IW3IPD 20m PSK31 QRP transmitter

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It is a small transmitter I designed and built for PSK31 operation on the 20m band.

It is based on direct amplitude and phase modulation of the carrier, instead of the usual "audio out over SSB" approach, which would require to build a full SSB transmitter.

At the moment the carrier is generated externally with an HP3325A synthesizer; this drives at +7dBm the LO input of a TFM-2 mixer used reversed: the IF port (working to DC) is the input and is connected with a trick to the PC audio out. The RF port is the output and goes to a chain of 3 linear amplifiers. The circuit ends with an RX/TX relay and a PI low-pass filter.

The modulation signal is baseband and is obtained by patching the PskCore.dll used by WinPSK in order to have the baseband signal at the output. Since the audio board doesn't carry DC (which is required), the output is split in stereo, modulating the positive side on one channel and the negative on the other at a rather high frequency (16 kHz, or 1/3 of the sampling frequency of 48kHz). This allows to reconstruct the baseband signal by rectifiying, filtering and adding the signals from the two audio channels. This is done on the small vertical board.

The baseband signal going to the IF input of the mixer is 0.23V peak max (-2.8dBm), giving an output level around -7dBm on the RF port. Stronger levels would cause unacceptable mixer distortion.

The 1st amplifier stage is a 2N2222A in class A, feedback design (50ohm in and out). This stage amplifies 15dB to +8dBm.

The 2nd amplifier stage is a 2N2219 in class A, feedback design (50ohm in and out). This stage amplifies other 14dB to +22dBm

The final amplifier stage is an IRF610 in class AB. This stage amplifies 15dB to +37dBm or 5W with good linearity at 12.8V supply for QRP operation. With 13.8V and increased input level, the stage is capable to deliver +39.2dBm or 8.3W, so there is good margin.

A small BJT driven by the PC serial port RTS line drives a DPDT relay. The relay switches power to the transmitter and operates as TX/RX switch. On the RX side, a PIN diode (BA243) is used to better block power to the receiver during transmission.


I have successfully operated this transmitter for the first time on March 5, 2006 using:

bulleta Sangean ATS909 receiver
bulletthe WinPSK program with modified PskCore.dll
bulleta very simple antenna: a ~4.5m wire going down from the 3rd floor, manually matched at 14MHz (large metallic structure used as cointerpoise)


Modified PskCore.dll

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