► Pass the twisted pair through one hole to
the other side, bend the wire back and pass it back through the
other hole (like a U turn). This is one turn, like this, make
similar 10 turns.
► Cut out the remaining
ends of the windings leaving about half an inch of the twisted
pair on each end.
► Scrap the enamel
off to about quarter inch, and tin the leads.
► Using a VOM at low
ohms setting, identify the two separate windings of the twisted
pair. If we call the two wires X and Y, each will have two ends
A and B. This you will have four ends AX, BX, AY and BY. Short
AX and BY together and use this as the center point of the transformer
in the circuits. Use AY and BX as the two opposite ends of the
Making a trifilar transformer
is similar, except that you have to use three wires twisted together.
Separate out the three wires as before, use the first two as described
above, and the third winding as the secondary.
filter and its associated IF circuitry is shared between the receiver
and transmitter. Although the crystals are inexpensive enough
to be able to afford separate filters for the transmitter and
the receiver, we noted that each filter would have a different
center frequency. This would make zero-tuning difficult for SSB
operation. Therefore, it was decided to share the same crystal
filter, carrier oscillator and the VFO between transmit and receive
filter requires 200 ohms impedance matching at both ends to provide
the correct bandwidth and low ripple. A regular practice among
hams is to strap a resistor of approximately
the same value as the terminating
filter impedance across the input and output ends of the filter.
This is incorrect. This looks like a resistor that is paralleled
with a reactive impedance of the rest of the circuitry attached
to the filter. When the crystal filter is not properly terminated
and sees reactive termination, ripple and ringing are introduced.
This will spoil the crispness of the receiver and spoil your on-the-air
The crystal filter is terminated
on both sides by ‘strong’ RC coupled amplifiers based on 2N3866.
This is slightly unusual. The 2N3866 is used mostly as a VHF power
amplifier. It has excellent low-noise characteristics, good gain
and using it as a small signal device is now an established practice.
The 2N3866 is an expensive transistor. It costs about Rs.20 in
the open market. We think it is a good investment.
Using RC coupled broadband amplifiers
makes the IF system a ‘no-tune’ affair. The output of the post-filter
amplifier is coupled to a two diode mixer. The two-diode mixer
uses a broadband bifilar wound transformer. It is next to impossible
to get toroids in India. We have evaluated using TV baluns as
substitutes for toroids. These baluns are available at most TV
Most designs we have studied couple
the RF input to the diode detector through the transformer and
inject the BFO at the center of the transformer. This is a wrong
practice. The diode mixer requires a minimum of 5mW of energy
from the transformer input to operate properly. There should be
enough energy to switch on both the diodes. This means about 1.2
v peak voltage. The received signals
are rarely this level. As a result, the product detector operates
like a regular envelope detector and the diodes act as distortion
devices to mix the BFO with the signal. The correct configuration
is to inject the BFO across the transformer
An unusual approach is taken here.
The IF amplification gain is just enough to maintain good noise
figure and recover the losses in the ladder filter. We measured
almost 10 dB loss in the filter.