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Fully Digital HF Radios

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

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Fully Digital HF Radios

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  1. Fully Digital HF Radios Phil Harman VK6APH Dayton Hamvention – 17th 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 LPF I BPF ~ 0 – 192KHz 90 LPF 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 14.100kHz, with 10kHz IF, then image will be at 14.080kHz

  6. Performance • Image rejection limited by analogue components Rejection Phase(deg) Amplitude(dB) 40dB 1.0 0.1 60dB 0.1 0.01 80dB 0.01 0.001 100dB 0.001 0.0001 • 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 on After 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 Data Digital Signal Processor A D Audio D A

  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 Data Digital Signal Processor A D Audio D A Data Rate

  15. Fully Digital Approach • Speed requirements • 16 bit samples @ 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 0 - 30MHz 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 Decimator (divide by n) 16 bit samples @ 63/n Msps A D 16 bit samples @ 63Msps

  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 • 3rd Order Intermodulation Products • Inject two equal amplitude signals in the antenna socket • Any non-linear stages will create 2nd harmonics • These mix with the fundamentals to produce 3rd order IP

  29. Performance • 3rd Order IP • Inject two equal amplitude signals f1 f2 dB 0 2 4 5 6 8 10 12 14 16 18 Input MHz

  30. Performance • 3rd Order IP • Inject two equal amplitude signals • Any non linear stages will create harmonics f1 f2 dB 2f1 2f2 3f2 3f1 0 2 4 5 6 8 10 12 14 16 18 Input MHz

  31. Performance • 3rd Order Intermodulation Products f1 f2 dB 2f1-f2 2f2-f1 0 2 4 5 6 7 8 10 12 14 16 18 Input MHz

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

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

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

  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 Digital Output Analogue Input

  37. Performance Performance • Real-world ADC Analogue Input Digital Output

  38. Performance 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……

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