The production prototype radio installed on it's test stand
The Quartzmite project started when I decided to build a surface mount version
of the popular 'Rockmite' QRPp CW transceiver. After talking with Dave Benson, K1SWL
the original designer and Chuck Carpenter, W5USJ who did some efficiency
modifications to the P.A. and low pass filter I came up with a variant of the
circuit that uses nearly all SMT parts.
Information on the Version 1 Quartzmite can be found here.
As well as the improved Tx low pass filter my design makes use of the second op-amp
in the LM1458 to
implement a Sallen-Key second-order active low pass audio filter for the sidetone, making it
sound much less harsh than the square wave of the original Rockmite.
There is also a pre-set pot in the sidetone circuit so the user can set the
sidetone volume independently of the Rx audio.
The version 2 Quartzmite also has a memory keyer and better Rx muting.
With a 12v supply the PA delivers about 1 watt.
On Rx it draws about 15mA and on Tx about 370mA.
Some effort was focused on increasing the power output by adjusting things in the driver and PA
part of the circuit to make them more efficient.
One area that took some work was the 'shift' function that
moves the local oscillator between two frequencies about 700Hz apart,
this is used to separate the Rx and Tx frequencies.
The original Rockmite design is rather basic in this area and it has proved
impossible to get a sensible shift with any of the available zener diodes.
After some experiments I decided that rather than use zener diodes to generate reference
voltages for the varicap diode I would use a D2A converter controlled by the microprocessor.
As well as making it easier to get the required shift this has the additional
benifit of allowing the use of the low end of the varicap capacitance range,
which means we are pulling the crystal less and so the oscillator is more stable.
The PCB is set up use HC49U wire ended crystals.
The output waveform on a 'scope is a nice sine wave and my spectrum analyser measured the
transmitter 2nd harmonic at -52dB so the low pass filter is performing as expected.
A PCB and board-level component kit is available, please contact me for details.
A schematic is available here,
a Bill of Materials here,
and a set of construction notes here.
The Quartzmite V2 keyer is based on the K8 shareware keyer design from Steven Elliott, K1EL.
If a straight key with a mono plug is connected at power up then the iambic keyer is disabled
and the rig operates in straight key mode.
With paddles connected, pressing and holding the control button enters command mode.
Full details are available in the Quartzmite V2 operating manual available
here,
