit a variable resistance to RF signals depending on the DC
current that flows through the diode junction. The bias current
for these diodes comes from the AGC (Automatic Gain Control)
circuitry further down the receive signal path. This circuit
continuously measures the strength of the received signal and
adjusts the overall gain of the receiver to keep the audio volume
relatively constant. If a received signal is especially strong, the
AGC will activate the attenuator to keep the amplitude within a
range that the receiver can handle.
1254 Reference - 9