Ten-Tec SSB-CW-AM Instruction Manual page 103

This brings us to the first mixer stage which is made up of JFET
transistors Q2 and Q3 and transformers T6 and T9. This circuit
takes an incoming signal and combines it with the local oscillator
to create a replica that is shifted up to the first IF frequency of
45.000 MHz. The output of the mixer feeds a crystal bandpass
filter, FL1. The bandwidth of this filter is about 15 kHz. This is
actually much wider than is really necessary for shortwave
reception but it is about the minimum bandwidth that can be
achieved with this type of low cost filter. The output of the filter
is amplified by transistor Q4. This is a dual-gate MOSFET stage
and was chosen because the input impedance is very high and
provides a good impedance match for the filter. The frequency
response of the crystal filter is very dependent on impedance
matching so we have included adjustments, L9 and L10, to
optimize the shape of the filter.
The next stage is the second mixer. There are several good
reasons to make this second frequency conversion. First, we
must get our desired signal down to a frequency where it will be
easy, and inexpensive, to further filter and amplify. There is a
type of filter component based on ceramic resonant circuits that
is widely available at 455 kHz. It makes sense to take advantage
of these parts to accomplish the narrow filtering we need for AM
and sideband reception. Furthermore, we still need a lot of
voltage gain to get weak radio signals up to a level that will be
easy to demodulate and gain is easier to accomplish at a lower
frequency. We also use this second conversion to provide fine
tuning. Since the first LO is synthesized and can only tune in
discreet 2.5 kHz steps, we've added a clarifier adjustment on the
second LO to fill in between the steps. This is not so important
on AM mode but it is on sideband. Proper sideband reception
requires that the receiver be tuned exactly to the frequency that
recreates the original harmonic relationships in the transmitted
voice.
The second mixer is built with the widely used "diode ring"
circuit. In this approach, a strong local oscillator signal supplies
the drive to alternately switch four diodes, D10-D11 and D15-
D16, on and off . This switching action, together with the
phasing of the two transformers, T4 and T5, causes the input
signal and LO to be multiplied together. For this stage we have
chosen an LO frequency of 44.545 MHz so that the difference
product between the first IF and this second LO will occur at 455
kHz.
The second local oscillator is crystal controlled. This Colpitts type
oscillator circuit is built around transistor Q6. The other
transistor, Q5, acts as a buffer amplifier to boost the output up
1254 Reference - 10