Menu:



Version:

Apr 14, 2025:
Revised: v1.0

The SugarCube "SC+M" Si5351a VFO

for the 80m/40m MRS-1 HF SSB Transceiver

The SC+M SugarCube-Plus is a Si5351a-based dual-output PLL module designed to replace the original channel and carrier crystal oscillators in the MRS-1 transceiver. 
This compact board lies at the heart of the converted portable MRS-1  SSB transceiver which operates on the 80m and 40m ham bands. 


Figure 1 : The Condor HF SSB SAR handheld tyransceiver
 (Note: Several versions of the Condor exist)


Background

The "SC+M" SugarCube-Plus module (Figure 1) is a new version of my SugarCube-Plus miniature synthesised oscillators specifically designed for the MRS-1 transceiver.That said, it can also be used in a variety of similar transceivers.

The SC+M provides the required variable frequency oscillator (VFO) signal for the 1st mixer and the carrier injection oscillator (CIO) for the balanced modulator and detector in the MRS-1 transceiver. The transceiver's frequency is tuned using a low cost rotary encoder mounted on a replacement front panel. The tuning step size can be selected using the encoder’s integrated push-switch.

The SC+M also includes a digital battery voltmeter and received signal strength meter. It also handles the inputs for the new Band and VFO Lock switches. All of the relevant information is displayed using a standard 128x32 pixel OLED which can be seen in Figure 2.


Figure 2 : The SC+M module connects to a 128 x 32 pixel OLED on the front panel of the MRS-1 transceiver

The module is easy to build with only the tiny Si5351a 10-pin SMD chip providing a challenge for the builder. All other components are regular through-hole components. It’s all build on a postage-stamp sized double-sided PCB.

SC+M Features

SC+M and the MRS-1 Transceiver

Figure 3 shows how the SC+M module is integrated into the MRS-1 transceiver. The VFO output replaces the crystal channel oscillator (“OUT0” in Figure 2) while the CIO output (“OUT2” in Figure 2) replaces the crystal carrier/BFO oscillator.

 Figure 3 : The original channel oscillator and carrier crystal oscillators are replaced by the SC+M module.

The new SC+M functionality is delivered with an 8-pin ATtiny85 microprocessor and a 10-pin Si5351a 3-output digital PLL chip. Some of the ATtiny's pins handle several tasks in order to achieve all the necessary functions using the few available pins. Most noticeable is single pin used for the rotary encoder and step switch.

SC+M Schematic Details

Figure 4 shows the schematic of the SC+M VFO used in the modified MRS-1 transceiver along with the circuit details of the interface used to connect the Si5351a outputs to the MRS-1.

Figure 4 : The SC+M module generates the 80m and 40m VFO output signal and the 10.7MHz LSB carrier oscillator signal using just two small ICs, a voltage regulator, and a few passive components.
Note: "Right click' to view a full scale view of this schematic.

The VFO output generates a ‘high side’ VFO signal exactly 10.7MHz above the operating frequency. For 3.5 – 3.9MHz (80m), the output ranges from 14.2 to 14.6MHz. For 7.0 – 7.3MHz (40m), the output ranges from 17.7 to 18.0MHz. This can be tuned in steps from 10Hz to 10kHz.

The MRS-1 transceivers were originally designed for USB operation but amateur radio uses LSB on 80m and 40m. The SC+M can, of course, generate either carrier frequency. In the modified MRS-1, it is programmed to generate a fixed 10.6985 MHz carrier on the CIO output for LSB operation.

All of the SC+M frequencies and band edge tuning limits are stored in the ATtiny85’s EEPROM and areable to be reprogrammed by the user.

“Single Wire” Rotary Encoder
The rotary encoder uses three resistors (R1, R2 and R5 in Figure 3) as a 2-bit D-to-A converter to allow multiple encoder functions to be managed with just one processor pin.  Any change in the encoder or step switch triggers a software interrupt. This provides very responsive tuning and tuning step selection with only one processor pin. The voltage subsequently measured on this pin indicates the direction of rotation of the encoder.

Figure 5 : The voltage on the ATtiny85’s pin changes as the encoder is rotated CW (Left) and CCW (Right). The interrupt routine is called each time the voltage falls below the Logic High voltage level.

AGC Voltage and S-Meter
The MRS-1 AGC voltage controls the gain of the receiver’s RF and IF amplifiers. This AGC voltage (Figure 6) varies from 4.5V with a weak input signal of -110dBm (“S1”) and 1.5V when the input signal is very strong, around -40dBm (“S9+20dB”).


Figure 6 : The wavy line in the graph is the AGC voltage response as measured on an
MRS-1 transceiver. The straight line is the S-meter OLED bargraph response.

