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HW-5400 (1985) Direct Digital Synthesizer Board (2005)

Removed and replaced the original CPU board that controlled the radio (including the front panel buttons, knobs, and display). Dual Flash programmed Freescale 9S12D64 chip(s) now runs the show (including Filter selection, DDS control, Display, buttons and knobs)

A Direct Digital Synthesizer replaced the original Heathkit designed PLL controlled analog synthesizer's. Feeding the DDS with a temperature controlled cystal oscilator eliminated the need for any PLL circuits. The idea is to use a predictible computer algorithm to generate a sine wave where the stability is now based on the stability of the osc. reference used.
The HW-5400 radio originally used 3 PLLs and VCOs to synthesize the LO reference sine wave on all bands. The circuitry was found on the CPU board, synthesizer board, and VCO boards. The IF signal was generated on the BFO board and was crystal controlled. The PLL arangement was fairly new technology for 1983 (for consumer use), but had a limiting slew rate and limited the frequency selection down to only 50Hz steps.
This project was to eliminate the PLL, VCO, BFO circuits while increasing the LO frequency resolution, increasing the slew rate, and keeping the frequency stability.

Left Side:
Audio Board contains Balanced modulator, audio processing, and switching circuits.
Microphone amp board (perf board) contains mic amplifier (replacing the original circuit on the Audio board.
Power Supply (container) contains switchmode supplies

Right Side:
DDS LO board (with Master Controller) Contains CPU to control entire rig along with front panel controls, Generates the proper LO sine wave for the mixer and controls the DDS IF board.


Click on pictures to Enlarge

The image above shows in 2 DDS boards and there connnection to the radio itself. The IF shift is now designed to shift TX IF only. The RIT is now used to shift the RX IF only.

The "9" button on the keypad allows you to cycle through the 3 IF Crystal filters

Right Click here to download the microprocessor code (1 for the LO and 1 for the IF BFO). This contains the C code source for the HCS12 CPU's

The C Compiler "Codewarror" is found at: https://www.nxp.com/support/developer-resources/software-development-tools/codewarrior-development-tools/codewarrior-legacy/codewarrior-development-studios/codewarrior-for-microcontrollers/codewarrior-development-studio-for-hcs12x-microcontrollers-classic-ide-v5.2:CW-HCS12X?tab=Design_Tools_Tab

The folders in the zip file will load directly into the Codewarrior. However to program the CPU's you would need to purchase a P&E HC12 programmer device

Right Click here to dowload the PCB schematic.

Right Clock here to download the PCB Gerber data.

Note: This is the original design using the 9850 chip (the BFO DDS still uses this). But for the LO DDS, I replaced the 9850 with the 9851 (as an experiment)


DDS Design
The DDS project is based upon a single chip Analog Devices AD9851 Direct Digital Synthesizer capable of resolutions under 1hz using a 32bit tuning word, and a range that runs through and past the HF spectrum (up to 60mhz), and a capability of changing frequency at a rate of 23million/second. Only a low pass filter is required
The DDS chip does require an external CPU to feed it the frequency as a 40bit tuning word. I chose the Freescale MC9S12D64 for its all-around performance and features: low speed external clock but runs intenally at 50mhz, flash memory, 10bit 8 channel A/D, and plenty of I/O. The CPU controls the DDS via 1 high speed serial pin line.

DDS LO board
The "LO DDS" 16bit CPU controls all of the radio functions, including the VF display, keypad, tuning knob, memory buttons, band selection, mode selection, and LO frequency generation. The tuning knob is fully optical and sends digital signals to the interrupt driven CPU for frequency changes. The keypad allows for direct frequency control as well as turning on/off special functions. The EEPROM stores 5 frequencies for each band (selectable from the keypad)

Manual "RIT" adjustments are made via the front panel knob. The original "RIT" control (10k pot)  is rewired to the CPU's A/D converter. The A/D information is converted into frequency shift information for RX only. The "LO DDS" board maintains the proper RF frequency by adding the exact same "IF DDS" frequency to its own LO frequency. This is achieved by making sure the "LO DDS" and the "IF DDS" boards see the same information.

The radios SBL-1 mixers require +7dBm LO, in order to operate properly. Output level (dBm) of the "LO DDS" is too low on the 10m band (LO = ~36 mhz), requiring an additional OP-AMP to boost this level.


Single Side Band Modulation

SSB generation is accomplished via the DDS IF board.
( This replaces the original BFO board.)


An idiots guide to SSB generation, via the FILTER method:

1) Apply audio and an IF sine wave into a balanced modulator. We not only want to mix the audio and IF to produce an audio modulated IF signal, but eliminate the carrier (the RF envelope generated when the transmitter is active, even when there is no audio present) by balancing out the audio modulated IF signal sent to the LO mixer. The idea is to have no output signal if your not talking.

