My transceivers usually don’t use any ready-made cabinets. To save space and have full and easy access to all parts of the radio during construction, adjusting, assessment and repair I prefer an open 2-layer sandwich method.
There usually is one centered frame that is fixed using M3 or M4 bolts to the front panel carrier and the rear wall or carrier (if the rear wall consists of a more complex structure):
The center carrier here consists of 4 aluminum bars that have quadratic cross section (7mm edge length) and are building up a rectangle. This a rugged basis for 2 aluminum sheets (0.8mm thickness). These aluminum sheets are fixed with bolts (M2 winding) fitting into screw threads inside the bars that have been cut in there before.
The single veroboards are bolted with spacers using M2 screw threads onto the aluminum sheets. These spacers are available from Chinese vendors on the internet and have a fairly low price. They have become my favorite mechanical aids when building compact radios. For higher demands concerning force I use the same devices in M3 or even M4.
The front and rear carriers are bolted into this center frame and support the front and rear panel:
At the top of the picture you can see the final LPF and the DC input for the transceiver mounted to the rear of the radio.
The front panel is made of an aluminum sheet metal (again 0.8mm thick) that has cutouts for the LCD, the controls, the push-buttons and the microphone jack.
The front panel light diffuser is made of a part of white translucent plastic bought from a shop that distributes material for architects. 3 strips of LEDs are mounted there also using the M2 spacers (3mm length).
The whole body then is placed into an outer cabinet that is composed of 2 halves of aluminum that have been bent into the correct form to fit the shape of front and rear panel.
The lower one has a height of only 1.5 centimeters. To connect the upper half on each side a strip of aluminum (2mm thickness) is bolted to the aluminum sheet close to the edge that carries screw threads to fix the upper and the lower half together to close the cabinet. These “sidebars” also affect the stability of the relatively thin cabinet in a positive way.
To avoid the interior section slipping out of the closed cabinet the two “sidebars” are cut into an appropriate length so that they “block” the inside from slipping out either to the front or the rear side.
Using aluminum has two major advantages in my point of view: First it is easy to be processed (in contrast for example when using metal sheets made of steel) and it is very lightweight what I prefer because I use my radios on frequent travel activities.
This unit is a very simple one. I did not want to use more relays than necessary. The consequence was to save at least the one commonly used in the transmit-receive switch unit. Here 2 p-channel MOSFETs do the job:
Hint: The “PTT” in the radio here leads to a PIN of the MUC switching the transmitter on. For general purposes a “PTT” has been drawn into the schematic.
Function: When Gate (G) is “hi” (i. e. close to VDD) the S/D channel goes to nearly infinite ohms. Resulting current is 0A apart from some uA leakage current.
When S is pulled to GND, or, to be more exact, some volts lower than VDD the S/D channels switches to a value very close to 0 ohms. Pushing PTT pulls G of the left MOSFET to GND thus switching on the transmitter. G of the right MOSFET is now pulled to VDD (via 10k) which means that the right MOSFET becomes non-conductive and receiver is turned of. A dual-LED (red and green) in the front panel shows the current status.
After having built this respective board with two NE612 ICs (one for DSB generator, one for the TX mixer) I was not satisfied with carrier suppression of the DSB generator. It turned out as only 40dB. Afterwards I constructed a new board with an old SIEMENS Mixer IC (S 042 P) that is still available NOS from various sources. With this one I gained carrier suppression rates of around 55dB. I think this is OK for a homemade transceiver.
The board looks as follows, set up on a 6x4cm 0.1″ veroboard:
The circuit starts with an AF amplifier equipped with a bipolar transistor where also a power supply for Electret microphones has been added. The radio now can handle dynamic and Electret microphones adequately.
Afterwards we see the S042P mixer IC where I have changed the circuit slighty to the one used in my 40-meter-QRO TRX. Audio input signal is now to PIN8 of the IC, Lo input on the rf side of the IC to PIN11 and PIN13. To reduce carrier level and enhance carrier suppression a 5.6pF cap is in series because the relatively high level of signal coming from the LO amp would deteriorate the performance of the DSB generator without countermeasures.
