Version: 13. September 2004
5
2. Local oscillator signals:
The LO signal needed for transmitting and receiving is created directly on
the frequency. Via a PLL the VCO is tied to a 4,0- 4,5MHz DDS- VFO. This
approach reduces the need for filtering the harmonic rich DDS output
before entering the transmitter/receiver mixer.
The MOSFET (T2) VCO in the band module oscillates on a frequency 8 MHz
(the IF frequency) higher than the actual working frequency. The oscillator
proper works a Hartley. D4 stabilizes the amplitude. The signal from P1/Dr3
is coupled via D2 to the common VCO bus. Because of the cascode coupling
of T2, the VCO is effectively decoupled from the oscillator tuned circuit. The
tuning diode D3 is coupled so hard via C20 to the VCO tuned circuit of L1-
C9-C11, that it gives the proper tuning steepness. This gives a good side-
band noise distance. The band set oscillator of the band module swings
with T1 and Q1 4 MHz lower to the band frequency of the VCO. C3-Cr1
avoids 1. harmonic function of the overtone crystals (necessary on 15, 12,
10m). The crystal signal of R4-Dr2 is coupled to the bus via D1. IC1 handles
the mixing of the VCO and band set XO frequencies. The mixer output cir-
cuit of L1-C5 is tuned to the difference between the two inputs; the fre-
quency range is the range of the DDS- VFOs (4,0- 4,5MHz) . The via L1
inductively coupled amplifier T1 serves to raise the output signal of the
mixer to the required input level of the following 64: 1 divider, IC2. At the
output of IC2 is some 66 kHs, to feed to the frequency/phase comparator of
IC3. The 2. port of IC3 gets another 66 kHz signal from the division of the
DDS VFO frequency by 64 in IC4. IC4 will give a tuning voltage, depending
on the size and direction of the difference between the output frequency of
IC2 and IC4. This voltage is smoothed in the loop filter of R3-R4-C1 and
passed to the VCO to give equal phase of the two ~66 kHz signals. If the
VFO frequency changes, the VCO follows the change. As both VCO and XO
are switched in a band change, it is possible to use the same VFO range for
all bands.
3. VFO
We use a direct digital synthesis (DDS) VFO. The AD9835 chip is the heart
of the circuit. With a 25 MHz clock from IC8 and serial data from the CPU
(IC7), the DDS signal is taken from pin 14 of the AD9835 and passed on to
the PLL. The low pass filter of L2-L3 dampens the side bands and phase
noise typical of the DDS. Tuning of the VFO is done by a rotary encoder, and
the frequency step can be chosen with a push button. IC7 also gives the
possibility to program a transmission (XIT) or reception (RIT) offset. IC7
gives a CW marking of the mode, using the keyer function of IC7. LED’s
show the frequency range of the VFO in 100 kHz steps. An optional 7 seg-
ment LED display is available. Transmission or reception offsets are sig-
nalled on a LED. The VFO has a tuning range of 4,000 to 4,500 MHz (+/- 5
kHz. The stability of the VFO is based on the 25 MHz clock, and thus is the
crystal stability needed for PSK31.
4. RX input:
The input signal from the antenna passes through the 33 MHz transmitter
low pass filter and goes via relay RL1 to the electronic switch of PIN diode
D8. D8 conducts and leads the antenna signal to the band module/preselec-
tor bus via C35. This is followed by an undercritical double band filter with
a out of band suppression (L2-L3). The antiserially coupled capacity diodes
D7-D8-D9-D10 changes the pass band for the SSB or CW band segment. Due
to this good preselector, the RF stage and the input mixer is free of large
BC signals. The steep preselector filter also gives a high mirror suppression
and a minimum of IF throughput. The output from the preselector is led via
