W6JFR 2008 20M SSB Transceiver Following the success of the 2007 SSB QRP Projects, I decided to build a 20 Meter version using a 9.8304MHz IF frequency. A 100 piece bag of these crystals is being offered ny All Electronics for $15. At 15 cents a piece this is a real bargain. The filter is a 6 pole and the coupling caps are 150PF. The basic architecture is a single conversion bi-lateral (or bi-directional) approach where a TUF-1 DBM is used both on transmit and receive, common amplifier stages preceed and follow the homebrew filter and a diode ring is used both as a balanced modulator and product detector. The advantage of this approach is that the LO and BFO/CIO are not switched and feed the appropriate devices in both transmit and recieve. Switching from Receive to Transmit is done with DC switching and Diode steering. All of these circuits are on the mainboard. Separate from the mainboard is the bandpass filter for the band of choice as well as the RF Amplifier Board. The RF Amplifier board contains both a Receiving RF amplifier as well as the Transmit RF Linear Amplifier. Here again Diode steering enables the the antenna to be connected to the LPF for the band of choice and DC switching steers the signal to either amplifier. In measuring the bandwidth of the filter it was ready confirmed that it favored the LSB and thus to get it to 14MHz would require a LO of 24 MHz. I decided upon a mixing scheme where a 4 MHz Hartley VFO would be mixed with a 20 MHz crystal oscillator and the venerable NE602 would be used as the combined 20Mhz Oscillator and Mixing device. Using the tables in the Solid State Design Manual (Hayward W7ZOI) I was able to calculate an appropriate filter that would follow the NE602 to provide the SUM frequency. This was followed by a single stage 2N3904 amplifier that provides a very stable 24MHz signal of about 0.5Volts PP which is sufficient to drive the TUF-1 balanced mixer. I was testing the lower level linear driver stage which is capable of producing about 500 milliwatts and had it connected to an antenna. AL7TC (Alaska) was signing and I could not resist giving him a call. He came back to me with a 5X3 report. From my QTH near Seattle that is a distance well over 600 miles. Imagine my surprise. With the full RF Linear Amp the unit produces in excess of 4 watts. I have driven my 100 watt SS linear amp with the radio and my first contact was with Brazil and the report was 5X9. The radio as it stands does not have an AGC circuit but that will be added later. I also have a digital display based on a design by I3KOL. The beauty of his design is that offsets can be introduced so that the frequency accurately displays the transmit frequency. This is a very small package and a worthwhile addition. I will be providing schematics of all of the circuits as shown below as soon as I update the schematics with the changes I incorporated. * RF Amplifier Board (Receive RF Amp, Transmit RF Amp) * Bandpass Filter * Mainboard (6 Pole Filter, 1st IF Amp Stage, 2nd IF Amp Stage, Product Detector/ Balanced Modulator, Carrier Insertion Oscillator/BFO, Audio Pre-Amp, Audio Amp, Microphone Amp) * 4Mhz Hartley VFO * 24MHz Mixer Board * Switching system * Frequency Display (PIC Based)
|