A Micro Mulibander – Step by step (Part I)

This project tries a new personal approach in designing a very small (i. e. a micro) QRP radio. And also new is the way I want to report about it. The blog entries will be published more or less simultanously to the building progress of the respective modules.

1 A brief project description

The main objective is to set up a SSB QRP transceiver for 6 HF bands (similar to my 5-bander introduced in 2015) now starting with 160m, then 80m, 40m, 20m, 15m and 10m at last.

Another idea I have in mind is to build the rig from separate modules for each single stage so that each main circuit (mainly receiver and transmitter section) is constructed with the needed stages on verobaords that are mechanically identical.

The idea behind that is that a board which does not show top performance in function (or even completely fails) can be changed quickly and an improved version can be installed easily without the need to throw the whole receiver (for example) into the junk box labelled with “failed projects”.

Band switching will be done by small relays again (I purchased 60 SMD relays for 40€ some months ago). The band filter section will be shared by tx and rx section this time. This saves space and reduces effort.

The transmitter will be a 4 stage unit again (more or less the same like in my 5 bander). Output power projected is 5 to 10 watts on all the bands.

The receiver is designed once again as a single conversion superhet because experiments with double conversion were not successful due to a large number of spurs audible in the receiver.

The first mixer is set to be an SBL-3 diode ring mixer. This will give the receiver a very good handling of strong signals, I hope. IF amplifier will be a two staged one with dual gate mosfets controlled by an audio derived AGV voltage. The rest? The usual suspects, I would say. Wait and see!

1.1 The VFO module

I have become quite familiar with SiLab’s Si5351 oscillator module the recent months. I first used it in my “Micro 20-3” trx which was a success. The module is very small, completely ready for use (I’m still using a breakout board made by Adafruit) and able to handle 5V. It provides 3 oscillators that can be programmed independently to put out something lieke a square wave ranging from 8kHz to 160MHz. I have developed a software that avoids any tuning noise, so, this oscillator (which is a clock oscillator by intention) can be used as a VFO for an amateur project.

To keep the effort simple, I reused the 1306 oled module that you can see in lots of my previous projects. Both boards (Si5351 and 1306 oled) are controlled by I²C-bus which allowes me to use a relatively simple micro controller. In this case again I have the Arduino Pro Mini containing an ATMega168 controller (16 MHz) on board. If it should turn out that I might need more memory, the same board here is on stock carrying an ATmega328 controller. Let’s see how this will work out.

This is the circuit of the complete VFO module:

Si5351 VFO for Micro Multiband QRP SSB TRX (C) DK7IH 2018
Si5351 VFO for Micro Multiband QRP SSB TRX (C) DK7IH 2018

The module will be placed behind the front panel.

Tuning will be done by a Bourns optical encoder that turns very smoothely. I purchased some for under 5 Euros each from pollin.de. An unbeatable price! Unfortunatley they have been quickly sold out.

The core of this module is the Arduino Pro Mini microcontroller centered on the diagram. It is connected to the Si5351 breakout board and the 1306 oled display by I²C bus.

Si5351: Output 0 is used as VFO terminal and output 1 carries the LO signal with 9Mhz. To avoid digital noise spreading on the +5V line a 100µF capacitor should be switched close to the VDD terminal. Proper and short grounding also is recommended to avoid spurs.

OLED1306: Also a 100µF electrolytic capacitor has been added including a 10µH rf choke forming a low pass filter together. I found that these oleds a very prone to distribute digital noise via VVD line, so this measure contributes much to keep your receiver clean from any unwanted signal spektrum generated by the oled.

Keep in mind to tie SCK and SDA lines to +5V via two resistors of 4.7kOhms each!

Band switching: It is software controlled. To save output ports I did not connect the 6 relay drivers for the 6 bands directly to the ports of the microcontroller. I’m using an open collector BCD to DEC driver (74LS145) instead. Ports PB0, PB1 and PB2 are forming a 3-bit pattern that is switched to 6 output lines (output 0 to output 5) of the BCD2DEC driver IC. 74LS145 is capable of handling switch voltages up to 15V thus working with 12V coil relays is easy.

User control interface: This rig has 4 different switches that will be explained later from the functional point of view. The operator can set nearly all functions of the transceiver by using these push buttons and the main tuning wheel. The buttons  switch to GND by 4 different resistors and are read by PC0 port of the micro. PC0 equivalents to  channel 0 of the integrated analog-to-digital converter (ADC) inside the ATMega168. This also saves controller ports to a large extent (using 3 instead of 6 ports!). So, all in all, I think I can dare controlling a multibander by a relatively small microcontroller.

(To be continued!)


5 thoughts on “A Micro Mulibander – Step by step (Part I)

  1. Hi Peter,
    Sounds like a great project, look very much forward to seeing the whole project.
    Keep up the great work you do
    John ZL2TCA

  2. Always loved your work Peter. Thank you !
    Will keep tracking this project.

  3. gOOd morning Peter,

    I am always glad that your blog is informing me about any news automatically. As I mentioned before it was your „QRP Baubuch“ which brought me to the path of homebrewing and qrp.
    I am looking forward to read about your new „Take 5“ project.

    As the lyrics of the original song “Take five“ are pointing it out ……. :

    “Start a little conversation now, it’s alright, just take five, just take five…“

    bon chance es 72 de sTef – DM5TU / VY1QRP

  4. Hi again Peter, your si5351 oscillator/controller looks FB. Some si5351 designers use CLK 0 and 2 for the two oscillators rather than adjacent clocks, on the basis that there is more electronic and physical separation. I suspect this may be folklore, I haven’t seen any actual measurement, but I suggest it as ‘common and/or good practice’. Look forward to future episodes in your serial. 73 VK3HN.

    1. Hi Paul, I agree. I think the Si5351 as an oscillator or a combination of VFO and LO has got other problems than signal separation. I have observed that in a VFO/LO combination there is some noise on the unused channel (in the range of some mV) but no detectable RF signal of the channel in use.

      And without knowing about the interior structures of the chip it is just guessing that using CLK0 and 2 might have any benefit. But what I found is that if you don’t set an unused channel deliberately to 0Hz and switch it off this way, this noise can be reduced to a certain degree. But that was just an observation which I haven’t quantified.

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