The AGC voltage is measured by the SC+M processor via a pair of scaling resistors. The S-meter OLED bargraph then displays the received signal strength over a signal range from S0/S1 to S9+20dB.

During transmit, the AGC line is clamped to ground in the MRS-1 (via Q12) and the S-meter display , for simplicity, shows a fixed full-scale bargraph.

Battery Meter
The supply voltage to the MRS-1 usually comes from the internal battery in the transceiver's enclosure. This voltage is measured using a resistor divider which is connected in parallel with the Lock pushbutton function. This divider converts the 10 to 16V supply voltage to a range of 2 to 3.25V suitable for the ATtiny85.

The resistor divider’s voltage must not fall below 2V to ensures the battery voltage is not confused with the H-to-L transition of the Lock switch sharing this processor pin. R6 (100k) was added to ensure pressing the Lock switch does not affect the supply voltage. This might occur if this button was accidentally held down for an extended time.

The ATtiny85 periodically measures this voltage and displays it in the lower right hand corner of the OLED display (Figure 2). It’s essentially an expanded range voltmeter measuring from 10 to 12V. The OLED also shows the transceiver's frequency, the current step size (The arrow icon), signal strength (An “S9+” signal level is shown in Figure 2) and the battery meter voltage. The Lock icon can also be seen.

Voltage Regulator
The Si5351a requires 3.3V and, for simplicity, the SC+M including the OLED runs at this voltage. The MRS-1 transceiver operates from 10 to 15VDC although it is normally operated using an internal set of eight AA batteries.

Since the SC+M only needs about 40mA, a simple 78L33 TO-92 linear regulator was used. It runs from the MRS-1 regulated 6VDC rail normally used for the crystal oscillators. This minimises heat dissipation.

SC+M PCB
The SC+M module mounts in the location normally used for the crystal oscillator. Four connectors handle the wiring to the various parts of the MRS-1 transceiver including the front panel.

The PCB layout can be seen in Figure 7. One of a number of prototype modules can be seen in Figure 1.

Gerber files for the PCB can be obtained from the Download section at the end of this page.





Figure 7 : The SC+M module measures just 33 x 23 mm,
the size of a large postage stamp. Four multi-pin 0.1”
DuPont-type connectors handle the MRS-1 wiring.


SC+M Module Construction

The first step is to mount the Si5351a, the tiny 10-pin SMD IC. It may be soldered by hand, with a hot plate, or with a hot-air gun. Note: The remainder of the module assembly can be completed using standard through-hole parts and the usual methods.

Next, solder in the 25MHz crystal.

AT THIS POINT, you can check if you have successfully soldered the Si5351a at this point BEFORE fitting any other parts – See Appendix A below.

The DIL-8 socket for the ATtiny85 should be fitted next. The socket allows the ATtiny85 to be removed for reprogramming if that should be necessary at some later date. This also permits easy calibration of the SC+M (Refer to Appendix B below).

Now fit the resistors and capacitors, and then install the regulator. Ensure it is mounted so that at least 3mm of the regulator’s legs are visible for better heat dissipation.

Finally, add the various connectors. These can be cut from a 20 or 40 way 0.1” DuPont-type male pin strip.

SC+M Alignment and Testing

See Appendix B for the calibration procedure if you wish to get your output frequency accurate to within 10Hz.

Appendix C describes how to program the ATtiny85.

Appendix D describes a test jig that can be built to very quickly test SC+M modules.

Kits for the SC+M VFO

This is currently under discussion at the local amateur radio club. Frequent visitors to my website will know that I do not offer kits. However, if this plan proceeds, let me know (via email) if you are interested in a SC+M VFO kit.

-----
 

Appendix A : Checking the Si5351a Soldering

It’s useful to be able to check that the Si5351a has been installed correctly immediately after soldering it (and the crystal!) into the SC+M PCB. No other parts should be installed on the PCB. This is a particularly helpful test when a number of these modules are being built at the same time.

I built the prototype using a second SC+M PCB which just contained the ATtiny85, encoder, switches, and the voltage regulator. I’ve called this the “Test & CAL Board” to make it clear what I’m talking about in the rest of this description. This board DOES NOT have a Si5351a chip mounted on it. The ATtiny85 on this board is programmed with the SC+M Calibration software.

A 4-wire “flying lead” runs from this Test & Cal board to the SC+M board with its Si5351a and crystal i.e. the module being tested. (I’ll just refer to this as the “SC+M”) These four wires are for +3.3V, ground, and the two I2C wires.

