The ‘Hilltopper-20’ Transceiver K1SWL rev. 15 October 2018
2
Features:
Frequency coverage: 14.000 to 14.350 MHz
Tuning: 100 Hz /20 Hz steps
Transmitter power output: 5W nominal
Receive current draw: approx.. 60 mA.
Size: 4.35” x 3.95” x 1.07”, weight 8 oz.
Fully-integrated packaging- no wires.
‘On-the-fly’ CW speed control.,
Iambic mode A/B (selectable), 8-35 wpm.
Adjustments: BFO trim cap, one-time
Frequency Calibration
Frequency readout: Audio Morse
SMT Parts (2): Pre-installed
Description:
Receiver
:
The receiver is adapted from K1SWL’s ‘SW+’ Series with minor modifications. The front-end circuitry
was revised to replace the now-vanished 10.7 MHz IF transformers. The output of 1
st
mixer U1 is transformed to a
220 ohm value by L3/C8. The following crystal filter has a -3dB bandwidth of approximately 400 Hz. L4 and C12
step the impedance back up into the 2
nd
mixer U2. Trimmer capacitor C53 provides adjustment of the BFO
frequency during the alignment process. The two op-amp stages following provide approximately 60 dB of audio
gain. The final audio stage is configured as a bandpass filter centered on 800 Hz with a Q of 2. The receiver
output is suitable for headphone use. An 800 Hz sidetone is injected into this final stage.
Transmitter:
The transmitter strip closely resembles Steve Weber’s – KD1JV- fine work. The frequency source
for both transmitting and receiving is an Adafruit Si5351 board. This board outputs a 3V p-p square wave. The
‘CLK1’ signal is enabled (turned on) during Transmit directly at the operating frequency. The ‘QSK’ signal further
‘gates’ the CLK1 signal to eliminate a ‘back-wave’ between code elements. U7’s three remaining gates are
paralleled for higher current drive into the PA devices Q5-Q7. The low-pass filter design is based on a drain
impedance of 10 ohms and was optimized using the ELSIE modelling application. The addition of C52- in parallel
with L7 provides a notch to reduce the 2
nd
-harmonic content to FCC-compliant levels.
Wave-shaping: Q4 is a P-channel MOSFET rated for 2A continuous duty -with proper heatsinking- and is turned on
when Q3 is on, conducting current to ground. Capacitor C45 serves to make the supply voltage rise and fall linear.
Rise time is 2 mS and fall time is 4 mS. Note: R17, R18 and R21 are ‘insurance’, ensuring that the PA stage and
supply bias are firmly OFF during initial power-up.
MPU Controller
:
The controller IC is a 28-pin DIP- the Atmel ATmega328P used in the Arduino UNO. It relies
on an external 16 MHz crystal (Y6) for its timing. The application firmware was written in the Arduino environment
(more on this later). An on-board rotary encoder outputting 24 pulses-per-revolution provides a tuning function.
The variable DC voltage provided by Speed pot R16 is read by an A/D converter and scaled for Morse code timing.
A pair of inputs are used for dot/dash paddles, and Straight-key mode is also available. The remainder of the I/O
provides various control signals and sidetone for a variety of operations.
Firmware
: The Hilltopper firmware was written in Arduino’s (mostly) C language and supported by its own
compiler. After power-up initialization, the main program runs in a fairly high-speed loop, awaiting keyer and
pushbutton inputs. Encoder phase A is handled by a brief interrupt routine. That routine flags the presence of a
new tuning input and reads phase B to determine its up/down tuning direction. Outputs ‘A4’ and ‘A5’ provide a
clock and serial data to the Si5351 board. During Receive operation, CLK0 is active, and its output equals the
operating frequency plus the IF. During Transmit, CLK1 is active directly at the operating frequency. The
changeover is a fairly slow process, requiring a number of data bytes be sent to the Si5351 at a fairly low bit rate.
To avoid having to keep switching back and forth between code elements, there’s a 50 mS ‘hang’ time on key-up.
The Hilltopper firmware is open-source
and can be downloaded from
https://4sqrp.groups.io/g/HilltopperKit
See the ‘file
s’
section
