This article will describe some design ideas that might be helpful in case the objective is building a medium power (i. e. 10 to 20 watts pep) radio frequency amplifier using a very compact design
The whole final amplifier for the broadband linear transmitter had to be packed into an area whose size is 5 by 11 centimeters.with approximately 2.5 centimeters in depth.
Because of limited space the power amplifier module had to be stacked using pin headers and connecting them with appropriate means. This effort resulted in two layers of circuitry:
The transformers for in- and output, the bias circuit and the radio frequency choke for the DC power line
The two power transistors (2SC1969)
Also a heat sink had to be planned.
Practical design of a compact medium power QRP RF linear amplifier
A large heat sink of aluminum covers the area of the underside of the transceiver:
When removed, access to the power amplifier section is possible:
When removing the thick aluminum block containing the screw’s holes we see a support construction that holds the connecting block and the final transistors in place. Here also the temperature sensor (KTY81-110) can be spotted, attached and the rear side. This aluminum structure is the only thermal connection between the small sheet metal holding the transistors and the heat sink. But it turned out that temperature of the pa transistors rarely rises above 50°C even when long test periods are performed for adjusting the amplifier. So, this idea has proved to be a mechanical, thermal and electronically good arrangement.
Between the pair of 2SC1969s the biassing diodes are visible which control the current for the current regulating transistor sited under the input transformer. Thermal contact is ensured by positioning the transistors extremely close to the diodes.
When removed, we can see the transistors mounted to a very small part of veroboard and connected to the “main board” with a row of socket strips.
Underneath we see the output transformer made up of 2 stacks having 3 toroids FT37-43 glued together with 2 component glue.:
Because of limited space the connection to the low pass filter board is done with hookup wire. With just a length of 2.5 centimeters this is not a real problem.
The five band switch relays are connected to 5 separate veroboards (which makes changes of e filter very fast) containing each one filter for one band. Every veroboard is held in place by 2 small bolts with M2 specification. Between the veroboards small brass tubing pieces can be observed which serve as “shielded” lines for the longer leads going to the end of a respective filter. Inside the tube there is another piece of 1.1 mm diameter PVC insulated hookup wire.
Color coding of the various bands is unique over the whole transceiver.
Very far on the left side the transmit-receive relay has been positioned. This one only is for switching the antenna socket between transmitter and receiver. DC switching is done with small p-channel MOSFETs sited behind the LCD display and will be discussed later.
The transmitter for this transceiver is designed to deliver a peak envelope power (PEP) of 10 watts on all bands from 80 to 15 meters. Due to its purpose (SSB amplification) it is a linear amplifier.
The circuit is equipped with 5 stages where the first one is a gain control stage containing a dual gate MOSFET whose gate 2 is controlled by a gain voltage from a digital-to-analog-converter (DAC), an MCP4725, and preset by values manually stored in the microntroller (MCU). This allows the user to compensate the decreasing gain when higher frequencies are used. The remaining stages are equipped with bipolar transistors.
The audio input stage of this transmitter uses a commercially designed integrated circuit (SSM2166 by Analog Devices) which is a microphone compressor for computer applications.
The DSB-generator succeeding the compressor is an integrated mixer of the AN612 type.
Afterwards the TX mixer section (NE612 active Gilbert Cell mixer) follows. After being processed by the band pass filter (BPF) the five stages of the transmitter push the filtered signal to the designed final power level:
Gain control stage (40673 dual gate MOSFET)
1st amplifier (2N2222)
Driver stage (2x2SC2078)
Final power amplifier (2x 2SC1969)
Audio stage and double sideband generator (DSB) and TX Mixer
This unit is designed for usage with an electrete microphone. Supply voltage is generated in a chain of 2 series switched 3.3k resistors, a 4.7V zener diode and a blocking capacitor. Following is an integrated circuite (IC), the SSM2166, which is a microphone amplifier and compressor circuit.
An AN612 integrated mixer forms the DSB generator in this circuit. There is no potentiometer for carrier suppression, in general carrier suppression of >45dB can be achieved with this simple circuit.
The resulting DSB signal is fed into the SSB filter that is placed in the receiver section. Usage of shielded cable with high shielding capacity is mandatory here for interconnecting the filter to the transmitter circuit. Even if stray coupling in high level radio frequency energy is not a severe issue on the interfrequency branch of the transmitter.
TX mixer is an NE612 with balanced output. This measure which will result in some extra dB concerning output gain.
Power amplifier strip
A lot of introductory research had been done on a multi band QRP power transmitter with 10 watts of pep power when building the experimental 6 band transceiver. The general issue for a broadband power amplifier is the gain difference that occurs when band switching is applied. 3dB gain loss per octave is the rule of the thumb that is stated in lieterature and has proven to be correct under practical examinations..
