A Mini 20 Meter Rig
Inspired by the many QRP rigs designed by Peter, DK7IH, I decided to try building a simple single band rig to experiment with some of his circuits. Pete's designs used some 'vintage' parts such as 40673 dual gate RF mosfets, MC1350 IF amplifiers, and SA612 mixers. Only the latter part is still being made today, but only the 40673 is really hard to get. I actually have a fair number of the dual gate mosfets in the junk box, so I'll use Pete's RX RF amplifier circuit in this rig.
Pete showcased a nice simple rig on his web site that used a pair of SA612's in a bilateral circuit with one as the tx/rx mixer, and the other as the product detector / balanced modulator with relay switching of the IF filter between the inputs and outputs of the mixers. The SA612 is a poor performer in the RX mixer position under strong signal conditions, but it works very well in the other functions. Also, without an IF amplifier, only the RF amplifier has AGC control, and varying the gain of the 40673 via gate two is marginally effective. I'll be making use of some of Pete's other designs, borrowing his MC1350 IF amplifier and AGC circuits, and I'll have two separate signal paths, one for RX and one for TX rather than making use of a bilateral design. I'll switch the IF filter between the two paths using diode switches, instead of the relays.
I'm not sure what I'll use for the RX mixer, but I'm leaning towards one of the dual JFET designs that have appeared in the ARRL handbook over the past ten years. There is also a similar mixer using a pair of 2N5179's from the handbook, and the dual FET 'KISS' mixer that can be found on line. (BTW, while current prices for the TO-72 type 2N5179's now hover at over $5 each, an improved spec version of the same transistor in an SMT package sells for 31 cents!)
I already have an enclosure for the rig, made from some surplus boxes thrown away by a former employer. I had to fill in some holes, drill new ones, and repaint it, as well as make a new top cover on my bending brake.
The end result can be seen below. What will be the "S" meter (and relative output power meter), is a surplus 100 ua meter with a general purpose scale. A 5 pin 'DIN' socket is for the microphone, but will have contacts for 'PTT' as well as power, and AF output so I can use my David Clark aviation headset with an adapter. The small knobs starting at the meter and going clockwise will be for RF gain, AF gain, and TX (microphone) gain. The tuning knob controls a rotary encoder, and the window above it is for a 128x32 OLED display.
Pete showcased a nice simple rig on his web site that used a pair of SA612's in a bilateral circuit with one as the tx/rx mixer, and the other as the product detector / balanced modulator with relay switching of the IF filter between the inputs and outputs of the mixers. The SA612 is a poor performer in the RX mixer position under strong signal conditions, but it works very well in the other functions. Also, without an IF amplifier, only the RF amplifier has AGC control, and varying the gain of the 40673 via gate two is marginally effective. I'll be making use of some of Pete's other designs, borrowing his MC1350 IF amplifier and AGC circuits, and I'll have two separate signal paths, one for RX and one for TX rather than making use of a bilateral design. I'll switch the IF filter between the two paths using diode switches, instead of the relays.
I'm not sure what I'll use for the RX mixer, but I'm leaning towards one of the dual JFET designs that have appeared in the ARRL handbook over the past ten years. There is also a similar mixer using a pair of 2N5179's from the handbook, and the dual FET 'KISS' mixer that can be found on line. (BTW, while current prices for the TO-72 type 2N5179's now hover at over $5 each, an improved spec version of the same transistor in an SMT package sells for 31 cents!)
I already have an enclosure for the rig, made from some surplus boxes thrown away by a former employer. I had to fill in some holes, drill new ones, and repaint it, as well as make a new top cover on my bending brake.
The end result can be seen below. What will be the "S" meter (and relative output power meter), is a surplus 100 ua meter with a general purpose scale. A 5 pin 'DIN' socket is for the microphone, but will have contacts for 'PTT' as well as power, and AF output so I can use my David Clark aviation headset with an adapter. The small knobs starting at the meter and going clockwise will be for RF gain, AF gain, and TX (microphone) gain. The tuning knob controls a rotary encoder, and the window above it is for a 128x32 OLED display.
Here you can see the back of what will be the front panel. In this photo I demonstrate how to make use of parts in SMT packages. The 32 pin QFTP device is an ATmega328P microcontroller, the very same device found on the Arduino UNO. It's soldered onto a through hole adapter that was purchased from a Chinese vendor on ebay for less than a $1 each. To the right of that is the Si5351A clock generator chip, also soldered onto a through hole adapter. At less than $1 from DIgikey for the Si5351A, and the same for the adapter board, this clock generator module is smaller and cheaper than the one from Adafruit. I used a much larger 25 mhz crystal, but I have over a hundred of them obtained surplus (a freebee actually). Not shown, mounted under the board, is a 128x32 OLED module, also from China, for under $5, and a mechanical rotary encoder with push button switch. The OLED board was 'mounted' to the back of the panel by double sided sticky tape.
There is a six pin connector on top for the usual USB-Serial adapter board for programming via the Arduino IDE, and under that a 6 pin connector to attach my Atmel ISP programmer (used to flash the Arduino bootloader). To the right side are three connectors for the clock outputs of the Si5351A, and below the processor board is a connector for the OLED (you can see the cable for that to the right of the perf board).
At the bottom of the perf board are two SMT 3.3 volt LDO regulators that are fed from an LM7805 regulator mounted on the back of the perf board and bolted to the bottom of the cabinet as a heatsink. The OLED board has a 3.3 volt regulator on it, so it is powered off of the 5 volt rail (through the RF choke). I've read that OLED's can cause switching noise, having each of the devices on this board powered by their own regulators and bypassing all power with 0.1 uf capacitors will hopefully keep this under control.
There is a six pin connector on top for the usual USB-Serial adapter board for programming via the Arduino IDE, and under that a 6 pin connector to attach my Atmel ISP programmer (used to flash the Arduino bootloader). To the right side are three connectors for the clock outputs of the Si5351A, and below the processor board is a connector for the OLED (you can see the cable for that to the right of the perf board).
At the bottom of the perf board are two SMT 3.3 volt LDO regulators that are fed from an LM7805 regulator mounted on the back of the perf board and bolted to the bottom of the cabinet as a heatsink. The OLED board has a 3.3 volt regulator on it, so it is powered off of the 5 volt rail (through the RF choke). I've read that OLED's can cause switching noise, having each of the devices on this board powered by their own regulators and bypassing all power with 0.1 uf capacitors will hopefully keep this under control.