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Hints

On this page you will find a random collection of hints, ideas and discussion to help with construction and operation.
Intended mainly for builders of earlier kits.

Case Speaker Holes
Here is a good suggestion supplied by Ron WD4GWK.Case speaker holes - very easy to make using 3 inch plastic kitchen sink drain as template. Holes uniform,smaller speakers just less holes for 2 and 2.5 speakers. All hardware stores have them for a dollar or two.Just for info Ron.

Transmit Distortion
As with any RF project it is important to keep all component leads as short as possible. This is particularly the case with the microphone amplifier transistors Q3 and Q4. If the leads, especially Q3 base, are left too long there is the possibility of RF from the antenna getting into the circuit and causing distortion in the transmitted signal. In really stubborn circumstances it may also be necessary to solder a 10nF (0.01uF) ceramic capacitor across the base and emitter leads of Q3.

Rotary Encoder Shaft
The rotary encoder supplied with the DDS VFO has a 6mm 'D' shaped shaft. Commonly available press-on and grub screw fitted knobs are designed to fit 6.35mm (1/4") shafts and so the knob will be a loose fit. One suggestion to make the knob a snug fit is to wrap a few layers of clear sticky tape (sellotape/scotch tape) around the shaft to build up the diameter.

Remember too that there must be small gap between the rear of the knob and the front panel to allow the rotary encoder switch to operate when pressed.

Here is another good suggestion supplied by Mark N7EKU.
I have a hint for bringing metric shafts up to english size: Tape used for enlarging shaft diameters often turns to goo in hot weather or with time. I prefer to use aluminum shim made from an aluminum beverage can (your choice!). The wall diameters of these cans is tapered, but if you take a piece from about 1/3 of the way from the top or bottom, the thickness should be about 0.1mm. Just cut a strip out with some scissors to a width equal to the depth of the hole in your knob. The circumference of a 6.35mm hole is about 2cm, so you will need about 3 wraps to bring the hole down closer to 6mm in size. Cut a piece that long and then wrap it around a 3/16" drill bit. Hold it as you slide it off the bit and then push it down in the knob hole. It should still have some spring left in it and will spring out to fit the walls of the hole. The aluminum will be strong and more durable than tape and should last a long time.

DDS VFO Configuration
The DDS VFO configuration was described in the construction manual, however the relationship between the carrier (BFO), DDS output and displayed frequencies can be a bit confusing for beginners. Here is a more detailed explanation with an example.
The balanced modulator takes the 10MHz carrier (BFO) signal and mixes this with the audio signal when you speak. The output of the balanced modulator is an upper sideband (USB) and a lower sideband (LSB) signal centered around the carrier frequency. The carrier is not present at the output (suppressed) and so we use the term DSBSC or Double Sideband Suppressed Carrier modulation. The DSBSC signal is applied to the crystal filter. The filter is designed with the upper skirt of the filter at 10MHz, and so the LSB is passed by the crystal filter, while the USB is blocked.The LSB signal is then mixed with the output of the DDS VFO to obtain the final transmit signal.

When the DDS is first powered on it uses the default setting: IF frequency of 10MHz, IF offset of Minus and display frequency of 7.1MHz. The DDS output frequency will be 10.000MHz - 7.100MHz = 2.900MHz.
The final transmitted signal will be 10.000MHz (suppressed carrier) - 2.900MHz = 7.100MHz.
Now let's assume that we want to move the carrier frequency 300Hz higher. This could be due to a slightly higher crystal filter response or after testing we find the USB is not suppressed sufficiently. The new carrier frequency will be 10.0003MHz.

Returning to our DDS VFO. If we have not changed anything it will still be programmed for an IF frequency of 10.000MHz and the output frequency will be 2.9MHz.
The displayed frequency is calculated by the DDS firmware and will be 10.000MHz - 2.900MHz = 7.100MHz. However the actual output frequency will be: 10.0003MHz (suppressed carrier) - 2.900MHz = 7.1003MHz. The LCD displayed frequency and the actual transmit frequency are not the same.

To fix this we need to measure the actual suppressed carrier frequency and re-configure the DDS VFO IF frequency parameter. The frequency can be measured with a counter connected to the emitter of the BFO buffer transistor Q2.
Using this example we would now configure the DDS VFO with the following: IF frequency of 10.0003MHz, IF offset of Minus and display frequency of 7.1MHz.
The DDS output frequency will be 10.0003MHz - 7.100MHz = 2.9003MHz.
The displayed frequency will be 7.100MHz.
The final transmitted signal will be 10.0003MHz (suppressed carrier) - 2.9003MHz = 7.100MHz.


DDS VFO output
The output of the DDS LPF is DC coupled and does not include a series capacitor. If using the DDS VFO in another application you may need to use a series coupling capacitor between the DDS VFO LPF and input of the other circuit.

DDS VFO frequency range
The DDS VFO was designed to be used as digital VFO. As such the minimum range allowed by the current firmware is 1MHz. Attempting to program a frequency lower than this will result in an error message on the LCD. The upper frequency range is not limited by the firmware, but there is a practical limit set by the hardware. The AD9834 is clocked by a 50MHz master clock and the maximum usable frequency is 1/4 of this or 12.5MHz. Also the DDS VFO hardware incorporates a LPF with a 10MHz cutoff frequency and any frequencies higher than this will be greatly attenuated.

The DDS VFO does not limit operation beyond the amateur bands. It is up to the user to ensure they do not transmit outside their freqency band allocation.

Thanks to Fred K3TXW HERE is an Excel spreadsheet listing the MST parts list along with Mouser part numbers.


A guide for identifying fixed capacitors that can be used in the MST kits is HERE.

The electrolytic capacitors are standard parts and available from all electronic retailers. Note that they are all 'RB' style, that is, their leads extend from the same end.
The MLCC (monolythic) ceramic capacitors should ideally have a 0.2" pitch to match the PCB and again are widely available.
The polyester film capacitors are specified to be 'MKT' style. These are small, have a rectangular body, and consistant pin spacing. If 'MKT' types cannot be found then older style'greencaps' can be used although they may not fit neatly into the board.
When purchasing the ceramic disk capacitors ensure that the values below 100pF are NPO types, especially those used in the crystal filter and BFO.
The transmit low pass filter capacitors need to be good RF types with low loss and have a minimum voltage rating of 100V. The parts list calls for monolythic ceramic C0G (NPO) capacitors which although quite small work well at QRP levels. Try RS components, Element14, Mouser or Digikey.
If preferred they can be substituted with more traditional polystyrene or silver mica types although due to their size may be difficult to fit in the board.

LCD. The variety of LCDs available is staggering, however the type supplied in the DDS VFO kit is a fairly standard type. If you wish to use your own LCD for the project please read the information below:
- 16 character x 2 line display employing the common HD47780 controller or equivelant.
- Size 80mm x 36mm.
- The solder pins need to be in a single line at the top of the board with pin 1 at the corner and the remainder numbered sequentially to 16. Pin 1 is earth, pin 2 +5V, pin 15 is the backlight + terminal while pin 16 is the backlight - terminal.
- Ensure the display backlight is a low current type. This normally means it uses white LEDs for the backlight. LCDs with yellow/green backlights draw considerable current, 100mA is typical, and are probably impractical.

Microphone. The MST is designed to be used with low impedance dynamic microphones. This type is very commonly used with CB radios and other two way radios.
The MST board does not provide a bias voltage for electret type microphones.
If you want to use an electret microphone you will need to modify the board to provide a bias voltage and capacitvely couple to the MIC gain control.
You may also need to decrease the gain of the mic amplifier by changing the value of R11.