1 Fully Digital HF Radios Phil Harman VK6APH Dayton Hamvention – 17 th May 2008

2 Overview Software Defined Radios are now providing performance equal to the best Analogue designs There’s is a new trend in HF SDR radios that eliminates most of the Analogue components. In effect the antenna is connected directly to an Analogue to Digital Converter (ADC). So how does this next generation of SDRs work? How well do they work?

3 Background Most current SDRs use PC sound cards or audio ADCs to provide analogue to digital conversion BPF ~ 90 LPF 0 – 192KHz I Q

4 SDR

5 Performance Bandscope width restricted to sound card sampling rate e.g. max of 192KHz Image response –e.g. Receiver tuned to kHz, with 10kHz IF, then image will be at kHz

6 Performance Image rejection limited by analogue components RejectionPhase(deg) Amplitude(dB) 40dB dB dB dB This accuracy must hold over each ham band and 300Hz- 3kHz, with temperature, component aging, vibration, voltage fluctuations etc

7 Performance We can compensate digitally for consistent phase and amplitude errors Automatically and manually

8 I & Q Error Correction Can provide >90dB of image rejection at a single frequency either manually or automatically But - image rejection will drop at band edges So - apply the correction at multiple frequencies

9 I & Q Error Correction ‘Rocky’ software (Alex, VE3NEA) ‘learns’ how to correct I and Q using off-air signals Switch onAfter one day

10 I & Q Error Correction Not the full solution since: –We need enough, strong signals, for the calibration to work –The calibration will change with SWR, temperature etc –Needs doing on each band –It’s time consuming This doesn’t mean it not a solvable problem – some really smart people are working on it!

11 Fully Digital Approach A DA D Digital Signal Processor Data Audio

12 Fully Digital Approach ADC requirements –Must sample > twice max receiver frequency –For 0 – 30MHz sample at >60MHz –Need >120dB of dynamic range –At 6.02dB per bit need 20 bits

13 Fully Digital Approach ADC – how much can we afford? For $100 –Linear Technology - LT2208 –Sample rate – 130Msps –Input bandwidth – 700MHz –Bits – 16 –Wide band noise floor - 78dBFS

14 Fully Digital Approach DSP interface A DA D Digital Signal Processor Data Audio Data Rate

15 Fully Digital Approach Speed requirements 16 bit 63Msps ~ 1000 Mbps i.e. 1Gbps Options –Firewire* = 400Mbps –USB2 = 480Mbps –Firewire800 = 800Mbps –USB3 = 4.8Gbps (Q2 2008) –Ethernet = 1 & 10Gbps –PCIe = 64Gbps * In practice Firewire is faster than USB2 due to Peer-to-Peer architecture

16 Fully Digital Approach DSP requirements PC – Quad Core PC –Processor speed OK, limitation is getting data in and out of the processors' main address space PlayStation 3 –Processor Speed OK, limited to 100T Ethernet or USB2 interface Expect to process 4~6MHz of spectrum

17 Fully Digital Approach Digital to Analogue Conversion (DAC) For Audio output need 16 bits at 8ksps = 128ksps Modern sound cards/chips do > 4Mbps

18 Fully Digital Approach Reality Check! ADC not meet our needs USB2 or Firewire will give 240Mbps to PC Enough for a 60MHz wide bandscope or 6 simultaneous receivers each 300kHz wide So we compromise!

19 Fully Digital Approach With Analogue radios we don’t process MHz simultaneously We process a single frequency and a narrow bandwidth e.g. 3kHz Can we apply the same process to a fully digital radio? Yes! We use Digital Down Conversion which is based on Decimation.

20 Fully Digital Approach Decimation A D Decimator (divide by n) 16 bit 63Msps 16 bit 63/n Msps

21 Fully Digital Approach ADC Output

22 Fully Digital Approach ADC Output – Decimate by 3

23 Fully Digital Approach Decimate by 3 Output data rate now 63/3 = 21Msps But, maximum input frequency now <10.5MHz What if we use superhet techniques?

24 Digital Down Conversion

25 HPSDR Mercury DDC Receiver LT2208 ADC sampling at 125MHz ADC output 0 – 60MHz Decimate by 640 Output = 125MHz/640 = 195ksps 24 bit samples 24 x 195,000 = 4.68Mbps Bandscope now 195kHz wide

26 HPSDR Mercury DDC Receiver By decimation we have eased the load on the PC but increased the complexity of the DDC But there is an additional advantage of decimation! Every time we decimate by 2 we increase the output SNR by 3dB

27 HPSDR Mercury DDC Receiver By decimating from 60MHz to 3kHz we improve the SNR from 78dB to 121dB

28 Performance Standard way of measuring receiver performance 3 rd Order Intermodulation Products Inject two equal amplitude signals in the antenna socket Any non-linear stages will create 2 nd harmonics These mix with the fundamentals to produce 3 rd order IP

29 Performance 3 rd Order IP Inject two equal amplitude signals Input MHz dB f1 f2

30 Performance 3rd Order IP Inject two equal amplitude signals Any non linear stages will create harmonics Input MHz dB f1 f2 2f12f2 3f1 3f2

31 Performance 3 rd Order Intermodulation Products Input MHz dB f1 f2 2f1-f22f2-f1

32 Performance Graph of IP3 for Analogue Receiver Input dB Output dB Saturation 3 rd order intercept point Fundamental (Slope = 1) 3 rd Order IMD (Slope = 3)

33 Performance Graph of IMD for ADC based Receiver Input dB Output dB Saturation Intersection has no practical significance Fundamental (Slope = 1) IMD Products (Slope = 1)

34 Performance Graph of IP3 point verses input level Input dB IP3 dB Saturation Analogue Receiver Digital Receiver

35 Performance What causes IMD to vary with input level? –Fewer bits are used at low input levels –Non ideal ADC performance

36 Performance Ideal ADC Analogue Input Digital Output

37 Performance Real-world ADC Analogue Input Digital Output

38 Performance Real-world ADC Analogue Input Digital Output

39 Performance

40 Performance

41 Performance Sources of dither –In band signals and noise –Out of band signals and noise –Internal pseudorandom noise –Added external signal –As long as all the external signals don’t add….. Then big signals are your friend.

42 Fully Digital HF Radios Summary –Fully digital receivers perform differently to analogue ones –IP3 measurements are not meaningful. –Large signals can improve the performance of digital receivers –In practice……