Output from this DSB generator is also symmetric and fairly high. Thus a low valued capacitor has been inserted prior to the SSB filter, sited on the RX board.
After that we see an amplifier with limited gain due to high emitter degeneration and the NE612 as TX mixer. The latter one also with an symmetric output to get more gain from it by using the two inherent output transistors.
TX-power amplifier stages
As I have described in the article of my “Give me 5“-Transceiver some years ago, building a broadband power amplifier is challenging due to one special problem related with the wide range of frequencies that this amplifier must be able to cope with. an extra gain of 5 to 6 dB is commen, when the frequency is divided by the factor of 2. Usually the necessary compensation is done by adding adequate capacitors and inductances using their frequency depending reactance.
With this radio I tried something new. I added an amplifier that is gain controlled by an adjustable voltage. Here a dual-gate MOSFET with gain control to gate 2 sets up the initial stage of the whole amplifier strip. The stage’s gain is set by a simple bipolar driver transistor controlled by a digital-analog-converter (DAC). A numeric value for each individual band is stored with in the EEPROM of the MUC. This numeric value is calculated during adjustment, then stored in the MUC and recalled whenever the radio is switched to a certain band. The DAC is an MCP4725 breakout board, containing a 12-bit device.
After that we see an amplifier that is common solid state technology. Preamp stage and predriver stage are set to A mode which requires a heat sink for the predriver stage. Here a 2N3866 is used as amplifying element.
Driver stage is single ended, operates in AB-mode and also is protected by a heat sink.
After that a somehow uncommon technique has been applied. Instead of using a broadband transformer to reduce the stages output impedance to the some ohms input impedance of the final stage, a set of 6 switchable low-pass-filters is used.
This filter section has been optimized to an output impedance of 50 ohms for each band thus enabling me to test and optimize the transmitter to a maximum with a defined output impedance (remember, this is an experimental radio! 😉 ).
After this filter section the final amplifier stage follows which is able to drive the output power up to 15 to 20 watts on all bands but depending on the DC voltage used for transmitting. The max. power gained during tests was 22 watts pep at 15V DC with two NTE236 transistors. Unfortunately the turned out not to be so rugged and blew in the tests. The eleflow 2SC1969 inserted later showed no problems at all. Thank God! When running on 12.0 V DC the amplifier puts out 12 watts at all bands.
The final part of the transmitter section is the last low-pass filter that is positioned next to antenna relay in the same compartment:
The whole transmitter looks like this:
The various units are:
1: DSB-Generator and TX mixer
2: Amplifier stages 1 to 4
3: MCP4725 transmitter gain controller
4: Intermediate LPF board
5: Power amplifier
6: Final LPF section
7: TX/RX switch board
Here a little bit of analysis to end with the article. First is the output of the SSB-Generator/TX-mixer board with maximum output (Around 500mV pp) set to the 40m band.
Nest we see the carrier suppression when dual tone audio in has been suspended. Carrier is about 55db under the signal peak.
And here an output signal with max. power at 3.5 and 7 MHz:
So, that’s all for today, thanks for watching and 73!
The receiver had to match a lot of requirements that should be described first:
Particularly on the lower bands and with effective long wire antennas the receiver front end will see high signal levels that it has to cope with. IMD always is a serious topic in this case.
Sensitivity particularly on the higher bands, where noise level is ow and signals are weak, is also an issue.
Dynamic range and extensive AGC gain compensation should be as high as possible.
