RDDS Frequently Asked Questions

 

Why is this unit called RDDS (Reverse Direct Digital Synthesis)

Normally a DDS chip is clocked with a lower reference input frequency than the required output frequency. The internal clock multiplier often being used when the highest output frequency is required. The DDS clock multiplier may compromise the phase noise and alias products may affect the signal purity. RDDS is not used to produce the required output frequency directly but is used as a very flexible PLL upgrade for existing OCXO systems. The DDS chip is clocked with the beacon base band VHF OCXO signal typically in the range 90 140 MHz via an input line bridging buffer amplifier and used to produce an output signal at 10 MHz (or whatever convenient available reference is available in the range 5-20 MHz) This lower output frequency produced by the DDS is formed the control PIC loading the DDS with a Hex control number at power on. Adequately fine frequency steps can be produced to produce oddball frequencies and produce the fine frequency stepping for F1 and JT4 modulation schemes. The system Reference Signal and the DDS output signal are then compared in an analogue comparator, the output of this being amplified by an op-amp which controls the varicap in the VHF OCXO to complete the control loop. The DDS chip is therefore used in reverse to its more commonly used function.

 

What am I able to do with the RDDS module

There are four main functions currently possible with this module.

The first PIC configuration is for use as a local oscillator PLL for microwave transceiver systems and contains 16 pre-programmed commonly used LO frequencies. These may be user defined frequencies by using the RS232 interface to modify individual memories 0-F. Again the PIC can be used for an F1 keyed beacon with external TTL keyer with appropriate memory settings. A more sophisticated four bit digital control connected to the expansion interface would allow up to 16 tones to be transmitted for special purposes. This 16 memory PIC is normally supplied with the RDDS kit and recommended for testing the RDDS after construction before moving onto a Beacon mode particularly JT4 where if the code does not match the GPS signals problems may arise.

The second PIC type for dedicated Beacons is pre-programmed with the CW message information. The PIC can be pre-programmed to output the CW message either as F1 or as a TTL signal for A1 usage. Whilst the message is fixed the frequencies for the F1 mark/space can be changed by RS232 connection. Source Code for the PIC may be downloaded here to be user customised and compiled. This is a two tone, mark, space single frequency configuration for F1 modulation unless configured for single tone using A1 keying. The .asm source code file once customised is used to produce a custom PIC and is not message modifiable using the RS232 interface.

The third arrangement allows sending JT4 modulated signals. The transmitted signals being Carrier, F1 CW ident with up to three tones and the JT4 FSK sequence. G4JNT has made the JT4 software available for creating the JT4 PIC or alternatively offers a programming service. GPS timing signals are required to be connected to the 20 pin expansion port for this mode.

Fourth arrangement in development, is as a VCO controller for test or personal low power beacons. Stability is excellent with the VCO multiplied into the millimetric bands.

 

Can I use the PIC clock crystal as the RDDS reference frequency input signal

No. The PIC clock is of low quality and stability used for loading the DDS chip with hex numbers. In some instances the clock runs only briefly at initialisation whilst it is needed. The Reference Frequency input should be of the highest quality as its phase noise and stability will have a direct effect on the overall signal quality. The preferred PIC clock for Beacons to implement improvements is now 11.2896 MHz.

 

Can I use a resonator instead of a crystal for the PIC clock

It was not intended to use a ceramic resonator and so there is no earth connection point for the third ground connection. Scrape away the PCB board resist to make a ground point. Also if the resonator contains capacitors omit C6,7 33pf from the PCB construction. In testing operation note the PIC clock only runs very briefly as above. Look at the LED sequence below which will verify the resonator runs ok.

 

How do I know the kit 16 memory PIC has initialised at power up

In the case of the 16 memory PIC supplied with the RDDS kit initialisation will cause the Green LED2 to flash briefly followed by the Red LED2 lighting. The clock oscillator also only runs briefly as the DDS is loaded with hex from the selected PIC memory location. Choosing another PIC memory frequency will repeat the above LED and clock oscillator sequence to reload the DDS with new numbers.

 

How do I calculate the Hex number to programme the PIC for a frequency not already pre-programmed

An online calculator has been created here. Insert your frequency data and it will calculate the required Hex numbers to insert into your chosen PIC memory location. Read further below for how to do it. A table of examples is also available here for beacons. Hex programming numbers are shown for both 10 and 5 MHz references together with the keying shift frequency resolution available when using F1 keying. Some further examples for Local Oscillators are also shown.

 

I am one digit short using the Online Calculator to generate the Hex number

If this occurs then add a leading zero

 

The Kit PIC is supplied with a number of useful frequencies pre-programmed, they are listed in the table below

A PDF copy can be downloaded for printing for easier comparison of PIC memory contents with those viewed using the RS232 interface.

 

 

Memory information and intended purpose for the pre-loaded PIC V2 supplied with the RDDS Kit

Examine and change memory contents using the RS232 interface and Hyper Term connected at 2400 Baud 8 n 1

An Online Calculator is available to calculate new hex numbers for other frequencies using references 5-20 MHz

 

 

PIC

Memory

No.

 

 

Hex No. in

Memory

10MHz Ref.