An easy and reliable way to compensate this common gain loss, can be achieved using a programmable gain set stage at the entrance of the transmitter chain. This onset here is achieved by using a dual gate MOSFET transistor whose gate 2 is controlled directly via an I2C programmable digital-analog-converter (DAC). This DAC (MCP4725) is 12 bit wide, thus software in the MCU allows the user to set the gain in 4096 steps via the controls and store this value in the MUC’s EEPROM. After each band switch the respective value is recalled and subsequently sets the stage’s gain.
The amplifier strip presented here includes 2 push-pull stages as driver and final power amplifier. In contrast to the 6-band transmitter there is no “in-between” low-pass-filter.
All coil data is stated in the schematic. Pig nose cores are used in the final amp stages.
After the power amplifier the circuit terminates with the final low pass filter section. Simple 5 element filters are used.
When setting up the circuit on a PCB or veroboard keep in mind that the 15m filter section should be placed in the closest position to the input/output connector to save lead length! Or to say in other words: Reverse the filter order in the schematic!
Sorry for having deferred the description of the transmitter. The recent days I have been concerned with a new frequency layout for the transceiver. I found that the 17m-band could be an interesting topic because when tuning on internet based SDR pages the last days I saw many strong signals appearing. This might be due to the fact that sun is higher now in the northern hemisphere and conditions will even be better with solar cycle #25 now about being to commence.
Based on these considerations I changed the band plan for the 5-band radio: 10m band has been removed, instead 17m has been added.
The new band layout now is 80m-40m-20m-17m-15m.
Here are the respective values for coils installed into the band pass filters (BPF) and the layout for the final low pass filter (LPF).
Hint: Inductance for the BPF coils have been measured with (probably) excess error ratio. Thus calculations are resulting in a different resonant frequency for the LCs when using Thompson’s formula!
Currently some additional tests with the the transmitter are pending, but full description will follow the next days. So, stay tuned! 😉
This is some sort like “Copy & Paste”, a useful mean if you want to create a doctorate, like the former German Minister of Defense Mr Guttenberg once did. 😉 I don’t want to achieve a doctorate but the receiver of this radio is more or less the same I have constructed for the Midi6-transcevier. So I just copied the schematics and put down the changes in this paper.
To see a full sized picture of the RECEIVER, please click here!
Starting the tour on the left you can see the band switch unit, beginning with a BCD decoder that converts a 3-bit pattern created by the MCU into a 5 line decimal output. The ULN2003 then is a driver designed for motor controls but it is very useful as a relay driver as well. Integrated clamp diodes and open collector circuit make it practical as a driver circuit for this unit.
Next is the band pass filter section. I still use relays for switching the respective filter because I found that it is the best way to keep unwanted signals low from passing the filter, provided you use relays that can serve this purpose., Here signal relays TQ2-12V by Panasonic have been applied. Coils are small TOKO style coil formers with 5.1 mm (2×2.54mm i. e. 2×0.1″) pin spacing.
RF preamp is equipped with a dual gate MOSFET like the BF900 or so. The “AGC” this time is to be manually, just connect the AGC input (which now is an “MGC” to say it correctly!) of the stage to a 10kOhm variable resistor allowing a voltage swing between 0 and 12 V and this will lead to a preamp stage with gain control in the range of 25dB. This variable resistor is to mounted into the front panel, just to be concise.
The receiver’s mixer is an SL6440 which has great IMD3 performance (about 30dB) and has been used instead of diode ring mixer. Some dBs of gain are achieved as well but not the amount you can expect from an SA602.
In practical terms the ic really proves what the manufacturer promises. On 40m e. g. with a large doublet antenna no IMD products are audible even when strong broadcast station are next to the amateur radio band. A really worthy trial with this receiver!
Due to the fact that the following SSB filter is used for the transmitter also, another signal relay switches the filter between the receiver and the transmitter branch.
Next the MC1350 video amp is installed to do the major amplification with the interfrequency signal. It is gain controlled by the AGC circuit on the right side of the schematic. Gain is minimum if AGC input is around 7V or higher.
The product detector is a dual gate MOSFET which is only there because this one has a slight amount of gain and does not consume much space on the tiny boards.
The audio preamp stage is also very simple, just a bipolar transistor with negative feedback applied via a large resistor (390k) also biassing the unit to an appropriate value.
The audio main amp here is not an ic (like the inevitable LM386 e. g.) but it is a push-pull arrangement using 3 bipolar transistors. The stage that enhances the voltage is designed with a BC547, the stage that is bound for current amplification uses a pair of complementary transistors (BD137 -NPN- and BD 138 -PNP-). Audio power is about 1 Watt which is suffice for a small radio.