This lead to a circuit that has proven its stability in lots of my radios:
Band filtering for each band with a double and loosely coupled LC circuits
Dual-Gate MOSFET (part of the AGC chain) as the first amplifier
Diode ring mixer (with Schottky diodes)
Post mixer amplifier with Dual-Gate MOSFET (part of the AGC chain)
SSB Filter (now 10.7 MHz) also used for transmitter (relay switched)
Main IF amplifier with MC1350 (part of the AGC chain)
Audio preamp with bipolar transistor
Audio final amp: (once again! 😉 ) LM386
Before describing the receiver itself we will have look at the band pass filter unit, that is shared between receiver and transmitter:
To minimize stray energy traveling from the input to the output of the filter, two SMD relays have been used on each side of the filter per band. And to reduce feedback fromt the transmitter (when the BPF is used to filter the TX signal after the TX mixer) the filter has been placed far away from the TX amplifier section.With an overwhelming result: The transmitter is nearly unconditionally stable now (compared to the TX section used in the “Give me 5”-Transceiver that had severe shortcoming in this aspect.
Control leads for the relays follow a designated coding scheme:
The receiver’s circuit
VFO signal is coupled into the DBM via a 10nF capacitor. The same is valid for the amplified RF signal from the output of the first amplifier stage using a Dual-Gate MOSFET (40676, BF900 or equ.).
Another Dual-Gate MOSFET is used as the post-mixer amplifier. All Dual-Gate MOSFETs so far are part of the AGC-Chain. This maximizes the possible gain swing to about 40 to 50 db. and enhances the receiver’s capability to handle even the strongest signal levels without distorting the output signal and the end of the audio chain.
Next is the SSB-Filter. Due to this is an “experimental” transceiver, the filter has not been soldered to the circuit board. Instead it is fixed with an aluminum clamp into two parts of header strips. Thus I can compare numerous SSB-Filters (9-, 10.695-, 10.7-MHz commercial ones, various home made ladder filters etc.). Here the different performance is very interesting to be explored.
The filter is accompanied by a special rf relay (manufacturer “Teledyne” with excellent performance concerning separation for the two channels) so that it can be used as the SSB filter for the transmitter section.
After the filter section the IF amplifier follows. This one uses an MC1350 video amp (old but good and still available, even in SMD!) and this IC also is controlled by AGC. The input is unbalanced (PIN6 to GND) the output is balanced and terminated with a tuned circuit.
Demodulator is an SA602 mixer IC.
After that the signal is handed over to the audio chain. But before the signal is processed in the next stage the frequency range is limited by a low-pass filter to reduce hiss. This filter also has two switched capacitors (controlled by MCU via NPN-driver stages) to adapt the sound to the preferred settings of the user. The software contains a respective function.
The audio amplifier consists of two sections: A preamp with a bipolar transistor and the inevitable and well-know LM386.
The full circuit on a 6×8 cm veroboard:
Starting from left top corner there is a 1:4 input transformer (not in the schematic), the preamp, the DBM, post mixer amp, SSB filter, relay, MC1350 as IF amp, demodulator and 2 stages of audio amp.
Performance is excellent. The circuit has no problem with high signal levels (in-band and out-of-band) especially on 40 meters. No IMD problems are noticeable even when used with high gain antennas like a 2×25 meter doublet with a tuner. On the higher bands noise figure is pretty OK what I think is based on the usage of Dual-Gate MOSFETs in 2 of the 3 amplifier stages. The MC1350 deteriorates this to a certain degree but is still very much acceptable for a shortwave radio.
This short article will describe the adapter board that is connected to analog data sources and that is converting the respective voltage data into suitable voltage levels for the ADC inputs PA0:PA4 at the microcontroller:
The following data will be converted and later shown on the display:
User keys (Key1:Key3)
TX power measurement
PA temperature (Sensor: KTY81-210 switched against GND)
AGC output (DC) from receiver => S-Meter
This article covers the remaining digital (or “analog to digital”) stuff, next on the agenda will be the receiver.
This 6-band transceiver has several stages where band switching will occur:
The band pass filter section (shared by transmitter and receiver)
A first section of low pass filters (LPF) between the driver stage and the final amplifier
A second section of LPFs at the end of the rf power amplifier chain.