The circuit is shown in Figure 8:

Figure 8 : Si5351a Soldering Test & CALibration System
(Right click to view full size)

This arrangement was used to test a group of ten SC+M modules, for example, each with ONLY the Si5351a and crystal soldered in place. I temporarily soldered the four wires onto the underside of each SC+M, one by one.

A useful feature of this test, aside from checking that the Si5351a has been correctly soldered in place, is that the calibration value for each SC+M’s crystal with its Si5351a can also be determined.

So, once everything is connected, turn on the power to the Test & Cal board and check the Si5351a and crystal on the SC+M board are operating correctly.

If you now wish to get the exact crystal frequency to calibrate your SC+M, read Appendix B.

------
 

Appendix B : SC+M Crystal Calibration

EITHER the test arrangement shown in Figure 8 (above) OR the Test Jig shown in Figure 10 may be used to determine the calibration frequency for the crystal in the SC+M board.

The procedure (Figure 9) only takes a few minutes to complete:

 
Figure 9 : Calibration Procedure

Using the USBasp programmer and Extreme Burner or a similar ATtiny programming software package:

1.    Connect your ATtiny85 to the USBasp programmer and start Extreme Burner (“EB”) on your PC.

2.    Load the Calibration software into EB i.e. The Calibration HEX file and Calibration EEP file, and enter the correct fuse settings into the Fuses page in Extreme (High = 0dfH, Low = &0e1H)

3.    Program the ATtiny85 flash/program memory with the Calibration HEX file
4.    Program the ATtiny85’s EEPROM with the Calibration EEP file
5.    Program the ATtiny85’s fuses (High = 0dfH, Low + &0e1H)
Note: In EB, steps 3, 4 and 5 can be done at once by clicking on the tab Write and clicking on All

6.    Insert the programmed ATtiny85 into the SC+M module.

7.    Select 40m with the Band switch.

8.    Connect your frequency counter to pin 1 of J4 on the SC+M module.

Note: The frequency counter should be temperature stable, accurately calibrated, and allow measurements to a resolution of 1Hz.

9.    Turn on the power to the SC+M. You should see the Calibration startup screen message. The screen will then display a frequency of 25.000.000 MHz. This is NOT the output frequency of the VFO. It’s actually the value of the crystal frequency that is currently saved in the ATtiny85’s memory.

10.    A small “X” character should be visible at the top RHS of the OLED screen. If the OLED shows a different frequency, usually 5.0MHz, and the character “T” (for “Test Mode”) can be seen at top RHS of the OLED, then switch the Band switch to 40m. The display should now correctly show 25.000.000 MHz.

In this “X” (i.e. “Xtal” mode), the VFO’s output of 25MHz is being calculated in software using the on-screen value. However, since the crystal value may not actually be the correct value for your crystal, the output frequency will likely be offset slightly from the correct value.

11.    Using the rotary encoder, adjust the value shown on the OLED until the output frequency of the VFO measured by your accurate frequency counter is exactly 25.000000 MHz. Use the step select switch in the encoder to select the required tuning steps.

12.    Write down the value on the OLED. It will probably be something like 25.003.158 MHz. This is the exact frequency of your crystal installed in your SC+M module.

Now we will update the EEPROM file for the SC+M module.

13.    Enter the new calibration frequency for your crystal into the MRS1_SCplus_EEPROM.xlsm spreadsheet. If necessary, this spreadsheet can also be used to amend the band edge and starting frequencies for each band as well as the BFO frequency for each band.

14.    Click on the green ‘Write EEP’ box on the spreadsheet. This will automatically create the new EEPROM file for your VFO. It is located in the same directory as the spreadsheet.

Program your ATtiny85 using the SC+M HEX software and new EEPROM EEP data. 

15.    Load EB with the MRS-1 HEX program software, the new EEPROM file “MRS1_SCplus.eep”, and the revised VFO’s fuse settings. (High = 05fH, Low + &0e1H) 

16.    Program the ATtiny85 flash/program memory with the MRS-1 HEX file
17.    Program the ATtiny85’s EEPROM with the “MRS1_SCplus.eep” EEP file
18.    Program the ATtiny85’s fuses (High = 05fH, Low + &0e1H)
Note: In EB, steps 3, 4 and 5 can be done at once by clicking on the tab Write and clicking on All

That’s it! Now insert the programmed ATtiny85 into the SC+M module and you’ll be right on frequency!

Note: If you need to reprogram the ATtiny85 in the SC+M module, the SC+M fuse settings require you to first erase the ATtiny85 using a HV programmer or erase tool. Since these programmers can be expensive to buy, you can make my simple chip eraser and fuse restorer (“CEFR”) for this task. The details are on my website.

