+ Block schema
+ Working of the SSB 6.1
+ Direct Digital Synthesis (DDS)
+ The DDS1 used as VFO
+ Testing the DDS1 VFO
+ Building the SSB 6.1 Tranceiver
As a project to get through the corona time properly, I started building the TRX SSB6.1 following by example of Nick (N.A.) Strong – G0CWA. I want to record here my adventures of building this TRX. Note: many others have preceded me!!!
So it must be a beautiful and well-functioning design.
It is an interesting project from the perspective of mechanic and also electronic view. For example soldering the SMD components in particular has proved to be a major challenge!! And not only the smd components, as the adjacent picture shows. Here you see one of the legs of the optocoupler is not soldered properly. Strict control in every step is really necessary to avoid disappointment later on.
The SSB6.1 TRX is delivered as a build project by Alie-express for example, where I bought it. In the package you will find everything needed, smd, fet, copper wire, coils, etc. But ….. the VFO (DDS) is NOT included. So you have to buy it separately. Nick has provided ideas for this, but it is good to orient yourself in advance.
As I mentioned the VFO is the weak point of this design. A DDS using the AD9850 CMOS, 125 MHz Complete DDS Synthesizer is used in the DDS 1 and also DDS 2. This synthesizer is problematic in the shape of its output signal. The signal produces a lot of harmonics, which of course affects the RX signal of the SSB 6.1. I will come back to this topic later with experiences to improve the quality of the output signal.
Discussion of the block diagram is a must for a good understanding of working of the TRX. For this I use the preliminary work that PY1EGG did by drawing a block diagram. I found this on the site “linuxquestions.ru” . Unfortunately it is written in Russian, but i can handle it a little.
Unfortunately Nick forgot in his build instructions to explain the working of the TRX however i can understand wy he did’n, but for the project good, understanding of the working is nesseccary.
You can download the block scheme from the link here.
Power supply: 12 volt
Range: 3.5, 7, 10, 14, 21, 28 MHz.
Modulation: SSB – LSB & USB; AM only receiving, CW
Power: 45 Watt, if connected with a power amplifier.
The device works according to the standard superheterodyne double conversion principe.
In the block scheme you find two basic colors: Green for the RX way and Yellow for the TX way.
In receive mode, the signal enters the input of the transceiver through the RX connector. Then, via relay JK1, it reaches a series of band-pass filters (circuit with L1-L12), which switch between ranges using direct-biased diodes D1-D12. The diode control signal comes from the synthesizer and is sent to them through the opto couplers. In the diagram the optocouplers (OPTOISO1) number 1 corresponds to the range of 80M and number 6 to 10M. After passing through the filters, the signal enters the preamplifier created with a dual gate MosFet M1.
Dual gate MosFet are widely used in the transceivers to implement automatic gain control (AGC, automatic gain correction in the AGC scheme).
After the amplification stage with the dual gate MosFet M1, the signal is sent to a balanced RX mixer IC1 and thereafter to first IF stage with L18. The signal then is amplified by the dual gate mosfet M2 and filtered in circuit C41-L13. Then it goes through the relay JK2 and reaches the quartz ladder filter form by quartz resonators JT1-JT5. The center frequency of the ladder filter is 8 MHz. This filter is controllable in the passband by supplying voltage to varicaps BB1-BB4 in a bandwidth circuit (bandwidth potentiometer). Then the IF signal is fed to a third amplification circuit with the dual gate mosfet M4.
Thereafter the amplified and filtered IF signal is sent to the input of a second balanced mixer with IC3.
It also has an IF generator. The IF generator (xtal BFO) is also controllable to shift the frequency up or down depending on the receiving mode. For the upper sideband (USB) the frequency of the Xtal shifts downwards by 1.5 kilohertz (8 MHZ – 1.5 kHz = 7998500 Hz), for the lower (LSB) it shifts upwards (8 MHz + 1.5 kHz).
After the mixer we get a normal signal that goes to the low pass filter of the C13-R6-C14 and goes through CON3 connector to the volume control (Potentiometer Volume) and to the low-frequency amplifier on the TDA2003 chip.
The VLF output is connected to the AGC circuit through a detector on D17 and a buffer stage with transistor Q4. This way of controlling the AGC is very problematic while the AGC will not working at a low low volume level. A reasonable solution would be to supply a control signal of R6 via an additional amplifier stage.
The transmit mode is activated by pressing the PTT button.
The signal from the microphone goes to the pre-amplifier with transistor Q3, from where it goes to the input of the balance mixer IC3. In the balance mixer IC3 the desired sideband will be select. Then signal goes to the quartz ladder filter to suppres the undesired frequencies. The (hopely) clean signal goes to filter section with C40-L14 and injected in the gate from a mosfet amplification stage M3 and circuit C37-L17.
After this, a second frequency conversion takes place in TX-mixer with IC2. Here the signal wil be injected with de VFO frequency for the desired band. After passing the mixer the signal will be filtered in the bandpass filter and goes thereafter to the TX-connector.
As mentioned before the VFO is no part of the tranceiver project. You have to develop a VFO yourself or buy it ready-made.