To keep the circuit simple and to save controller output ports I have decided to code the band number (0 for 160m up to 5 for 10m) in binary and send this pattern to pins PA0:PA2 of the MUC. This is pattern is lead to a BCD to Decimal Decoder integrated circuit (HCF4028) that converts the binary pattern to a set of individual output lines. The respective part of the truth table used is:
The 6 lines are fed into an ULN2003 integrated circuit, which is a relay and motor driver.
The outputs of this driver are switched against GND thus the relay coils have to be supplied with VDD (+12V in this case). The IC also contains a clamp diode for each output. That makes the circuit fairly simple. The full circuit of this unit:
The heart of this transceiver is an ATmega128 microcontroller (MCU). It controls the vast majority of functions within the radio. E. g.: Frequency generation of the 2 DDS systems, audio tone and AGC decay time, T/R-switching, the presets for transmitter gain on the 6 bands independently, display and panel lights etc. etc.
And, due to usage of a parallel interface for the LCD (8 data lines and 4 control lines) an MCU with sufficient ports had to be used.
First I started with the SPI version of the LCD (ILI9341). This LCD has a high resolution of 240×320 dots. Driven by a relatively slow 8-bit controller like an AVR and the LCD driven in serial mode the performance was inferior.
Next I found that the same LCD is also available with a parallel interface. Then called CP11003. This one uses 12 lines (8 data and 4 control lines minimum), which made it mandatory to use an ATMega128 controller. To enhance speed and performance this one is clocked by a 16 MHz crystal. A touchpad is also integrated, but not used in my application.
Source code in C programming language can be downloaded from Github.
The two DDS oscillators are mounted to the side of the cabinet. They are sited close to the microcontroller board to keep leads short.
Right on the left you can see the small dual-tone oscillator for testing and tuning. Next is the AD9834-equipped local oscillator (LO), centered the AD9951 that serves as the VFO. Right the ATmega128, mounted to a 64 lead breakout board can bee spotted behind the varios cables going to and from this section.
The Dual-Tone Oscillator
This one consists of two simple phase-shift audio oscillators. I have introduced this circuit a longer time ago for testing purposes here in this blog.
The capacitors and resistors in the phase-shifting chain have been chosen to put the two different frequencies to values of about 700Hz and 1900Hz, thus they are not harmonically related. A variable resistors allows the user to set the balance between the two signals so that they are equal in voltage.
Two transistors (a PNP-NPN pair) are switched by Pin PB7 from the microcontroller. There is a respective function in the software that activates the transmitter together with this oscillator for comfortable tuning and testing.
The Local Oscillator (LO)
This one again uses the “good old” AD9834, overclocked to 100MHz. I found that some chips from the “grey market” have problems when being overclocked and therefore produce spurious signals. In case this occurs, it is recommended to step back to the clock frequency of 75 MHz which is high enough for the purpose of the LO.
The oscillator comes with an balun output transformer (will reduce spurs!) and a low-pass filter plus a simple amplifier. The latter basically is not necessary because the LO will only have to drive the inputs of SA602 integrated mixer circuits (200mV RMS) used as SSB generator and rx demodulator. I had another mixer type in mind before, that one needed higher voltage. Thus the coupling to PIN6 of SA602 is only via 5.6pF capacitor to avoid overdriving the mixer and improve signal purity. This will be shown later when we are about to discuss receiver and transmitter circuitry.
Here the AD9951 DDS again comes to operation. This one has got a 14-bit DAC which makes it less prone for spurious signals. The clock rate has been pushed to the limit of 400MHz which, according to datasheet, is the max. clock rate for this DDS module.
You can download a datasheet of a suitable clock oscillator. This device is very small but it can be soldered to a 2 by 2 hole piece of veroboard and then mounted to a piece of headerstrip by soldering wires to the underside of the board:
A voltage divider will reduce the 3.3 V to 1.7V that is acceptable for the clock input of the AD9951 chip.
