►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
transformers.
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.
IF
sub-system
The crystal
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
functions.
The crystal
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
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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 quality.
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 spare shops.
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.
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