A dual-band transceiver with a
crisp receiver and a clean SSB signal is described. It started
its life as an investigation of the excellent S7C receiver described
in EMRFD. This transceiver was specifically designed to use components
that are easily available in TV and Radio spares shops. The receiver
sports an above average dynamic range, very clean signal and noiseless
performance. Although the components are easily available, and
every detail about making it is covered here, this is not a weekend
project. The design is elaborate and invites improvisation.
We decided to pursue the following
rules in designing this transceiver:
►† Use what is easily available. Very often, we find designs
that look good but they use exotic parts like TUF-1 mixers that
are simply impossible to get hold of in India and other countries.
Instead, we have tried using those spares that are universally
►† Keep impedances and gain low: Often, we try coaxing maximum
gain out of a stage making it difficult to duplicate and stabilize.
We chose to take only modest gain out of each stage, using extensive
feedback to make the circuit stable. Most of the interconnections
between modules are for 50 ohms termination. In fact, the rig
was a number of discrete board connected using RCA audio cables
and sockets before we hooked it all up together to work.
►† No PCB. We directly solder the components over a plain
copper clad board (un-etched PCB). It is an excellent way to experiment,
physically robust and has a quick and dirty appeal. You can usually
solder up a whole circuit as you think it out in a few minutes.
See the pictures.
►† Broadband. We wanted to be able to use broadband design where
applicable. We have found that the television balun cores are
an excellent and very cheap (about Rs. 2 per balun, that is 5
cents) way of making broadband transformers.
►† Modest cost.
While we didnít want to use very expensive components.
We didnít want to compromise the performance either. You will
see that we have used 2N3866 exclusively. This was because we
found that the BF195/BF194/2N2222 series transistors available
in the market were consistently inferior in the HF range and performed
below their stated specs. The 2N3866 is commonly
used in cable TV equipment and has a good HF
performance: both as a low noise small signal transistor as well
as driver up to 1 watt level. 2N3866 is expensive (about Rs.20
each, but well worth the expense). It is used in a number of critical
►† Measure what you have built. We used a 12 volt 1.5A power
supply, a frequency counter, a test oscillator (to measure the
crystals and coils) and a high impedance voltmeter with an RF
probe to test and measure the design. All these test equipment
were homemade. The transmitter design did require a PC-based oscilloscope.
It helped us identify the spurs and harmonics using the in-built
FFT functionality. But now that the design is complete, just an
RF probe and a 14MHz receiver are enough to align the rig.
►† Quality over quantity. A better signal is preferred to a bigger
signal. This is a 6 watt design that will work off a simple 12V,
1.5A supply (using a single 7812).
ladder crystal filter
A good filter is central to the
crispness of a receiver and the quality of the transmitter. There
are two types of crystal filters possible, the lattice filter
and the ladder filter. The lattice filter requires ordering crystals
with 1.5 KHz frequency difference between them. This was ruled
out, also procuring readymade filters
from BEL India and other sources was ruled out as it is too expensive
to do that. Instead, a ladder filter was chosen. The ladder filter
offers results as good if not better than a lattice filter. However,
the design is crucially dependent upon internal parameters of
the crystals used. It is not possible to suggest any generic values
for the capacitors to be used in the ladder filter. Rather, a
method to measure each of the crystals and calculate the capacitor
values has been worked out. We present this here. This design
procedure will work only for 10 MHz crystals. 10Mhz is the chosen
IF of our filter as the crystals are easily available and it sits
comfortably between 7 and 14 MHz amateur bands. We have followed
the Butterworth design methodology given in the new ARRL book
ĎExperimental Methods in RF Designí.
The circuit centers around a four
crystal ladder filter. Each lot of crystals from each manufacturer
differs from the others. We will describe a way to experimentally
calculate the values of the capacitors for the filter. You should
probably buy 10 crystals and select 5 of them.