Nick went for a ready-made DDS VFO with the AD9850 synthersizer as a basis.
You have two types of this VFO: the DDS 1 and the DDS 2. Both for a reasonable price for sale in a distant country. I chose the DDS 1 and will share my experiences with this VFO with you.
Direct digital synthesis (DDS)
Direct digital synthesis (DDS) is a method employed by frequency synthesizers used for creating arbitrary waveforms from a single, fixed-frequency reference clock. DDS is used in applications such as signal generation, local oscillators in communication systems, function generators, mixers, modulators, sound synthesizers and as part of a digital phase-locked loop.
A basic Direct Digital Synthesizer consists of a frequency reference (often a crystal or SAW oscillator), a numerically controlled oscillator (NCO) and a digital-to-analog converter (DAC) as shown in Figure.
The reference oscillator provides a stable time base for the system and determines the frequency accuracy of the DDS. It provides the clock to the NCO, which produces at its output a discrete-time, quantized version of the desired output waveform (often a sinusoid) whose period is controlled by the digital word contained in the Frequency Control Register. The sampled, digital waveform is converted to an analog waveform by the DAC. The output reconstruction filter rejects the spectral replicas produced by the zero-order hold inherent in the analog conversion process. (Bases on the WiKi page Direct digital synthesis)
The DDS 1 used VFO
The DDS1 is a 6 bands VFO based on the AD9850 CMOS, 125 MHz Complete DDS Synthesizer from Analog Devices.
The features are:
- Supports 0~55MHz continual adjust at 1Hz step.
- Power supply 8-9 V. Input current at 200 mA.
- The 5p / 6p connector connect to encoder as picture shows.
- The middle 7p connect to digital matrix input.
- The 14p connector powered the VFO from main board. (8V + Gnd no extern supply needed) and send a signal to the band switch
- 6 frequency bands are provided: (3.5 MHz, 7 MHz, 10 MHz, 14 MHz, 21 MHz, 28 MHz).
- The four mode choices are AM, LSB, USB, CW.
- Formed the transmit DT signal for main board.
The DDS need to be feed with 8-9 volt from a power supply or be feed from the transceiver board by 14 pins connector.
Beware the supply voltage via the two-pole connector is connected to the 14-pole connector and is only intended for external power with 8-9 volt.
As explained later, this has a significant effect on the output voltage of the VFO.
Below is the detailed schema of the DDS1.
Unlike in DDS2, the frequency compensation of the DDS1 is formed by six transistors in the emitter of the output transistor 2CS3357, which transistors are driven independently of used frequency band. In the two highter frequency the emitter is compensated.
Further the basic frequency of the DDS ia 125 MHz which means that the influence of the temperature and the supply voltage on the output frequency is limited.
Another important difference of the DDS1 from the DDS2 is that the supply voltage of the output transistor is 5 volts stabilized. Which means that the VFO frequency can never exceed that level. The DDS2 uses the supply voltage of 12-13.8 for this, which is more common for transceivers. A considerably higher VFO frequency is the result of this. A big difference that certainly affects the balanced mixers of the tranceiver and therefore the whole transmitter.
Here you can download the schema DDS 1 VFO schema PA3BYB
Testing the DDS1 VFO
Frequency measurements were made on the DDS to evaluate the output signal. This was done at a load of 180 Ohm. The images below are screenshots of the scope.
Immediately noticeable is that the 3.5 MHz signal has a low level compared to that of the other frequencies. Furthermore, the frequencies are definitely not free of harmonics. This can be seen very well with the 21 MHZ and 29 MHz signal. Which means there is still some work to be done.
Building the SSB 6.1 Tranceiver
To get the best possible result, Nick has compiled a very extensive building description. that you can find here.
To build SSB 6.1 I quite strictly followed Nick construction description as mentioned before.
I started soldering the smd components. With a preliminary exercise this gain is not exactly luxurious. After soldering each smd component, everything is neatly marked on the list. Only when there was no other option, the active smd components were also soldered.
It is very wise to carry out a strict check after every step. It is a big task to fix errors later !!
Now an interesting step follows: winding the coils. Some skill is certainly nessecary. I don’t find Nick’s building description very clear here. I therefore gladly used the description that YT2FSG has given.
After this step, it is wise to adjust first the bandpass filters. Here the use of the NanoVNA is useful.
The alignment of the bandpass filter with the NanoVNA then is very easy.
In the same way we use the NanoVNA for the cristal filter. Special for the exact frequency it filters. This frequency you can use in the DDS later on.
And so, after every component has found it place on the PCB the moment is there to align and test the TRX. But before……. a control of the soldered PCB is not a luxery!!!
A other problem before you can start testing of the TRX is that there is a VFO to connect.
And in case of the DDS1 you have to connect also a nummeric keybord, potmeters, stepper and so on. Wich means you need to construct a base to place all these component.
People who use the DDS2 have less of that problem.
I used as contruction material for these mentioned component double side PCB material. It easy to handle and form also a good grounding.
Shown at the top image it looks more and less as a nice tranceiver already.
Will be continued with more experiences later.
Scheme SSB 6.1 Scheme