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Version:

Jan 9, 2024:
Revised: v1.2

Part 2 : The SugarCube "SC+C" Si5351a VFO

for the 80m/40m Condor QRP HF SSB Transceiver

The Condor is a  two channel crystal controlled 1W HF QRP SSB transceiver designed  for Search and Rescue in New Zealand in the early 1980s. It is possible to modify these for use on the 80m and 40m ham bands. 
This page, the second part of a series on this radio, describes the Si5351a-based PLL VFO module designed to replace the original crystal oscillator hybrid in the Condor thransceiver.. 

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


Background

The "SC+C" VFO is a highly modified "application specific" version of my well-known SugarCube+ VFO. This new module, designed for use in the Condor transceiver, provides the required oscillator functionality as well as supported the required user controls and display.

Due to the limited space available in the Condor (and that's just about the ultimate understatement!) and the need to design a drop-in replacement module for the original crystal oscillator board, the VFO mostly uses SMD components. Exceptions to this include the ATtiny85 processor (DIL-8 package) and regulator (TO-92 package). Using the standard DIL-8 package makes reprogramming much easier but getting the chip into a socket so that it fitted the tiny space available was...a nightmare. The TO-92 packaged 78L33 regulator allowed improved heat dissipation.

The "SC+C" VFO module is shown in Figure 2.


Figure 2 : The Condor's original three-crystal oscillator hybrid is replaced by this "application-specific"
version of the ZL2PD SugarCube-Plus VFO. With SIL-11 pin connections to match the transceiver and supporting connections to the OLED display (4 wires) and rotary encoder and step switch (1 wire!), the module measures just 30 x 20mm and is only 8mm thick.  

To show how this module is used in the modified transceiver and identify the necessary functionality, let's first take a quick look at the internal details of the original transceiver and the requirements of the modified transceiver.

Technical Overview of the Condor Transceiver

The Condor SSB transceiver uses the phasing SSB method to generate and detect SSB. Figure 3 shows the block diagram. The I/Q transmit audio pair is mixed with the I/Q 455kHz carrier signals generated by the carrier crystal and quadrature divider in the original crystal oscillator module. This signal was mixed with the desired oscillator crystal to give the required RF output. The reverse process was used in receive mode.

 
Figure 3  : Block diagram of the Condor phasing SSB handheld transceiver

The full circuit diagram of the original Condor (PDF) is available in the Download section for those interested in the detail.

Adding a SugarCube+ VFO

At first glance, it appears a standard SugarCube-Plus ("SC+") VFO could directly replace the original crystal oscillator hybrid in the Condor transceiver, albeit with a new PCB layout to allow it to fit the oscillator hybrid's socket. The original hybrid module contained two crystals, one per channel, for converting signals between the 455kHz IF and RF channel frequencies. Incidentally, it also contained a divider for channel crystals on frequencies less than 3MHz to allow a physically smaller crystal to be fitted for those lower frequencies.

However, a critical problem was the fact the oscillator hybrid also contained the 455kHz carrier crystal oscillator. A 1.820MHz crystal and an additional divide-by-4 CMOS chip on the oscillator hybrid generated the critical 90o RF phase shifted outputs for the phasing SSB system.

My standard SC+ VFO has three programmable outputs, two of which can also support a 90o phase shift for output frequencies above 3.5MHz. Since the Condor required the phase shifted oscillator outputs at 455kHz, and the Si5351a in the SC+ cannot deliver phase quadrature on output signals below 3.5MHz, the standard SC+ VFO could not be used..

It was therefore necessary to add an new chip to the circuit of the SC+ to deliver the phase quadrature outputs at 455kHz. Further, if USB/LSB mode switching was required on different bands (i.e. 80m and 60m), a further chip would be required.

Defining Other VFO Module Requirements

To determine the other required changes in the Condor-specific SC+ VFO, Figure 4 was prepared to outline how the new VFO would be integrated into the modified Condor transceiver. This identified some of the other control, display and  functional requirements for the new VFO.


Figure 4 : The fixed crystal oscillator hybrid was replaced by an extensively modified SugarCube-Plus VFO to give dual band digital PLL VFO coverage on the 80m and 40m amateur bands

Some standard SC+ controls and outputs were required, several requiring special treatment:

I also wanted to add a simple battery voltage display because this is critical in portable applications, especially with AA-size alkaline batteries.
In order to support all of the expected inputs and outputs on the SC+C's ATtiny85 controller and the special requirements of the Condor, it was necessary to implement some significant hardware and software changes. Most noticeable is the reduction in processor pins used by the rotary encoder and step switch, from three to just one (excluding ground).

