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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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Fully Digital HF Radios

Phil Harman VK6APH

Dayton Hamvention – 17th 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.

  • 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?


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


SDR


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


Performance

  • Image rejection limited by analogue components

    RejectionPhase(deg) Amplitude(dB)

    40dB 1.00.1

    60dB 0.10.01

    80dB 0.010.001

    100dB 0.0010.0001

  • This accuracy must hold over each ham band and 300Hz-3kHz, with temperature, component aging, vibration, voltage fluctuations etc


Performance

  • We can compensate digitally for consistent phase and amplitude errors

  • Automatically and manually


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


I & Q Error Correction

‘Rocky’ software (Alex, VE3NEA) ‘learns’ how to correct I and Q using off-air signals

Switch on

After one day


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!


Fully Digital Approach

Data

Digital

Signal

Processor

A

D

Audio

D

A


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


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


Fully Digital Approach

  • DSP interface

Data

Digital

Signal

Processor

A

D

Audio

D

A

Data Rate


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


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


Fully Digital Approach

  • Digital to Analogue Conversion (DAC)

  • For Audio output need 16 bits at 8ksps

    = 128ksps

  • Modern sound cards/chips do > 4Mbps


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!


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.


Fully Digital Approach

  • Decimation

Decimator

(divide by n)

16 bit samples

@ 63/n Msps

A

D

16 bit samples

@ 63Msps


Fully Digital Approach

ADC Output


Fully Digital Approach

ADC Output – Decimate by 3


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?


Digital Down Conversion


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


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


HPSDR Mercury DDC Receiver

By decimating from 60MHz to 3kHz we improve

the SNR from 78dB to 121dB


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


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


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


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


Performance

  • Graph of IP3 for Analogue Receiver

3rd order intercept point

Saturation

Output dB

Fundamental

(Slope = 1)

3rd Order IMD

(Slope = 3)

Input dB


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


Performance

  • Graph of IP3 point verses input level

Analogue Receiver

Saturation

IP3 dB

Digital Receiver

Input dB


Performance

  • What causes IMD to vary with input level?

    • Fewer bits are used at low input levels

    • Non ideal ADC performance


Performance

  • Ideal ADC

Digital Output

Analogue Input


Performance

Performance

  • Real-world ADC

Analogue Input

Digital Output


Performance

Performance

  • Real-world ADC

Analogue Input

Digital Output


Performance


Performance


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.


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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