 

 

RF Frequency

Locked by this

Selection MHz

 

Crystal

Frequency &

Multiplication

 

 

Band & IF

 

 

20 Pin Connector Selection Links

Pins

3-4

Pins

5-6

Pins

7-8

Pins

9-10

 

0

1

2

3

4

5

6

7

8

9

A (10)

B (11)

C (12)

D (13)

E (14)

F (15)

 

1AAAAAAA

152D2D2D

12DEA9D3

15E15E15

18099D72

148FEF8C

14F0D168

19ED41D0

14B36831

171024E6

18BBFB0D

14D0214D

1511E8D2

1A068F85

183A2525

18099D72

 

1152

2176

3256

5616

10224

23904

46944

75832

2968

5328

9936

23616

46656

75544

1268

10224

 

 

96.0000

120.8888

135.6667

117.0000

106.5000

124.5000

122.5000

98.7396

123.6667

111.0000

103.5000

123.0000

121.5000

98.3646

105.6667

106.5000

 

x12

x18

x24

x48

x96

x192

x384

x768

x24

x48

x96

x192

x384

x768

x12

x96

 

23cm

13cm

9cm

6cm

3cm

24GHz

47GHz

76GHz

9cm

6cm

3cm

24GHz

47GHz

76GHz

23cm

3cm

 

144 IF

144 IF

144 IF

144 IF

144 IF

144 IF

144 IF

144 IF

432 IF

432 IF

432 IF

432 IF

432 IF

432 IF

28 IF

144 IF

 

Y

Y

Y

Y

Y

Y

Y

Y

N

N

N

N

N

N

N

N

 

Y

Y

Y

Y

N

N

N

N

Y

Y

Y

Y

N

N

N

N

 

Y

Y

N

N

Y

Y

N

N

Y

Y

N

N

Y

Y

N

N

 

Y

N

Y

N

Y

N

Y

N

Y

N

Y

N

Y

N

Y

N

 

When an additional frequency is to be added place it in F and no links are required for its selection. When used for an F1 beacon with external keyer, place mark and space frequencies in E & F and connect the TTL CW message to Pin 9 and ground to Pin 10 on Con 1. If keying is reversed swap the E & F memory contents.

 

 

I need to re-programme or change the PIC in my 16 Memory RDDS unit

The Hex code required to programme the 16F628A PIC can be downloaded here as a zip file. Read the Readme file.

 

How do I enter RS232 programming mode using the kit 16 memory PIC

Connect your RS232 Com Port cable, three wires needed, to the RS232 3 pin header on the PCB. The connection between PC Com Port ( or USB Com Port adapter ) is direct, no MAX232 interface is needed as the RDDS PIC will talk directly to the Com Port via R3 and R5. Use Hyper Terminal 8,n,1 running at 2400 Baud. Most USB to 9 pin Com Serial Adapters will also work. However your PC may allocate a new Com Port number each time the USB adapter is plugged into the PC. Solve this in your PC Control Panel.

Power up the RDDS and programming mode is automatically entered presenting a menu of actions on the PC screen. Follow the menu options to examine existing stored hex numbers or change existing numbers for new ones. End with G to go to normal RDDS operation. Unplug the RS232 cable on completion of menu operations or programming mode will be entered at next power up preventing normal RDDS operation.

 

 

The Hyper Terminal Screen showing Hex numbers stored for each of the 16 memories 0-F after pressing R to retrieve them.

 

 

 

 

 

Click here to view the RS232 Read, Change Frequency
Hex Data and Save operations.

 

May only play using IE

Ok any gripe messages to view

 

The power up LED sequence with RS232 cable connected to the 3 pin header is as follows:

Power on, LED1 Red flashes once

Menu appears on Hyper Terminal

Press R to read memories 0-F, LED2 Green lights and stays lit.

Any key to select memory location to change, LED2 stays lit.

Enter memory location, eg. F, LED2 green stays lit.

Enter new Hex hex number for F, press W for Write, LED2 Green stays lit but may flicker during entries.

Press G to go to normal RDDS operation, LED1 Red lights.

 

 

Can I program my own 16F628A PIC with the standard 16 memory software

Obtain the latest Hex file from the G4NNS website here.

Alternatively a zip file is available for download above.

Use a programmer such as WinPic to load and verify the 16F628A using the Hex file.

An example of the programming operation using WinPic here.

 

How do I obtain a JT4 PIC for RDDS

PICs are custom programmed either by DIY or by contacting G4JNT whom offers a service. To DIY programme the PIC download the file toolkit from G4JNT. You will need to generate content for two .inc files to hold the JT4 specific information. First .inc file is the JT4 message, Second .inc file the CW message of up to 40 characters and Frequencies required in Hex format. Up to three frequencies can be set, key up, key down and optionally carrier if a frequency other than the key up is required. Additionally several flags need to be set to determine whether GPS NMEA or Motorola Timing Protocol is used. Also flags for TTL or RS 232 levels and whether the GPS 1pps signal is to be used in addition to the normal data stream. Also the keying speed is adjustable to cater for longer CW information. These inc files above are then compiled with the standard .asm file and used to programme the specific purpose PIC.

 

How do I change the stored data in the JT4 PIC option

There is no RS232 menu option in JT4 operation for modifying data. You should not attempt to connect to the RS232 connector in JT4 mode. For changing data proceed as above, obtaining or making a DIY JT4 PIC.

 

Can A1A keying be employed with JT4G

A further development by G4JNT now allows Version 2 JT4G software users to set an additional flag to those mentioned above so that the main CW message of up to 40 characters can be sent using either FSK or A1A.

An example of the A1A and JT4G FSK keying sequence here. A reminder, JT4G requires GPS timing data to insert the 13 character FSK information into the keying stream. The A1A CW and JT4G carrier enable for the beacon PA is output from the PCB pads alternatively used for TTL keying input in other software variations. The enable is +5v active. This enable is not available or required using the fully FSK keying option.

 

Last update 4 August 2011 G8ACE