Single-wire Rotary Encoder Interface

I originally developed this interface for a pocket audio test oscillator and a multitone CTCSS tone generator. The encoder uses three resistors configured as a 2-bit D-to-A converter. Any change in the encoder or step switch will also trigger a software interrupt. This provides very responsive tuning and tuning step selection yet requires only one processor pin. The voltage that is subsequently measured on pin 2 indicates the direction of rotation of the encoder.


Figure 5  These two graphs show the voltage on pin 2 of the ATtiny85 as the encoder
is rotated. Left: CW encoder rotation    Right: CCW encoder rotation. When the voltage
initially falls below a Logic Low voltage level in each case, the interrupt routine is called,

OLED Display

A display is also essential on the transceiver to check the operating frequency of the transceiver. Because the transceiver may be used for CW (Morse) in future, tuning to a resolution of 10Hz required a 6-digit frequency display.

The Condor front panel limits the possible display size, and the 64x32 pixel 0.49" OLED was selected. This is driven using the 2-wire I2C bus which is connected in parallel with the Si5351a PLL chip.

AGC Voltage and S-Meter

The Condor's audio processor hybrid produces the AGC voltage for the receiver RF amplifier and first mixer. This AGC voltage varies between 3V with a strong input signal of -60dBm and 8V when the input signal is weak, around -120dBm.. This AGC voltage is actually superbly linear across this 60dB receiver input signal range.

Consequently, this voltage is used by the VFO module's processor, via a pair of scaling resistors, to drive the S-meter display for the modified Condor. It corresponds to a signal range from S0 to S9+20.

Battery Meter

I was eager to provide a battery meter on the OLED display since battery voltage is usually critical for portable operation. Despite saving pins with the D-to-A encoder approach, there were no free input pins on the ATtiny85. In addition, space was limited for additional components on the PCB. However, need drives invention.

I managed to combine a minimal battery measurement circuit with the Lock pushbutton function. The 9 - 12V supply rail is connected to a resistor dividerwhich reduces this to 2.5 - 3.3V. This voltage range is not a random design choice.

First, the resistor divider’s voltage must not fall below 1V. If it does, the ATtiny85 will see a H-to-L transition and trigger an interrupt that’s reserved for the Lock switch. In fact, the lowest voltage from the divider must also remain above 2V. This is the lowest voltage the ATtiny85 sees as a valid logic High. This ensures that after the release of the Lock switch, the processor sees a valid L-to-H transition which in turn indicates to the processor that the user has released the Lock switch.

A further high value resistor was added to isolate any impact of the Lock switch on the supply voltage when it is pressed. Should this button be accidentally held down, for example at the bottom of a pack. The ATtiny85 periodically measures this voltage and displays it in the lower right hand corner of the OLED display. It’s essentially an expanded range voltmeter measuring from 9 to 12V.

Important!
The absolute maximum supply voltage for the Condor is 12V! The CA3020A will be destroyed if the Condor is accidentally operated at higher voltages i.e. from a vehicle “12V” battery which typically delivers 13.8V or more. (Affordable) replacements for this device are no longer available!!


Figure 5 : Prototype front panel showing the OLED with frequency and step arrow indicator with
the lower display 'line' showing S9+ signal level and, in the lower right corner of the OLED, the
battery meter bar. The Lock icon, displayed between the S-meter and battery meter when
enabled, is not shown here. Note: If sufficient demand exists, this 'agricultural' front panel will
be redesigned and 3D printed with a better layout of display, RF connector and user controls.


SC+C Schematic Details

Additional controls and functions could potentially be added but the front panel area is almost at capacity with these user controls and the added OLED display. Figure 6 shows the schematic of this application-specific version of the SC+ VFO for the Condor transceiver.


Figure 6 : The schematic of the Condor SC+ VFO is a highly mmodified and hardware/software
specific variant of the "standard" SC+ VFO. The PCB also contains a voltage regulator (IC5),
a quadrature oscillator divider stage (IC3), and a sideband selection switch (IC4).
Note: "Right click' to view a full scale view of this schematic.

Mode Selection

Initial work and testing suggested operation would only be possible on 80m and 60m. This introduced a further VFO design complication. LSB is used on 80m but USB is used on 60m! Therefore, the Condor SC+ VFO would require automatic sideband selection determined by the band selection.

Later work allowed the Condor to operate on 80m and 40m, bands where LSB is the standard SSB mode. However, the ability to select USB and LSB has been retained in the design for those wishing to use other band or mode combinations i.e. 80m/60m or 60m/40m.

Band Selection

As noted earlier, the Condor originally used a toggle switch or a rotary switch (depending on the version) to select the required channel. This switching selected the correct crystal in the oscillator hybrid and the appropriate BPF in the output coupler and the mixers. This switch could serve as a Band switch for the new VFO.

