2020-12-13

Next Generation Beacons

DDS AD9912


Overview VCO-PLL DDS GPS reference Distribution amplifiers Service management Sequence PI4 Software

 

Peter, G8BCG - "Close-in noise is low and in-band spurii (from SMPS) are around -140 dBc.
The beacon is line of sight about 20 km west of here - with my 4 x 8 el. it is quite loud so I wanted it to be clean ;-).
The 2nd and 3rd harmonics are the only significant out of band signals and these are less than -90 dBc"

Russ, K0WFS - "Glad I found this group - this board is a turbo version of what I've been doing with an ATTiny85 and various VCXOs and AD DDS chips....cool."

Emanuele, IW9GDC - "... I can really say that this hardware is actually the best ever made ..."

The DDS unit can also be used for scientific purposes such as a Hydrogen Maser RF-driver

 

The OZ7IGY DDS board is the brain in the Next Generation Beacons hardware. It is clocked by the VCO-PLL board at 1 GHz and a GPS signal controls the timing. The use of GPS synchronization ensures accurate frequency and timing that is used to perform the right modulation at the right carrier frequency of choice. An output level exceeding 10 dBm can then be fed into an amplifier chain.

The DDS board uses an Analog Devices AD9912 as DDS that is controlled by an Atmel ATmega128A. The DDS board may be used up to 440 MHz without modifications as it is not frequency specific over a very wide frequency range. By using over-sampling (per Analog Devices application note 939 "Super Nyquist") it is possible to use the DDS board at 1,3 GHz which is the case for the OZ7IGY beacon. Above 1,3 GHz either multiplying the output signal or multiplying the 1 GHz clock and mixing with the DDS output signal can be used. At 1,3 GHz the OZ7IGY beacon uses the filter doubler unit. Above 2 GHz the OZ7IGY beacons use multiplication of the output signal from about 300 MHz to 400 MHz (3,4 GHz to 10 GHZ), the filter doubler (2,3 GHz) or a combination of the filter doubler and a subsequent multiplier (24 GHz). This still ensures that the phase noise is kept below -100 dBc/Hz at 10 kHz.

The micro controller can also be used to control other devices such as the power level of the amplifier chain vs. time.

All timing inputs and outputs are galvanically isolated to protect against static discharges. If no galvanic isolation is needed the optical connections can easily be replaced by a 2 or a 3 pin Molex connectors.

The DDS board also features an in-circuit serial programming (ICSP) connector and USB interface for updating, configuration and service management. An 4 bit DIP switch may be used for additional configuration.

The DDS board comes in a standard tinplate box.

Software

The hardware does not put any constraints on the functionality of the AD9912 DDS. A couple of application specific software packages are available:

Software support is provided on the above packages. From time to time new functionality is added under tight version control. The packages are available with both source code and compiled .hex files. This way specific modifications may be applied for any given application.

Parameters

DC characteristics Unit Notes
Voltage 7,5 V - 8 V Up to 12 V for a shorter period of time. Otherwise the 5 V regulator will overheat. Units without the onboard 5 V regulator has a maximum voltage of 5,3 V. Units produced before 2017-02-21 does not have the on-board 5 V regulator
Current ~420 mA  
Power consumption 3,2 W  
RF characteristics Unit Notes
Lowest frequency  137 kHz Tested at 137 kHz but it should be possible down to 4 µHz. However, AC coupling becomes a problem
Highest frequency 440 MHz/1,3 GHz Per application note AN 939
Frequency resolution 4 µHz
48 bits
At 1 GHz clock
Phase resolution 0,02°
14 bits
 