-----
 

Appendix C : Programming the ATtiny85

Background

The SC+M HEX file contains the software required to drive the Si5351a and the functions required for the modified MRS-1 transceiver. This is loaded into the flash (program) memory of the ATtiny85.

The EEPROM in the ATtiny85 contains the user-programmable frequencies used by the SC+M band limits (i.e. the upper and lower band edges where the VFO stops tuning) and the starting frequency for each band i.e. The frequency the SC+M module initially uses when the power is first applied.

The EEPROM also holds the crystal frequency for the specific crystal in the SC+M VFO. Adjusting this value allows you to calibrate the VFO frequency so that it will operate within, typically, 10Hz of the frequency reported on the OLED. These values are programmed into the ATtiny85’s EEPROM using the EEP file.

If any of these EEPROM values needs to be changed or if you need to set the exact crystal frequency for calibration, there is a Excel spreadsheet available to make this easier. This Excel spreadsheet allows you to enter these values yourself. It will then "automagically" create the EEP data file for you with the click of your mouse. See the Download section at the end of this page for this spreadsheet.

Finally, the ATtiny85 internal hardware is configured using “fuses”. These are programmable settings that determine, for example, the type of oscillator being used and the function of various pins on the chip.

Programming Process

The ATtiny85 must be programmed with the SC+M HEX and EEP files.  The procedure goes like this:

1.    Program the ATtiny85 flash memory with the HEX file
2.    Program the ATtiny85 EEPROM with the EEP file
3.    Finally, program the ATtiny85 fuses.

I use and recommend the well-known low cost [USBasp] programmer and Extreme Burner ("EB") programming software. The following procedure assumes that:

1.    USBasp (or your programming hardware and driver) has been installed in your computer, AND
2.    You have started EB (or your programming software of choice), AND
3.    You have selected the ATtiny85 as the device to be programmed in EB.

The HEX (program) file and the EEP file (with the default SC+M frequencies) for the ATtiny85 in the SC+M can be obtained by sending me an email. My email address details are on the main page of my website. In your email to me, please confirm that the software is for your personal use only and that you will not copy or distribute it to any third party. If your group or club wants to build a number of these, include that in your email. I usually reply within 24 hours.

Step-by-Step Programming Procedure

Here are the recommended fuse settings:

Table 1 : ATtiny85 fuse settings for the SC+M
 
IMPORTANT!!
Once you program these fuses, and in particular, the HIGH Fuse byte with &5Fh, you must use a special HV programmer or an eraser/fuse reset tool to reset the chip if you subsequently want to change the EEP settings (i.e. the 80m and 40m band edge frequencies etc). A simple low cost HV device to do this can be found here on my website.

When all of this is completed, insert the programmed ATtiny85 into the SC+M PCB.

--------


Appendix D : Testing the Completed SC+M VFO Module

If you wish to test your completed SC+M before installing it into the MRS-1 transceiver, the schematic of a suitable test jig is shown in Figure 8. The VFO draws around 40mA.

 
Figure 9 : A test jig allows rapid testing of a completed SC+M module. This can
be particularly useful when making a number of modules in a group build.

My test jig was made using prototype board (Figure 10). I only need one of these and for that, this construction approach is fine.


Figure 10 : The assembled test jig with its two cables to connect to power and the various inputs. The OLED display plugs into the SC+M module being tested.  There is a test point (lower centre) which connects to ground. It is a useful tie point for scope probes and volt meters.

Using this test jig, you can monitor the VFO and carrier/BFO outputs with a frequency counter as you adjust the tuning control, band and step switches. The S-meter range can be tested by adjusting trimmer VR1, and the power supply voltage can be adjusted from 10-15V to check the operation of the OLED 3-digit battery voltmeter.

-----------

Downloads

arrowGerber files for the SC+M PCB (ZIP file)

arrowSC+M VFO software (ZIP file with SC+M HEX program file and the MRS-1 standard EEP file for the SC+M ATtiny85)

arrowSC+M CALibration software (ZIP file with ATtiny85 Calibration HEX program file and the Calibration EEP file)

arrowMRS1_SC_plus_EEPROM_Calculation_Sheet_VerB.xlsm EXCEL™ spreadsheet for calculating the EEP file for your VFO (ZIP file)

The HEX (program) file and the EEP file (with the various SC+M frequencies) for the SC+M's ATtiny85 and/or the SC+M CALibration HEX and EEP files can be obtained by sending me an email. The email address details are on the main page of the website. In the email, please confirm that the software is for your personal use only and that you will not copy or distribute it to any third party. If your group or club wants to build a number of these, include that detail in the email as well. I usually reply within 24 hours.




Want to go back to the main page? Click here to return directly.