2) Now you have a DSB (double sideband) signal in which the audio modulated IF signal only appears at the input of the LO mixer when you apply audio (i.e. talk into the mic). If you keep your IF freq at the center of the IF filter, any AM radio can properly detect your audio signal, since AM is DSB with a carrier all the time.

3) In AM, the carrier is in the center of the IF crystal filter and allows audio signals both above and below the IF center freq. In SSB there is no carrier, so a phantom carrier IF freq is located on either the low edge or upper edge of the selected IF filter. The idea is to use the crystal filter to allow audio signals either above or below the phantom carrier freq, but not both.

This example is for a 4Khz wide  8.830MHz crystal filter

Lower edge                     Filter center                       Upper edge

IF 8.828     ________    AM IF 8.830  ____  ___          IF 8.832

||  -2khz                        AM Passband center                  2khz  ||    

                                       || 4khz             USB signal between the lines                0hz   || <-phantom carrier point

Phantom carrier -> || 0hz              LSB signal between the lines                4khz  ||                                         

4) This method requires that the LO freq be adjusted to the IF freq your using. That is the RF freq. of interest is obtained by selecting a LO freq = IF (carrier or phantom carrier freq) + the RF freq.

5) Shifting the IF freq, shifts the phantom carrier freq, which in turn moves the audio passband.

With a USB signal, moving the IF up in freq. will raise the freq. response audio received because your moving your virtual 0 HZ audio point (phantom carrier point) away from the actual received audio signal. Moving the RF up in freq will lower the freq. response audio received because your moving the received signal back towards the 0Hz audio point (phantom carrier point), or maybe even past.


Here's a better description of what goes on inside the HW-5400 radio to generate SSB using a 4k crystal filter:
Click to enlarge picture



Stability of the LO DDS comes from using a rather expensive $100 reference OCXO (Oven controlled crystal oscillator) at 19.44mhz (+/- 0.25ppm). The DDS multiplies this by 6 for a 116mhz +/- 29hz (worst case). The DDS max output is (10meters) 30mhz+IF=38.830mhz, so output/input ratio is 38.83/116 = .335. The DDS output will vary worst case of +/- 10hz. The IF DDS output is 8.83mhz and so only requires a standard inexpensive $5 100mhz +/- 25ppm crystal since the output/input ratio is so low. The other way to achive stability would be to use a DDS with a built in PLL circuit and a low freq crystal reference.

Base IF 8.830 mhz: The 2.8kHz filter was moved off the IF board and placed on a new circuit board that allows mechanically switching in/out either the original 2.8kHz filter or a new 4kHz filter. This requires the IF freq. to be shifted to different phantom carrier frequencies (depending on the MODE and FILTER width) for SSB operation. Using the 2.8 kHz IF crystal filter:  "IF DDS" at 8.828550 LSB & 8.831450 USB (+/- 1450 hz from center) Using the 4.0 kHz IF crystal filter:  "IF DDS" at 8.828000 LSB & 8.832000 USB (+/- 2000 hz from center)

The "LO DDS" also shifts its LO freq according to the MODE and Filter width to keep your RF freq. constant. Digital signals sent from the "LO DDS" board indicate the operating mode (AM,CW,LSB,USB) to the "IF DDS" The "IF DDS" board produces the correct IF frequency based on this information.

Manual "IF shift" adjustments are made via the front panel knob.  The "IF shift" POT is wired to the CPU's onboard A/D converter. The A/D information is converted into frequency shift information. The "IF shift" POT is also sent to the "LO DDS" board to keep the RF freq. constant.

The DDS IF output level is adjusted down to ~200mv p-p (via a POT) for use by the balanced modulator in both RX and TX circuits of the AUDIO board.


DDS IF Generation Board

The HW-5400 is a single conversion, 8.830 mhz IF radio.

Stability of the IF output comes from a standard (inexpensive) 100mhz crystal reference osc +/-25ppm for a worst case output of +/-2500hz. The max freq IF DDS output is 8.83mhz for a ratio of 8.83/100 = .0883. The worst case DDS IF output of +/- 220hz. Actual reference varies +/- 250hz, so the worst case DDS IF (when divided down) output is actually +/- 22hz

Left Side:
DDS IF board
generates the IF signal used by the balanced modulator



DDS IF board Design


This board is identical to the DDS LO board except there are fewer connectors since this board does not control the radio like the DDS LO board. The DDS IF CPU monitors the mode via signals from the DDS LO board as well as the Crystal Filter selection, and the user selectable IF shift via the front panel analog control connection to the CPU's A/D convertor. The "Phantom Carrier" IF freq is shifted from its base 8.83mhz to the correct value knowing the mode, filter, and user selected IF shift.

This new IF board (shown on right image) has a 4k & 6k INRAD as well as the original 2.4k Heathkit filter.