Dual Digital DDS VFO for Ten-Tec Omni, Argosy, Corsair, Drake TR-7, Heath SB-104(A)  and
Other Display Equipped Transceivers Using a 5.0 - 5.5Mhz VFO
 adjunct picture
1. Introduction - provides frequency changes in 10 hz. steps, is drift free and is and is rock solid.

    This webpage describes a standalone, PIC processor based DDS VFO designed for the reasonably experienced homebrewer.  It can be mounted within a Ten-Tec, Drake (or any other solid state radio that uses a 5.0 - 5.5 Mhz VFO) or housed in a small metal enclosure. The Dial FREQUENCY LOCK and EEPROM storage of the last used frequency features are included.  Please review the whole web page if you are interested in building this VFO. 

    The latest version (4) Version 4 board allows the on-board mounting of the 100 mh inductor (L5)  used in the TR-5 / TR-7 applications. 

    Here's a schematic of the Version 4 PCB, the Version 4 PCB artwork,  connection points, and a parts list. 

2. How It Works - 30,000 Foot View - All-in-all, the program is very 'well behaved'.

    The AD9850 is a highly integrated device that uses advanced DDS technology coupled with an internal high speed, high performance D/A converter and comparator to form a complete, digitally programmable frequency synthesizer and clock generator function. The AD9850 generates a spectrally pure, frequency/phase programmable, analog output sine wave.  The AD9850 board comes with an integral 125 Mhz clock oscillator and a 7th order eliptic filter with a 42 mhz cutoff.  To further eliminate the possibility of spurious signals, an additional 7th order Butterworth filter with a 7 mhz cutoff frequency has been provided on the printed circuit board, all followed by a 2 transistor amplifier with an adjustable and stable output of up to 4 vac.  This DDS VFO has a fixed operational range between 4.950 and 5.550 Mhz.

    The PIC sends serial commands to the AD-9850 to update the frequency.  The program runs in a loop, constantly checking the optical encoder.  When either the Toggle or Split buttons are depressed, or when a transmit signal is sent by the served radio, demand interrupt routines are called to service them.  A timed interrupt routine is used for RIT (50 ms).

    The software driving the AD9850 provides 2 VFO's (A&B), split frequency capabilities, Receiver Incremental Tuning (RIT), (optional) power up retrieval of the last used frequency and encoder locking.

3. Building the Board  -  Construction Suggestions for All Radios - Parts List     Use a grounded (3 wire), fine tipped, ESD safe soldring iron to build up this kit.  Some kind of magnifying glass would also be advisable. Consider using 30 gauge solid conductor wire and an appropriate wire stripping / wrapping tool.  It's manageable and works very well.

    Standard components from reputable suppliers (like Mouser) are used.  The only exception is the DDS unit which is available on eBay or on Amazon (very fast shipping and excellent return options).  This software has been written for the AD-9850 DDS only. Optical encoders are usually available on eBay, as well.  Try to get one with a 128 to 256 quadrature pulses per revolution. They're relatively inexpensive.

    When soldering the components to this board, use a fine tipped iron and apply only as much solder as is needed to make the connection - nothing more. Remember the Bryl Cream commercials from the '50s?  A little dab will do ya! Give the solder ample to time to fill the space between the wire and the PCB plated hole.  Before soldering the components double check to ensure that you have the right one.  If you make a mistake and solder in the wrong or a defective component, don't panic.  Almost all components can be harmlessly removed without damaging the PCB, as described below.  

    Use your diagonal cutters to snip the component on the top side of the board. Next, use your needle nosed pliers to grip the legs (one after the other) of the severed component, turn the board over and then apply heat from your soldering iron, after which the parts will easily pull out.  There will probably be some solder residue which will inhibit placing the proper part.  The holes will need cleaning out.  

    To do this, place one lead of a scrap component (like a resistor or capacitor) on either side of the hole and heat the wire as close of the board as possible without burning it. Very soon, the wire from the scrap componentd will begin to slide thru the hole.  If the part drops all the way thru and can be freely turned, just snip off the excess lead on the other side of the board and you're done with this hole.  You may have to do this several times with several scrap components to achieve the desired result.  Don't fret - it's dooable. Properly done, they'll be no significant evidence that a component was replaced.