AGC uses an operational amplifier, any type like the LM358 will work great. The LM358 contains two identical amplifier stages. The first is used to bring the audio signal to a certain level, then rectifying this voltage and subsequently bringing it into a time constant consisting of a charged capacity (2.2uF) and a discharging resistor (3.3M), The circuit has very fast response, so there is no annoying “plopp” when a strong signal breaks in) and the decay is very soft.
The second stage just works as an instrumentation amplifier putting out up to 12V to control the input of the MC1350 at PIN5.
To end this article let’s have a look at the practical setup of the receiver:
As mentioned in the introductory article to this radio the digital components in this transceiver are pre-manufactured modules that have only been put together in a more or less sensible way. ;-). These modules are:
AD9850 as variable frequency oscillator (VFO), China made no-name board,
Si5351 as local oscillator (LO) produced by “Adafruit”,
Arduino Pro Mini w. ATMgea328p as Microcontroller Unit (uC, MCU), no-name
ST7735 colored LCD, no-name,
MCP4725 as digital-analog-converter to preset transmitter gain via MCU, “Sparkfun” clone from China, no-name
All units are able to run on 5V which made it easy to layout the schematic because only one 5V/1A voltage regulator had to bu used.
To watch a high resolution version (4.2MB!) of the wiring scheme, please click here!
a) The lines for ISP (MOSI, MISO, SCK, RESET and GND) have not been drawn but the location to the respective ports is mentioned in the table sited in the right top corner. Reset rquires 10kOhms to +5V and a 0.1uF cap to GND.
b) Certain clones of the MCP4725 DAC module will produce conflicts with the I²C/TWI-address of the Si5351 LO module. Original “Sparkfun” boards come with I²C/TWI-address 0x60, 0x61 or 0x62 (depending on literature/web resource you get this information from).
This address is set by the manufacturer AD inside the hardware on customer’s demand. On the other hand the Chinese made modules I am using have basic address 0xC0 which is the address of the Si5351 also. Thus this leads to conflicts on the I2C/TWI-Bus. One solution is to close a solder bridge to +VDD on the very tiny DAC-board which will set address to 0xC2.
c) For the 4(!) user switches (not 3 like in the photo above!) the pull-up resistor on PORT PC0 is set on. There are is a resistor (in the range between 560 Ohms and 2.2 kOhms) with each switch, that pulls voltage to GND when the respective key is pressed. This leads to a voltage drop at the analog input that will be detected by the ADC channel.
This voltage drop depends on the pull-up resistor and on some other factors so it must be determined for every controller setup individually. To solve this, in the respective functionthat returns the numeric value for the key pressed there is a small commented code that you have to de-comment temporarily:
Restart the software, press every key, put the indicated key value into the code (line 4) and re-comment the orange lines when fnished. Next re-upload the software to the controller.
d) Source code in C is available on my Github repository. Please note that even if an Arduino Pro Mini MCU board is used, the code is not designed for the Arduino “world”. It does not use functions of the Arduino environment and may not function with the Arduino bootloader.
To compile the C source and generate the HEX-File you need the GNU C Compiler either for Windows or Linux.
A compact SSB transmitter/receiver will be presented. This unit covers 5 bands within the amateur radio spectrum (3.5, 7, 14, 21 and 28 MHz). Receiver is a single conversion unit with an interfrequency of 9 MHz. Transmitter uses 5 stages and has got a power level of 10 watts PEP output.
Frequency generation is done by integrated ready made modules like an AD9850 as VFO, and an Si5351 as LO. Microcontroller is an Arduino Pro mini AtMega328 driving a colored TFT LCD with ST7735 chipset.
The whole device has been constructed in SMD but can also be setup by using “thru hole” techniques or mixed installations.
The unit is built into into a mounting frame of aluminum sheets of standardized width. Size of the whole radio is 17 x 12 x 5 centimeters. It is, to a certain degree, the “Little Brother” of the “Midi6“-Transceiver that had been designed mainly for experimental purposes.
Multiband QRP transceiver projects are a challenging undertaking for the radioamateur. The even more challenging matter is to build it as neat as possible.
The “Midi6” transceiver has been an interesting step which made me learn a lot of things. But it is a much too bulky for my needs (producing compact and lightweight portable gear for traveling, hiking etc. ) On the other hand I found that I don’t really need 160m installed in the radio (due to antenna problems here at my site) which defined the next multibander having a “classical” (i. e. 70s) layout with 80, 40, 20, 15 and 10 meters.