DAC resolution 14 bits
84,3 dB
 
Output power See table
Frequency
[MHz]
DAC current 511 DAC current 1023
Power
[dBm]
Power
[mw]
Power
[mw]
Power
[mw]
<100 17 50 19 80
150 16 40 19 80
250 15 32 19 80
350 13 20 17 50
440 12 16 16 40
Second harmonic <-20 dBc Fundamental frequency 50,4 MHz
Third harmonic <-20 dBc Fundamental frequency 50,4 MHz
Clock suppression -15 dBm  
In-band spurious <-90 dBc  
Out of band spurious <-80 dBc  
First image See table
Frequency, f0
[MHz]
Power, f0
[dBm]
Image, f1
[MHz]
Power, f1
[dBm]
Power, f1
[dBc]
50    17 950    <-40    <-57   
100    17 900    <-40    <-57   
150    17 850    -29    -46   
200    17 800    -26    -43   
250    17 750    -22    -39   
300    16 700    -7    -23   
350    15 650    2    -13   
400    13 600    4    -9   
440    12 560    7    -5   

DAC current set to 511/0x1FF.

Frequency, f0
[MHz]
Power, f0
[dBm]
Image, f1
[MHz]
Power, f1
[dBm]
Power, f1
[dBc]
50    19 950    -27    -46   
100    19 900    -20    -39   
150    19 850    -30    -49   
200    19 800    -12    -31   
250    19 750    -7    -26   
300    18 700    -5    -23   
350    17 650    5    -12   
400    16 600    7    -9   
440    16 560    10    -6   

DAC current set to 1023/0x3FF.

Phase noise See table
Frequency
[MHz]
Phase noise at 1 kHz
[dBc/Hz]
Phase noise at 10 kHz
[dBc/Hz]
Phase noise at 100 kHz
[dBc/Hz]
28    -133 -138 -138
50    -131 -138 -138
70    -130 -138 -138
144    -124 -144 -148
220    -120 -140 -142
297    -120 -139 -141
432    -118 -133 -137

DAC current set to 511/0x1FF.

Measured on a HP 3048A phase noise system. Measurements below 100 MHz are limited due to the HP 8662A's own phase noise that is -138 dBc/Hz. The DDS unit was clocked by a 1 GHz VCO-PLL with a Crystek CVCO55CX-1000-1000 VCO. 

Ports Connector Notes
Supply 2 pin Molex  
Clock SMA female Older versions, PCB 0.d to 0.g, have a BNC female
RF out SMA female  
Doubled RF SMA female Optional SMA connectors, one for each output
NMEA In TOSLINK Optional. Can also be replaced by a 2 pin or a 3 pin Molex connector if relevant
AUX In 1 TOSLINK Optional. Can also be replaced by a 2 pin or a 3 pin Molex connector if relevant
AUX Out 1 TOSLINK Optional. Can also be replaced by a 2 pin or a 3 pin Molex connector if relevant
AUX Out 2 TOSLINK Optional. Can also be replaced by a 2 pin or a 3 pin Molex connector if relevant
ICSP 6 pin header Standard Atmel configuration
Service USB B female  
Mechanical Unit Notes
Size 54 mm x 145 mm Fits into a standard tin plate box
Weight 200 g Incl. tin plate box. The DDS unit alone is 80 g

Best spectral performance is achieve using a low noise power supply. Please see the Analog Devices application note 927 "Spur related to power supplies." Power supplies with low suppression of the mains, rectifier or switching frequencies or other noise components may cause unwanted spurii on the DDS unit output. For the same reason the DDS unit is mounted with low noise voltage regulators.

Availability

The price for an assembled, tested, boxed and ready-to-use DDS unit with one SMA connector and in pre-punched tinplate box is 2334 DKK* (~315 EUR). The TOSLINK receivers and transmitters and the DIP switch are optional.

The time used for manual assembly and testing is five hours using solder paste stencil and oven soldering.

The price for a DDS unit 4-layers PCB is 162 DKK* (~22 EUR).

Please contact us for a partially assembled unit or hard to find parts.

Additional shipping and money transfer cost applies.

*: Prices are subject to change due to exchange rate fluctuations.

Documents

Operational troubleshooting

Here is a guide to troubleshoot an operational beacon including retrofits.

Further information

Please contact Bo, OZ2M, at my callsign @ this domain.

 


Bo, OZ2M, www.rudius.net/oz2m