1. Ensure that you are mounting the components on the silkscreened side of the blank printed circuit board.
2. Mount the first 20 pin integrated circuit socket (for the PIC processor) - the notch points to the top. 3.   Install the 5 volt regulator - LM-7805.  Leave enough 'slack' in the leads to allow it to be bent back flush with the chassis (heat sink) on which it is to be mounted with heat conducting grease.. Proper  heatsinking is vital for consistent RIT operation.
4.   Install reverse polaity protection diode D1 (1N4001 - or equvv)  Note the polarity shown on the printed circuit board.
5.   Install diode D2 (1N4148) -  transmit interface - buffer between the PIC and the transmitter transmit signal.  Note the polarity shown on the printed circuit board.
6.   Install nine (9) .1 mf capacitors (C1, C3, C4, C6, C8, C9, C16, C17, C18)    - C8 is a decoupling capacitor whose value was just increased on the latest PIC1618344 data sheet.
7.   skip this step.
8.   Install one (1) .01 mf capacitor (C15)
9.   Install the 100 uh inductor (L4) - If you plan to connect the DDS VFO to a Drake TR-5 or TR-7, install L5.
10. Install  5 (five) 1K, 1/4 watt resistors (R1, R2, R4, R9, R11)
11. Install 1 (one)  3.9k 1/4 watt resistor (R3)
12. Install 4 (four) 10 K, 1/4 watt resistors (R5, R6, R15, R16)
13. Install 1 (one) 27 ohm, 1/4 watt resistor (R12)
14. Install 2 (two) 470 1/4 watt resistors (R10, R19)
15. Install 1 (one) 100 ohn, 1/4 watt resistor (R8)
16. Install the 10 K trimmer resisor (R18) - used to adjust the DDS amplifier output level - place the adjusting screw at the bottom of the PCB. - max stable output possible is 6.0 VAC, peak-to-peak.
17. Install 2 (two) 100 ohm 1/4 watt resistors (R7, R13)  
18. Install 2 (two) 390 pf capacitors (C10, C12)  
19. Install 2 (two) 100 pf capacitors (C7, C13)  
20. Install 2 (two) 2.7 uh inductors (L1, L3)  
21. Install 1 (one) 4.7 uh inductors (L2)  
22. Install 2 (two) 10uf 25 volt electrolytics (C2, C5) - observe the polarity
23. Install 2 (two) PN2222A (or equivalent) transistors (Q1, Q2)

Important Suggestion - when soldering the pin headers (next step), you might first want to first place them in a spare integrated circuit socket or in a female header pin set (usually available on eBay).  This  way, the heat of your soldering iron will not melt the plastic and cause the pins to seat unevenly.  I've soldered hundreds of pins this way.  It always works.    

24. Install pin header connectors at the following points:
4. Testing the Board - you should be working in a static free environment

    Before installing the PIC processor and before installing the AD9850 DDS board  - Measure from the heat sink of the voltage regulator and verify that you have ground on pin 20 of the PIC processor socket and on pin 6 of connector DDS-1 and on pins 4,5,6,7,8,10 of  connector DDS-2.
  1. Apply 13VDC to JP-1, and verify that there is an approximate +5VDC on pin 1 of the PIC processor, on pin 1 of connector DDS-1 and on pins 1,2,3 of connector DDS-2.
  2. Verify approximately 13VDC on both sides of inductor L4.
  3. Remove power from the PCB.
  4. Install the pre-programmed PIC (notch goes at the top) in the previously installed DIP socket.
  5. Carefully (once again) remove the AD9850 from its static free packaging and install it on the board ensuring that all the pins are seated properly and that the RED LED is in the center of the board.
  6. Apply 13VDC as you did before, and the RED LED should light.
  7. Connect a frequency counter to the FREQ OUT pin and
  8. Adjust R18 until you see an approximate 5 Mhz signal on your 'scope, or hear it on your radio (it may sound a bit warbly because the RIT pot has not yet been connected). R18 will be adjusted for the proper output level when the VFO is connected to your radio.  If R18 is installed with the adjusting screw at the bottom of the board, turning the screw clockwise increases the output level.
  9. Remove the power and install the DDS VFO either in a stand alone enclosure (highly recommended), or in your radio proper.
  10. Connect the power source, the optical encoder, the RIT control, and the TOGGLE, SPLIT, and FAST buttons using 30 gauge wire and a wire wrapping tool. 
  11. Verify that both the optical encoder and RIT control tune in the proper direction.  If not, just reverse the connections.
  12. If you are building the DDS standalone, you'll need a suitable enclosure.  Both Mouser and Digikey have some nice sloping ones (recently constructed example).
    5) Drake TR-5 / TR-7 -  Application

