New single band superheterodyne transceiver
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Here's my latest rig (see pic below). It is a 10 Watt PEP single conversion transceiver using 6 printed circuit boards in a copper clad PCB enclosure. I've been having a lot of fun talking to my friends on 40 meters with this rig.
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The boards are as follows:
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1. VFO board. (purple, front) This uses a raspberry pi pico. The pico has a really inexpensive debug probe you can get for $15. Debuggers make it really easy to find problems with your code. The board has an Si5351 and a 26 MHz 0.5ppm TCXO reference oscillator.? I used Platformio IDE to develop the C++ code for this rig. I chose the Arduino framework in Platformio.
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2. TX amp and filter board (left wall of enclosure). This board takes the output of the product detector (though a high pass filter) and amplifies it by 40dB. It has 2 stages.: a 2N3904 followed by a 2SC3357.? It then filters the output with a triple-tuned bandpass filter for 40 meters. The purpose of the triple tuned bandpass filter is to filter out any spurs from the mixer before they get to the predriver and final amplifier. I thought about relay switching this band pass filter with one used in the front end board, but that would have over-complicated things and maybe introduced some paths to cause oscillation. I wanted to avoid bi-directional amplifiers as I wanted the rig to have AGC.
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3. Predriver and final amplifier board (red, rear). This board takes the -9dBm output from the TX amp and filter board and amplifies it to +40dBm (10 Watts PEP).? It has 2 stages. A pair of 2SC5706's followed by a pair of RD06HHF1's. If you push it you can get 12 watts PEP out of it, but I don't like running things at their limits. ?There is only one relay in this rig and it is on this board. The nice thing about push-pull amplifiers is that they attenuate the second harmonic significantly. The 5th order low pass filter on this board? is good enough to attenuate all harmonics below the -43dBm limit.
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4. Front end and crystal filter.? (Green board closest to the rear and close to the pre-driver and final amplifier board).? This board has a triple tuned bandpass filter before the mixer and also injects transmit audio into the mixer (which serves as the balanced modulator) during transmit. The output of the mixer is amplified using a 2SC3357? and then put through a 6dB pad before being presented to the 6 pole crystal filter. The crystal filter uses 12.288 MHz crystals. One thing to note about the 2SC3357: You can run it with 40mA of quiescent current, but you need to keep the the Vce below 12 volt absolute maximum in the datasheet. I power all the 2SC3357's in this rig with a switched 9 volt regulated supply.
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5. Hycas IF amplifier board with AGC. (Green board closest to the right side of the enclosure). This is a variant of the Hycas IF amplifier featured in QST: ?? with some switching modifications to allow adjustable gain on transmit.
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6. Product Detector/TX mixer/Audio Amplifier board. (Green board closest to the left front mounted on the bottom of the enclosure). This board contains a mixer, an NE5534 op-amp, and an LM380 audio amplifier.
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What is your I.F. frequency?? I'm curious because I have a similar superhet receiver using the HYCAS board and an 8 MHz I.F.? I'm currently using a Si5351a for both L.O. and BFO. It sounds pretty good but I have a lot of birdies. Do you have issues with receive birdies? My Si5351a board (Adafruit clone) has a 25 MHz crystal. One step I might take is to go with a different reference oscillator frequency. Where did your 26 MHz Si5351a come from? Do you use a module, or start with just the IC? Moving my 8 MHz I.F. would be painful because I went to a lot of trouble designing and building CW and SSB bandwidth crystals for that frequency. 73/TU, Nick, WA5BDU On Sat, Apr 26, 2025 at 12:13?PM hwstar via <steve=[email protected]> wrote:
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I rolled my own for both the mixer and the product detector. Its a diode ring DBM.? It consists of 2 trifilar wound FT37-43's with #30 AWG magnet wire, and 4 BAT54 diodes. The first mixer has a 100 ohm balance pot to help null out the carrier when it operates as a balanced modulator. The BAT54 diodes need to be matched for forward voltage.
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There is a footprint for an ADE-1 mixer in the product detector/transmit mixer on the PCB, but I didn't use it.
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Steve
WA6ZFT
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The IF is 12.288 MHz which works well for 40 meters because it far away from the LO and band frequencies. For LSB the LO is in the 5 MHz range.
The crystals I used for the filter are CTS MP122's: Regarding crystal filter design: yes, it is a pain.
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There are birdies, but they're mostly low level and manageable since they get covered up by atmospheric noise. 7296, and 7261 are a couple of big ones.
7168 seems? be an emission from the Raspberry pi Pico and it is modulated at ~50 Hz.
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The 26 MHz reference oscillator is a TG2016SMN 26.0000M-MCGNNM3 made by Epson. Digi-key part number SER4426CT-ND.
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No SI5351 module used here ;). The SI5351 chip is soldered down to the VFO board I designed.? The only module on the VFO board is the Raspberry Pi Pico. That has a QFN package and I don't have the equipment nor the dexterity to solder QFN's properly.
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Steve
WA6ZFT
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hwstar
Nick Kennedy