An important point was to use ready made modules or breakout boards for the major digital and analog circuits:
First I thought about using the Si5351 as VFO and LO because it contains 3 oscillators on one chip. But I gave that idea away very fast because there were to many spurious signals and the thus the receiver had to many “birdies” which I don’t accept. Having had some of the Chinese made AD9850 boards still here on the shelf I gave that one a try and was finally relatively happy with receiver performance.
The microntroller and its application also has been a challenge because for a multiband transceiver an Arduino Pro Mini might be a little bit weak because the number of ports is very limited. But it finally worked out when planning is carefully done and optimizing is brought to its limits. The port usage is as follows:
ISP leads are used for controlling the DDS and for uploading the software to the controller. This is done because the inputs of the DDS are high Z inputs that do not affect the ISP data transfer. On the other hand the programmer goes to high Z if there is no data to be sent to the controller. Thus testing the radio is possible when programming leads are connected.
LCD is an ST7735 TFT colored display because I found the OLEDs with 1306 and 1106 drivers to noisy on the higher bands where band noise is weak and therefore digital noise produced in the radio comes more into the foreground. And, above all, a colored display makes much more impression than an ordinary b/w one. 😉
Mechanical construction and transceiver units
For this radio I ordered aluminum strips holding a width of 5 centimeters via ebay. Thickness is 1.5 mm. From this material a very rugged frame has been constructed that gives the whole rig a very good mechanical stability.
Major units in this construction
The rig is very much unitized, each functional of a module section is soldered to a very small piece of veroboard that has been cut out from a larger piece of material. It is fixed to the aluminum basis by using inserted nuts with M2 screw thread. The main advantage is: If one unit fails it is easy to reconstruct it and put it to the place the predecessor has been mounted and second grounding is excellent because the small single units don’t require long grounding leads because the boards are very small in size and the 4 corners all have ground potential. Particularly for the transmitter I can say that I had never any unwanted oscillations.
The transmitter is 100% stable on all the 5 bands, which was not the way with the first “Gimme 5”-Transceiver that had severe layout problems in the transmitter having the initial BPFs very close to the final rf power stage. But in the end you should be knowing more than in the beginning pf a project. So is true here. 😉
The picture shows a close-up of the receiver section that consists of 5 single units (from the left)
Dual-gate MOSFET preamplifier (in the picture veiled by shielded cables) and rx mixer (SL6440)
interfrequency amplifier (MC1350) and product detector (dual gate MOSFET)
audio preamp (BC547) and main amp (3 transistors, the 2 finals in push-pull circuit)
AGC with OP (LM358) and bipolar transistors as voltage regulators.
The same technique has been used for the transmitter:
Starting from the left you notice an SSM2166 microphone compressor ic by Analog Device which also is the main microphone amplifier. Next is an AN612 mixer as DSB generator, followed by an NE612 serving as transmit mixer.
The second board from the right is a 3 stage unit to bring the transmit signal to a power level of about 150mW (Dual gate MOSFET, 2N2222 and 2SC2314 as active semiconductors in this order). On the right a push-pull stage equipped with 2 2SC2078 and relatively high emitter degeneration (2 Ohms for each transistor) brings the power up to 500mW.
Transmitter gain can be controlled with an MCP4725 DAC that is set for each band individually and helps much to compensate gain increase on the lower bands. This DAC is also connected to the microcontroller’s I²C-bus and data for each band is saved in EEPROM and is being recalled if a certain band is switched.
Tha main amp is centered on the center side of the mainframe:
On the left side of the tx pa unit there are 2 power transistors (2SC1969 by eleflow) mounted to a small strip of 3mm thick aluminum that is connected to another much thicker block of Al. Here a large heatsink can be mounted when the device is under test or finally fixed into the cabinet when using the aluminum cabinet as heatsink. Connected to the aluminum block there is the temperature sensor (KTY 81-110) that allows permanent check of the transistors temperature and that will lead to a warning on the LCD when excess temperature is detected.
The output transformer can be found under the two PA transistors and therefore is not visible here. This “stacked” construction saves very much space. PA transistors are connecting to 2.54 mm socket strips which makes the pair of semiconductors removable and allows access to the power transformer underneath.
On the right of the PA section there are the low pass filters for each band switched by a single relay.
Band filters are shared for transmitter and receiver and are switched to the respective branch by using relays. Left of the BPF unit there is a logical unit (HCF4028 BCD encoder and an ULN 2003 relay driver integrated circuit). This allows switching 5 relays by just using 3 binary coded controller output ports.
Software is written in C for AVR controllers using the GNU C compiler under Linux. The code will be discussed in the respective article that is going to follow this introduction.
I strongly recommend to stay tuned for the next articles covering this transceiver and giving details for each unit! 😉