    a) Building the Enclosure 

    This enclosure can be used with either the Drake TR-5 / TR-7, with Ten-Tec radios, with the Heathkit SB-104 - 104A -  and / or with other units that have a 5.0 - 5.5 Mhz VFO / PTO.  The only significant differences will be the how the unit interfaces with the radio.  I used a 2 piece, sloping panel enclosure from Mouser Electronics.  With this enclosure all the connections to the PCB, optical encoder, RIT pot, switches, push buttons, input / output and power connections can be made on the sloping panel potion itself.  The PCB is mounted herein, as well.  Here are the steps that I took:

    i) Hole Drilling / Component Mounting - Measure Twice ---------- and Drill Once
  1. determine where you want the controls and the LED's and drill the holes to suit your preference, 
  2. install the TOGGLE, SPLIT, FAST push buttons, the BIDEN and RIT switches, the tuning encoder, the 20K RIT potentiometer, the 3 LED's, and - on the rear of the enclosure - the transmit RCA phono jack and the female 12 VDC power connector (for Ten-Tec and similar radios)  - the Drake TR-5 / TR-7 application needs a grommeted hole for the connecting cord.  
  3. drill a hole and mount a ground tie point near the bottom of the chassis.  This will be used to terminate the backup ground wire from the cord and serve as a tie point for other ground wires.
  4. mount the completed and tested PCB on the rear panel using 4-40 hardware.  It will be necessary to insulate the back of the PCB to avoid any shorts.  I used a hunk of stick-on floor tile to accomplish this although a piece or non conductive perf board would work just as well,
  5. carefully drill a hole to heat sink the LM-7805 voltage regulator and use a dab of heat sink compound to secure it to the chassis. Without the chassis heat sink, the LM-7805 may fail,
  6. This particular unit was weighted down internally with 3 steel outlet covers (Home Depot).  It rests very securely on my desktop.
  7. If this enclosure is being constructed for Ten-Tec and similar radios, advance to Wiring the Box.  The LM-7805's ground tab must be connected to the box or chassis to ensure good thermal conductivity. 
  8.  Also, if the LM-7805 is not properly 'heat sunk', its output voltage may drift to the point where RIT accuracy could be degraded - - as RIT uses the processor's internal A/D converter.
    b) Preparing the Connecting Cord - for the Drake TR-5 / TR-7

    As an external VFO for your Drake TR-5 / TR-7, refer to the schematic for the extra wiring.  You'll need a Cinch-Jones Plug P-308-CCT (eBay), another 100uh inductor (L5 which mounts either within the P308 plug (between pins 2 and 7) or preferably on the PCB), some RG-174 coax and stranded wire.  When done, the DDS VFO plugs into the TR-5 / 7's Remote VFO connection and is powered by the TR-5 / TR-7 (RV-7, pin 3) which is always 'hot' when the TR-5 / TR-7 is powered up..

    When the Biden switch is operated, +13 VDC (fom the radio) is sent to connector JP-1 - pin 1 and - thru inductor L5 - to RV-7, pin 2 - disabling the internal TR-5 / TR-7 PTO,
(the RV-7 external VFO would disable it this way).  At the same time,  DDS VFO is enabled by the same +13 VDC. Inductor L5 ensures that the DDS VFO's signal to the TR-5 / TR-7 will not be compromised by the +13 VDC from RV-7's pins 7 / 8 when the Biden switch is operated.

    With the latest PCB design, inductor L5 mounts right on the PCB, see schematic.  Pins 7 and 8 of P308 are connected tpgether.

