Mini Transceiver
Description.
This is a prototype design that was developed from several ideas associated with the popular
Pixie micro transceiver. The basic fundamental concepts remain the same as in the
original Pixie, functionality with few parts.
An internet search using Pixie or Tixie search phrases will reveal various sites that explain the theory behind this popular QRP concept.
Q1 is the oscillator, Q2 is the TX PA, and DC RX Detector/Mixer.
The DC RX is not balanced, and thus will exhibit common DC RX problems.
Using a balanced detector/Mixer such as SA/NE612 will remove some of these problems. Q2 should bring
the output power close to 1W, and for this reason should have a heat-sink. C11, and C12 allow the
frequency of operation to be changed slightly, and together with Q3 implement a pseudo XIT/RIT.
These capacitors should be adjusted to give the desired offset, which is applied during key down.
The PA is keyed via T1. The 78L12 is needed to provide a stable regulated power source.
The Keyer can be any suitable PIC keyer that provides side-tone, and a postive keying voltage for T1 to apply to ground or close to it.
Suitable PIC keyer MPASM firmware based on the free K8 by K1EL can be downloaded
here. You will need a an assembler language compiler such as Mirochips MPASMWIN found in the MPLAB IDE to compile the assembler code to Hex, and a Hex programmer such as
Picall. Other 8 pin PIC keyers are available from other sources, as are freeware/shareware or commercial products.
TL071 is a low noise OP AMP used to implement an audio CW band pass filter, centered on 750Hz. LM386
implements an audio amplifier. The low pass RF filter used here, has 5 elements, which can be
changed for the band desired. This filter together with the desired Xtal gives multi band
capability. Values for the RF low pass filter are shown below.
The basic circuit can be enhanced by using a RF band pass filter on to minimize BC Band
interference to the mixing process. A shielded enclosure should be used.
Links.
ARRL Dos software for OP AMP low pass, and high pass audio
filter design.
Windows software for OP AMP audio band pass filter
design.