However, the original front panel toggle switch used +12 and Ground to indicate the band selection, and this is still required to control the internal switching. Adding a diode into the VFO (D1) resolved the potentially destructve voltage level problem.

VFO Outputs and Quadrature Switching

Refering to Figure 5, Si5351a output 0 is the VFO injection frequency. It is offset by the IF frequency of 455kHz. This goes directly to the VFO input on the transceiver. Output 2 generates the 1.82MHz carrier. The added 74HC74 CMOS flip-flop produces the required 455kHz quadrature outputs while also dividing the Si5351a OUT2 frequency by 4. It may be replaced by a 74AC74.

The optional 74LVC1G3157 uses the Band switch input to select USB and LSB modes if required. The necessary pullup resistor on PB4 for the Band switch is programmed inside the ATtiny85. This also serves to pull up for IC4. This device is not normally fitted. Instead, the required output signal is bridged by a tiny wire link connecting pins 3 and 4 of U4.

Voltage Regulator

The Condor SC+C VFO's Si5351 requires 3.3V and, for simplicity, the entire module is designed using this supply voltage. The Condor transceiver runs from 12VDC supplied by the internally fitted set of eight AA batteries or via the front panel external power supply jack.

A switching voltage regulator is the best option to deliver the regulated 3.3V voltage required. However, several different switching regulator circuits tested during development all generated considerable noise and reduced overall receiver sensitivity.

Finally, a simple 78L33 linear regulator and dropper resistor combination was used, a recommendation from Eric ZL2BMI, the man behind the successful ZL2BMI QRP DSB transceiver. This arrangement ensures the regulator operates at a reasonable temperature.

SC+C Module Mechanical Design

The new Condor VFO is designed as a plug-in direct replacement for the original crystal oscillator hybrid, Of course, since a number of additional features have been implemented on the SC+C, some additional connections are required to the SIL-11 socket used for the original crystal oscillator. These will be described in the next part of this Condor transceiver series.

The location of the oscillator module, at the base of the transceiver enclosure, is a very small volume that lies between the batteries, the outer case wall and the speaker-microphone. The inner surfaces of the Condor case are covered in a semi-flexible conductive material, almost certainly to provide some shielding. Since the original oscillator module was not encapsulated (to allow for fitting the crystals), it was instead given a thin folded plastic cover. This was held together with adhesive tape. It must have been a good quality adhesive tape because these covers are still holding after 40 years! This cover just sits over the crystal oscillator.

This cover cannot (easily) be reused with the new module due to the presence of the OLED cable. Instead, the new module, after programming and testing, should be covered in a layer of kaptan tape.

The PCB layout and the assembled prototype PCB can be seen in Figure xxx.


Figure 7 : Component side PCB view of layout and the prototype SC+C module. This side faces in
towards the centre of the transceiver when inserted into the SIL-11 oscillator module socket in the Condor.

Gerber files for the PCB can be obtained from the Download section on this page. The component details are on the schematic.

SC+C Module Construction

Once the PCB has been obtained, the first step is to mount the Si5351a. The method at this point forward depends if you are hand-assembling the module or if you are using an oven or hot plate SMD soldering system.

If the latter approach is being used, a stencil should be used to apply the solder paste. Place the SMD components including the Si5351a on the component side and complete the SMD soldering cycle. Passive parts and D1 are 0805 type except for R9 and R10 which are the larger 1210 type.

If hand-assembling the PCB, after fitting the Si5351a, the remainder of the SMD components on the component side may be fitted, one by one.


Figure 8 : PCB Component location details

Now fit the DIL-8 socket pins for the ATtiny85. These are obtained from a SIL machine-pin type socket strip. These are typically 20 or 40 pins ilong. Eight individual pins must be carefully trimmed from the black plastic support.

Locate these into the larger than usual holes for IC1 so that IC1 may later be inserted from the component side. Use a reject eight pin IC or  jig to hold the machine-pin sockets at an equal height and in alignment. They must be located such that IC1, when fitted, sits tight against IC2 and the upper labelled face of IC1 sits no higher than the top of IC5 (78L33). Now solder the machine pin sockets in place.

IMPORTANT
If this step is not done correctly, the SC+C VFO module will not fit into the available space.

Solder in the Si5351a crystal and then fit the regulator. Ensure it is no more than 1mm above the PCB.

Add the jumper from pin 3 to pin 4 (IC3) if IC3 is not fitted, the usual scenario.

Flip the PCB over and install the underside SMD parts.

Prepare the module connector pins that will allow the module to plug into the original socket for the crystal oscillator hybrid. The prototypes, and there were a number, used pins cut from long-obsolete DIL ICs. These must be cut from the IC body using very sharp cutters absolutely tightly against the IC body.