Preparing the interface cord:
  1. Get the Cinch Jones P-308-CCT plug  from eBay,
  2. Get an appropriate length of heat shrink tubing (I used Dorman 3/16 inch #85265 - purchased from Lowes),
  3. Fabricate (3) 4 to 5 feet lengths of color coded insulated wire.
  4. Cut one RG-174 (or equiv) miniature coaxial cable,
  5. Insert an opened coat hanger or some other piece of straight thin metal into the tubing so that it's accessible at both ends,
  6. Use this to pull one thin strong stranded wire (aka the 'messenger') thru the tubing,
  7. Solder the coax and other wires to the end of this messenger wire and carefully pull them thru the heat shrink tubing,
  8. When done - sever the 'messenger' wire.
  9. Mount the P-308-CCT connector on the one end (connections shown below), leaving ample wire at the other end to connect up within the enclosure (described later),
  10.  Use a hair dryer to shrink the tubing - the shank of a hot soldering iron will also work  if you are very careful.
    Four (4) connections are made on the P-308 plug.  (the pin numbers are marked on the inside of the plug),  viz;
    c) Wiring the Box - refer to the schematic , the PCB artwork (Version 4)...and to the connection points for the Version 4 boards.
           Steps Unique to a TR-5 / TR-7 Installation:

    d) How It Works
   
    When the TR-5 or TR-7 is powered on, the DDS-VFO does nothing.  The radio's internal PTO will tune the radio.  However, then the VFO switch is activated, the DDS VFO mimics the operation of the stock RV-7 VFO, viz: 13VDC is sent (through the inductor L5) to disable the radio's internal PTO, to power the DDS VFO and to 'tell' the radio that the DDS VFO will be used for both transmitting and receiving.

Note: Please double check your connections before trying to use the DDS VFO.

Caution:  When using the External DDS VFO with a Drake TR 5 / 7, make the power and RF output connections BEFORE turning the radio on.  If you attempt a power on installation and accidentally ground the power lead coming from the RV7 connectior, you may either burn out the L5 100 mh inductor on the PCB or blow the 5 amp fuse in the TR 5/7 - or both.  Replacing the fuse is a PITA.

    e) Setting the DDS VFO Output Level - It's important to set the output level of the DDS VFO to match that of the internal PTO.

    To easily accomplish this, place either a scope probe or another high impedance measuring decice on the Drake TR-7 mother board as shown in this picture.  With the DDS VFO turned off, measure the voltage at this point.  It should be very close to 1 VAC point-to-point. Then, turn the DDS VFO on and note the reading.  Simply adjust the DDS VFO R18 control until these two measurements are very close, you can easily switch back and forth.    Here are pictures of the enclosed DDS for the TR-5 / 7 - front, back.

  f) Birdies and Spectral Purity

    Here's how the testing was done.  The TR-7  was connected to a dummy load and the DDS VFO (in a sealed enclosure) was powered by the TR-7's RV7 jack.   Frequency scans were done from 1.8 Mhz. meters to 29.7 Mhz with both the TR-7's  PTO and with the DDS VFO for comparison purposes.
g) Operating SPLIT with the TR-5 / TR-7 and / or Using the RIT Function While Transmitting

    To operate in the SPLIT mode (say, using VFO A for receive, and VFO B for transmitting), you have to 'tell' the DDS VFO when you are transmitting both for the SPLIT function to work and for the RIT (if activated) to disengage.  The simplest way to accomplish this switching so to connect the DDS VFO's TMT lead to the matching power supply's  (PS-7) VOX relay jack, et voila!

    If you are not using the PS-7 with your TR-7, you have several options to access the DDS VFO TMT lead:
6. Interfacing Ten-Tec Radios

    You may refer to the following website articles describing how to connect a DDS VFO within the following radios.  Although the initial DDS VFO used an earlier processor, the conversion and connection sequences are similar:
Note: Connect the DDS VFO transmit lead to the proper point withing the radio so that the SPLIT function works properly and RIT functionality is disabled in the simplex transmit mode.
   
