QRSS Transmit Equipment at OE5EEP


Picture of the full set-up including DDS-VFO and amplifier:
DDS-VFO and amplifier in loose set-up

The DDS-VFO is a stack of three circuitboards with a LCD display on the front




The amplifier is built on a piece of circuitboard serving as copper plane




Encoder and switches for the DDS-VFO on loose cables




Detail of the amplifier:
QRSS amplifier


Input is on the bottom right corner, output to the left side (both BNC connectors)


The black test clips to the left go to the oscilloscope for on-line output measurements


The output transformer is a two hole ferrite bead of Amidon No 43 material, the input transformer is a FT37-43 ferrite toroid




The DDS-VFO was built from a G-QRP-Club kit. The design comes from Trevor Jacobs, K6ESE , (previously KG6CYN).  It uses a AD9850 DDS Chip from Analog Devices clocked at 120 MHz to generate sine wave signals in the range of 0 - 40 MHz in 1 Hz steps. The output is buffered with a ERA-1 monolithic amplifier to give 1 Vpp. These kits were available at a moderate price to club members. As far as I know they are sold out now.

The amplifier uses a BC547 as a driver and two pairs of BC547's in a push-pull final. The circuit is from Harry Lythall, SM0VPO (look under projects for the 500 mW HF linear). I get 200 mW output with 13.5 V DC input. This is less than what Harry claims, but more than enough for QRSS work. With 6 V DC input it still gives 100 mW out. Lower power levels are obtained with fixed 50 Ohm attenuators.

The whole transmitter including the keying section is built over a copper plane with small islands of double-sided circuit board soldered to the plane. Some people refer to this construction style as "ugly construction", but I nevertheless like it. This way of circuit construction is very advantageous, because you can use any size of component, whatever you find in your junk box, you have flexibility for future circuit changes and it offers a large copper plane on ground potential, which is helpful for the stability of HF circuits. If you discover that a certain component needs shielding, it is fairly easy to add some sheet metal dividers later on.

last modification: October 26, 2004

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