Take a spare "machine screw" socket pin strip and place eleven trimmed (and flattened if necessary) IC pins into it. Rotate these trimmed IC pins so they are all positioned correctly for fitting into place on the pads along lower edge of the PCB. Then, holding the assembly temporarily in place (Blu-tac is a wonderful aid for this) solder the faces of the IC pins onto the edge pads on the SC+C module. When complete, do a test fit in the U7 Condor oscillator socket (Make sure the power is NOT turned on in the transceiver!)

Using four very thin insulated stranded copper wires, make a cable for the 4-wire OLED display cable. Terminate the free end with a four pin DuPont socket to match the pins fitted to the OLED display. The wires in this cable in the prototype measured 87mm before soldering.

Software

The SC+C VFO firmware was developed from the foundation of the well-proven SugarCube-Plus software. The most significant changes resulted from the use of the single wire encoder and step switch interface.





Figure 9 : At power-up, the software currently displays
the original Condor logo briefly before proceeding
to display the frequency and other functions.




Although the software is quite functional and satisfactory for this application, I want to highlight two issues, one of which I will spend some time addressing in the near future. Firstly, it's critical the encoder that is used be one of the (cheaper) pulse type encoders, the ones that give a momentary closed contact as they rotate. If the encoder outputs remain closed to the ground at the detent, they will not function correctly with the current code. It's easy to drop in the alternate routine. Supporting both types of software is too great a burden at this stage.

Secondly, as the encoder gets older and the contacts become worn, the software does not work as well. When this occurs, different rotation speeds can also give erratic tuning at times. It's unlikely you will see this. My encoders are used/abused (mostly by very very lengthy testing cycles) until they wear out completely. Regardless, I'm not satiisfied with this and I will put some effort into improving this. But, essentially, it's more of a longer term issue, Interestingly, I've seen this same effect on some well known commercial products.

The HEX (program) file and the EEP file (VFO settings) for the ATtiny85 can be obtained by emailing me. 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 you want to do a club kit run of these, email me.

Currently Supported "SC+C" VFO Features

The features of the modified Condor transceiver currently include:

•    3.50 – 3.75MHz (80m) and 7.00 – 7.30MHz (40m) continuous coverage
•    User programmable high and low band limits and start-up frequencies on each band
•    User selectable VFO tuning rates – 10 and 100Hz, 1 and 10kHz steps 
•    SSB (A3J) with optional automatic USB (60m) and LSB (80m) mode selection
     (Required if 60m is used instead of either 80m or 40m)
•    Supported 64 x 32 pixel OLED display
•    Full 6-digit digital frequency display with 10Hz resolution i.e. 3.68453 MHz
•    OLED tuning step indicator shows selected tuning rate
•    S-meter bar-graph display (Linear from -110dBm to -60dBm i.e. S0 to S9+20dB, for 2 - 8VDC in)
•    9-12V battery level meter display (on OLED)
•    No-noise linear regulator on board
•    Supports one wire encoder tuning control, tuning Step switch, Band switch and tuning Lock switch
•    Tuning lock icon status indicator (on OLED)

EEPROM *.EEP File Generation

The EEPROM in the ATtiny85 contains the band limits (the upper and lower band edges where the VFO stops tuning) and the starting frequency for each band. It also holds the crystal frequency for the crystal in the SC+C VFO. Adjusting this value allows you to set the VFO frequency within, typically, 10Hz of the actual frequency.

At present, I have not created my usual Excel spreadsheet to "automagically" create the EEP data file. I've hard coded these for now in the firmware. If there is enough interest, I can create a spreadsheet.

Programming

See the procedure described on my SugarCube-Plus web-page.

Testing the SC+C VFO Module

If this is to be done iniitially outside the target Condor transceiver, an 11-pin machine pin socket strip should be wired to temporary switches, an OLED display, etc as shown on the schematic. Connect to a power supply of 9 - 12V. The VFO draws less than 40mA.

You can monitor the VFO output and the quadrature 455kHz outputs on the appropriate pins as you adjust the user switches and encoder. If necessary, reprogram the EEPROM setting for the crystal to align your oscillator on frequency. The low cost Chip Eraser and Fuse Resetter (CEFR) that I designed for erasing the ATtiny85 may be required if you do not have access to a suitable HV programmer.

Installation in the Condor

See the next part of this series. It describes the modifications required to the Condor and the fitting of this module.

PCBs and Kits for the SC+C VFO

This is currently under discussion. Frequent visitors to my website will know that I do not offer kits. However, in this special case, let me know (via email) if you are interested either in PCBs, SC+C VFO kits or Condor conversion kits.



Downloads

arrowGerber files (compressed in a ZIP file) for the SC+C VFO PCB


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