    Before populating your board determine where you will mount it. Use it as a mounting hole drill template.  The board can be mounted within the radio itself or in a separate, stand alone enclosure. The DDS VFO will work well in either situation. I've done it both ways with another PCB and earlier processor version, as noted on my website, viz:
a) Radio Connections   - see picture
  1. The DDS VFO needs a source of 13 VCD.  For Ten-Tec radios, this can be secured from a rear mounted phono jack, the
  2. DDS VFO's output is connected to the VFO input on the radio's rear panel by removing the factory 'jumper', and the
  3. Transmit input is connected to a lead within the radio which provides ground when the radio is transmitting.  
  4. For Ten-Tec radios, this is the 'R' lead on the Control Board (on the top side of the radio just under the optional filter board). This lead can be brought out by 'sparing' up one of the unused phono connectors on the rear panel. For my OMNI conversion, I used the EXT/TR jack (after first removing and insulating the attached wire).
b) Setting the VFO Output Level

   The DDS VFO output level should be set as close to the output of the Omni's PTO as possible as the Omni's circuitry was designed around this value.  Setting the DDS VFO's output higher than this may generate some 'birdies' and needlessly increase the receiver's background noise level.  The Ten-Tec's PTO's output is between .5 Volt to .75 volt, peak to peak, and you must match this output level with the DDS VFO unless you want to realign your Ten-Tec Omni, Corsair, etc.  A resistor trimmer (R18) has been provided for this purpose.  

    If you have a 'scope, you may use it to set the DDS VFO's board right on the money, ensuring that you are measuring the DDS VFO's output over the coacial cable connection where it enters the radio.   If you don't have a 'scope, just turn R18 slowly until the radio begins to receive AND transmit SSB  properly on all bands, but no higher.  

c)  Birdies and Spectral Purity - Ten-Tec Omni D, Series B - DDS VFO installed in an EXTERNAL enclosure
    
    The OMNI was connected to a dummy load and the DDS VFO (in a sealed enclosure) was powered by the OMNI's 12VDC phono jack.. The DDS VFO was connected by a 3 foot length of coax to the rear of the OMNI (VFO IN Jack).  Frequency scans were done from 1.8 Mhz. meters to 29.7 Mhz with both the OMNI's PTO and with the DDS VFO for comparison purposes.  

    As shown below, the UNMODIFIED OMNI generates several internal birdies on its own.  These internally generated birdies are shown in black, while the additional birdies attributable to the new DDS VFO are shown in red.  Those birdies that can be heard with an antenna connected, with the preselector properly tuned, and which could possibly interfere with a weak signal are shown in bold type.
Note: One way to cut down on spurious mixing products is to follow the instructions in the service manual to adjust both R23 and R2 on the oscillator / mixer board. Perform the adjustments shown in Step 3 and in Step 7 (Mixer Balance).
Also Note: If left powered on while an external DDS VFO is connected, the OMNI's PTO is constantly running and can produce an infrequent birdie on the upper frequencies.  If you want to retain the internal PTO, simply move the OMNI's PTO tuning dial should one be encountered.  Alternatively, the power lead to the PTO can be clipped.

        Spectral Purity: The RF output from the Ten-Tec OMNI using either the internal PTO or the external, enclosed DDS VFO were comparable when viewed on a TINY_SA (Tiny Spectrum Analyzer)

7. Heathkit SB-104 - SB104A - See Picture

    Interfacing the DDS VFO with the Heathkit SB-104 / SB-104A is very similar to  the Ten-Tec radios, preferably housing it in a separate enclosure (but it could be mounted within the radio itself, if desired).

Steps:
  1. assemble the DDS VFO in a separate enclosure as described here.  On the rear of the DDS VFO cabinet, provide:
8. Replacing the SB-104 (A) Burroughs Digital Display with a Newer, State-of-the Art Unit 
  
   Note: I'm just posting some suggestions that may be of interest to anyone contemplating the conversion.  No mention is made here about the physical conventions of mounting the display within the radio, although it could be mounted in a separete enclosure, if desired.

    As shown on the SB-104(A) schematic, the PREMIX signal consiste of the Heterodyne Oscillator frequency less that of the VFO's frequency.  This premixed signal is sent over to the SB-104 A's Frequency Display Circuit Board on coaxial lead J-101.   With this 1970's counter logic, the BFO frequency is not actually measured, but assumed.  The switched leads on pins 22, 23 and 24 mimmic the value of the BFO to be subtracted when +5 VDC is applied to the proper lead for the selected mode.  The service manual describes this process in excruciating detail.

