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zeroing the frequency of the VFO to calibrate
it to the proper range. The remainder of the
capacitors. C I and C3. should be NPO ceramic
or polystyrene. These provide the necessary
stability. Regular ceramic capacitors
will cause the VFO to drift.
The 5 pF capacitor off the tap ofLI couples
the tank circuit to the diode that provides RIT
and the transmit frequency offset. II is
switched in and out by Q7. The offset shifts
the frequency of the rig in the transmit mode
approximately 750 Hz down. This is the standard
offset in thc amateur community. If
there were no offset, you would be exactly
zero beat with the other station, and neither of
you would hear the other!
RIT: The Receiver Increme ntal Tuning
(RIT) circuitry is probably the handiest of
" bells and whistles" on this rig. This provides
a method of moving the receiver frequency
while the transmit frequency stays
put. This is useful for adjusting the frequency
of the tone ofthc station you are listening to,
but leaving the transmit frequency stationary.
Consequently, your frequency will not appear
to the other station to crawl up the band.
Driver: The NPN driver transistor (04) is
driven at a fairly high level from the VFO and
is matched to the final through n .The transforme
r is parallel with a 5- 50 pF trimmer
capacitor (and an additional
22 pF or 33 pF ca pacitor for
40 meter operation) which.
together with the inductance
of the pri mary winding of T2.
provide a resonant circuit.
This tuned circuit is more
complicated than a broadband
design, but allows more power
outpul.
Amplifier: The final specified
is the much written-about
2N3553. It gives plenty of
output but, unfortunately , it is
expensive and nor always easy
10 find . Radio Shack carries a
2N3053 which will suffice,
but thi s d rops the output
to about one and one-half
Watt s. Other possibilities
are a 2N3866. 2SC2075. or
MRF 476.
The output transistor must
have a heat sink! If you neglect
this, it will overheat and
destroy itse lf! The output
from Q5 is fed into a conventional
double Pi low-pas-s fil ­
ter network to reduce harmoTIlCS.
Receiver Protection Circuit:
The transmitter and receiver
sections connect directly
to the antenna. You need
circuitry to protect the receiver
portion from the power of
the transmitter. Otherwise,
the 3 Watts from the trans-mitter
will ruin the front end of
the receiver. The diodes, D3
and D4 , together with the resonant
combination of C44 and
22 73AmaleurRadio . June, 1989
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Figure 4. Pans placement guidefor the QRP transceiver.
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