“Gimme Five” reloaded – A compact 5 band QRP SSB transceiver in SMD technology – NEW BAND LAYOUT

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).

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - Coil data
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – Coil data

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! 😉

73 de Peter (DK7IH)

“Gimme Five” reloaded – A compact 5 band QRP SSB transceiver in SMD technology – THE RECEIVER

Introductory article of this project

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.

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - THE RECEIVER
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – THE RECEIVER

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.

An MAV-11 monolithic amplifier follows the filter to lift the signal a little bit.

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:

mini5_qrp_ssb_trx_dk7ih_receiver_close_up_2

Click to enlarge!

Vy 73 de Peter (DK7IH)

“Gimme Five” reloaded – A compact 5 band QRP SSB transceiver in SMD technology: VFO, LO, MCU etc.

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.

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - Digital unit with VFO, LO, MCU, DAC and LCD (low res.)
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – Digital unit with VFO, LO, MCU, DAC and LCD (low res.)

To watch a high resolution version (4.2MB!) of the wiring scheme, please click here!

Hints:

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:

//Read keys via ADC0
int get_keys(void)
{
    int key_value[] = {39, 76, 103, 135};
    int t1;
 int adcval = get_adc(0);

    //TEST display of ADC value 
    /*
        lcd_putstring(0, 5, " ", 0, 0); 
        oled_putnumber(0, 5, adcval, -1, 0, 0); 
    */
    for(t1 = 0; t1 < 4; t1++)
    {
        if(adcval > key_value[t1] - 10 && adcval < key_value[t1] + 10)
        {
            return t1 + 1;
        }
    }
    return 0;
}

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.

73 de Peter (DK7IH) and thanks for watching!

 

“Gimme Five” reloaded – A compact 5 band QRP SSB transceiver in SMD technology

Follow-up articles published so far:

Abstract

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.

DK7IH Multiband QRP Transceiver for 5 Bands 2020
DK7IH Multiband QRP Transceiver for 5 Bands 2020

Description

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:

Design considerations

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:

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - Port usage on AVR Pro Mini Microcontroller
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – Port usage on AVR Pro Mini Microcontroller

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. 😉

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - Front panel with backlight
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – Front panel with backlight

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. 😉

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - Receiver section (clos-up)
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – Receiver section (clos-up)

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)
  • SSB Filter 9 MHz by box73.de
  • 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:

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - Transmitter section (close-up)
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – Transmitter section (close-up)

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:

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - Power Transmitter section (close-up)
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – Power Transmitter section (close-up)

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.

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - Power Transmitter section with heatsink for testing purposes
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – Power Transmitter section with heatsink for testing purposes

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.

DK7IH Multiband QRP Transceiver for 5 Bands 2020 - Band pass filters
DK7IH Multiband QRP Transceiver for 5 Bands 2020 – Band pass filters

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! 😉

73 de Peter (DK7IH)

An experimental HF 6-band SSB transceiver – Part 7: The Transmitter

This unit basically consists of two parts:

  • SSB-Generator and TX-mixer
  • TX-power amplifier stages

The SSB-Generator and TX-Mixer  Board

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:

DK7IH 6 band QRP SSB TRX 2019 - SSB-Generator and TX-Mixer board
DK7IH 6 band QRP SSB TRX 2019 – SSB-Generator and TX-Mixer board

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.

DK7IH 6 band QRP SSB TRX 2019 - SSB Generator and TX mixer
DK7IH 6 band QRP SSB TRX 2019 – SSB Generator and TX mixer (Full size schematic)

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.

DK7IH 6 band QRP SSB TRX 2019 - Power transmitter
DK7IH 6 band QRP SSB TRX 2019 – Power transmitter (full size picture)

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.

DK7IH 6 band QRP SSB TRX 2019 - Intermediate LPF section
DK7IH 6 band QRP SSB TRX 2019 – Intermediate LPF section

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:

DK7IH 6 band QRP SSB TRX 2019 - Low Pas  Filter Unit for TX
DK7IH 6 band QRP SSB TRX 2019 – Low Pas Filter Unit for TX

The whole transmitter looks like this:

DK7IH 6 band QRP SSB TRX 2019 - Practical setup of the transmitter board
DK7IH 6 band QRP SSB TRX 2019 – Practical setup of the transmitter board

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.

DK7IH 6 band QRP SSB TRX 2019 - TX-Mixer's output signal
DK7IH 6 band QRP SSB TRX 2019 – TX-Mixer’s output signal

Nest we see the carrier suppression when dual tone audio in has been suspended. Carrier is about 55db under the signal peak.

DK7IH 6 band QRP SSB TRX 2019 - TX-Mixer's output signal, suppressed carrier only
DK7IH 6 band QRP SSB TRX 2019 – TX-Mixer’s output signal, suppressed carrier only

And here an output signal with max. power at 3.5 and 7 MHz:

DK7IH 6 band QRP SSB TRX 2019 - TX output at 80m band
DK7IH 6 band QRP SSB TRX 2019 – TX output at 80m band
DK7IH 6 band QRP SSB TRX 2019 - TX output at 20m band - Pout = 12 W PEP
DK7IH 6 band QRP SSB TRX 2019 – TX output at 20m band – Pout = 12 W PEP

So, that’s all for today, thanks for watching and 73!

Peter (DK7IH)

An experimental HF 6-band SSB transceiver – Part 3: The Microcontroller

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.

DK7IH 6 band QRP SSB TRX 2019 - ATMega128 microcontroller
DK7IH 6 band QRP SSB TRX 2019 – ATMega128 microcontroller

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.

Vy 73 de Peter (DK7IH)

An experimental HF 6-band SSB transceiver – Part 2: The Oscillators

The two DDS oscillators are mounted to the side of the cabinet. They are sited close to the microcontroller board to keep leads short.

DK7IH 6 band QRP SSB TRX 2019 - Oscillators
DK7IH 6 band QRP SSB TRX 2019 – Oscillators

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.

DK7IH 6 band QRP SSB TRX 2019 - Dual-Tone Generator
DK7IH 6 band QRP SSB TRX 2019 – Dual-Tone Generator

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.

DK7IH 6 band QRP SSB TRX 2019 - Local Oscillator with AD9834
DK7IH 6 band QRP SSB TRX 2019 – Local Oscillator with AD9834 (Click for full size image)

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.

The VFO

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:

DK7IH 6 band QRP SSB TRX 2019 - 400 MHz clock ocillator of DDS VFO
DK7IH 6 band QRP SSB TRX 2019 – 400 MHz clock ocillator of DDS VFO

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:

DK7IH 6 band QRP SSB TRX 2019 - DDS VFO
DK7IH 6 band QRP SSB TRX 2019 – DDS VFO (Click for full size image!)

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.

Thanks for reading and stay tuned! 😉

Peter (DK7IH)