    Now, the Chinese 6 Digit frequency counter (available on eBay) has provisions for two separate frequency offsets.  One is active with pin 4 on the rear of the unit ungrounded, and the second becomes active when pin 4 on the rear of the unit is grounded (to pin 3),.  On the rear of the unit there is a row of 8 sequential pins,   Pin 1 is the one right by the triangle, pin 2 is above it, pin 3, then pin 4, etc.  One offset is active when pins 3 (ground potential) and 4 are open, the other is active when they are shorted together, or when pin 4 is simply grounded. It's that simple.

    Now, according to the pictures in the assembly manual, the mode selection switches work in a mutually exclusive mode - that is - pressing one down releases the others.  Furthermore, there appear to be spare onntacts on each of the switches so one would not need to disturb the existing wiring.

   So......the first step would be using a frequency counter to measure the actual frequency of the LSB and USB oscillators on the Carrier Generator Circuit Board and write each of them down (CW will not be needed as the USB oscillator will be used when receiving CW).

    Next, program the respective offsets into the Chinese 6 Digit Frequency counter for the USB/CW and LSB modes. Ground pin 4 when entering the LSB offset.

    Then, wire up the spare contacts on the mode switches so that the LSB button grounds the LSB offset and is open for the USB offset.

    Finally, connect the input of the 6 LED Chinese Frequency Counter to the SB-104 (A)'s PREMIX signal.  Use a small value capacitor so this connection doesn't load down the circuit.

    Once this has been proven to be working satisfactorily you might consider removing the display board and that annoying DC / DC converter.

Also Note:  This technique would most probably work with the HW-104, although it would probably be prudent to wire it up in a separate enclosure, providing, power and mode selection leads from the radio along with the PREMIX signal.

dit...dit

   

 
9.  DDS VFO Commands and Controls

There are 3 working buttons, one tuning knob, two switches and a smaller knob (RIT) on the front of this box, the:
Note: Holding the VFO Toggle Button down for  about one second will LOCK the encoder.  The three LED's will light to indicate the LOCKED status.  While locked, the radio will still transmit based upon the previous settings and the RIT will continue to function unless the radio had been in the SPLIT mode before being locked.  Tapping the VFO Toggle Button will restore tuning.
  Important Note: FREQUENCY STORE capability is optional.  To enable it, ground the FLASH pin on the DDS VFO PCB.  If not grounded, upon power up the frequencies will be set to  5.5 Mhz for VFOA, and 5.0 Mhz for VFOB
10. Using it On-the Air

    Using the DDS VFO controller is simple.  For SIMPLEX operation, there are two VFO's to choose from - VFOA and VFOB..  The only concern here involves CW operation.  Most CW transceivers provide some kind of a frequency offset (usually 600 to 750 hz) from the transmit signal.  This is done automatically in the TEN-TEC series and in the Drake TR-7.  Because these offsets differ from radio to radio, it would be difficult (but not impossible) programming the radio specific offset to be used in the DDS VFO software.  Furthermore, an additional front panel switch would be required to indicate when in the CW mode.

    So, when simplex CW operation is contemplated, the RIT control can be used to provide this offset, if desired ----------- or if even needed.

    For SPLIT operation, you'll need to set both VFO's to the same frequency by tapping the SPLIT button.  Then, use either VFO (say, VFO A) to receive the station who is working in the SPLIT mode. If the station operating SPLIT instructs you to call on a particular frequency, just toggle to VFO B and set the desired frequency in it, and then toggle back to VFO A.  When you are all set up, push the SPLIT button and the SPLIT LED will light.  So, you'll continue to receive on VFO A, but will be transmitting on VFO B - the VFO lamps will flash accordingly as you transmit.

    Another, less publicized benefit of having  2 VFO's is the ability to keep track of 'conversations' - say, during a contest.  If you come across a desired DX station in QSO with another ham but want to make another contact in the meantime, simply switch to the other VFO and tune around.  You can always spot check your original 'target' by just tapping the TOGGLE button - very simple, n'est-ce pas?

11. Drake Service Manuals and Alignment Instructions
                                                                                                    Copyright (PCB and Software) 2021, 2022, 2023, 2024 K3JLS