The DDS circuit is common for frequent readers of this blog:
The low pass filter has been left out because when examining the output signal of the DDS it turned out to contain only very little quantum of harmonics. The max. frequency of this VFO will be 29.7 MHz + 9MHz which equals to 38,7 MHz.
An SSB radio for the HF bands will be presented. Featuring 12 to 20 Watts of output power (depending on DC supply), full DDS frequency generation, covering 6 major frequency bands (1.8, 3.5, 7, 14, 21 and 28 MHz) within the short wave amateur radio spectrum. The rig also features colored LCD and front panel backlight.
In this upcoming series of articles a relatively complex project will be discussed. It is some sort of „remake“ of my last multi-band QRP SSB transceiver that has been entitled the „Gimme Five“-Transceiver and that was finished in 2015. „5“ in that case stands for the 5 major (i. e. „classical“) RF bands: 80m, 40, 20m, 15m and 10m the radio covered. This new project (called the „Midi6“, because it is not a “Micro” or a “Mini” transceiver 😉 ) covers one band more, the range has been extended to 160m.
The basic features of this construction are:
Dual DDS frequency generation (AD9951 as VFO, AD9834 as LO),
Colored LCD (CP11003) with resolution 240×320 pixels,
Single conversion superhet receiver, interfrequency 9 MHz,
5 stage high quality transmitter, Pout=20W (max. at 15V DC) , featuring a microcontroller driven regulated gain stage to ensure absolute constant output on all bands,
Integrated 2-tone oscillator for testing and tuning,
Front panel full backlight.
“Experimental radio” means that there is enough space inside the cabinet to change boards and test new ideas in the same space. Also certain components like the SSB-filter have been made as “plug-in” components to enable quick change of the part. Also the connector between the various transmitter and receiver stages have been done by “jumpers” and header strips so that resistors and capacitors can be changed quickly to experiment with other values.
The radio has been realized with standard veroboards (0.1″ pitch), SMD components and been put into a homemade aluminum cabinet using 2 layer sandwich construction inside the cabinet.
Here a snapshot of the operational transceiver. Cabinet size, by the way, is 7.5 x 16.5 x 19.5 centimeters (2.95 x 6.5 x 7.68 inches). These dimensions are in the range of other multiband QRP transceivers like the Elecraft K2 (larger) or the Icom IC703 (a bit smaller).
For my current project, a compact sized multiband transceiver, I wanted to have a colored LCD module as display. On the web I found the ILI9341 LCD. This display has got up to >200000 different colors (depending on the respective mode you chose) and a resolution of 320×240 pixels. It can be driven in various parallel and serial (SPI) modes, therefore it is very versatile. Price for the LCD is about 10 US$ (11€).
First I developed code for a 4-line SPI interface. The display worked, but I found that it was much too slow. A lot of data has to be transferred because due to the higher resolution of the LCD I chose a 12×16 pixel font. That is very much for a small microcontroller (I am using an old ATMega128) clocked to 16 MHz via serial transmission
But I loved the colors and the luminance of the LCD. After a brief research I found that there is also a PCB available for parallel driving. This is sold as a “CP11003” display. The ATMega128 has plenty of ports and that made me think of driving it in parallel mode.
This display has 16 data connectors (DB0:DB15) of which 8 can be used for driving it in parallel mode. As common for parallel bus type LCDs these are the higher 8 bits of the data bus, thus DB8:DB15. DB0:DB7 are not used and therefore not connected.
As control lines there are “RS” (Data or command indicator), “WR” (write operation indicator),”RD” (read operation indicator) and “RES” (reset) are used for control.
CS (chip select) can be connected to GND when the LCD is the only device connected to the 8-bit bus.
Software development was easy using the GNU C compiler vor AVRs. (I still don’t use Arduino libraries! ;-)) The code can be found after this article.
Final hint: The module also has a touchscreen integrated but that is not in use here!