3. Background ... PlayStation 3. Processor Speed OK, limite

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
• Theres 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 gt90dB 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
Switch on
After one day

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

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
• Its time consuming
• This doesnt 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 gt twice max receiver frequency
• For 0 30MHz sample at gt60MHz
• Need gt120dB 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 _at_ 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 46MHz 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 gt 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 dont 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 _at_ 63/n Msps
A
D
16 bit samples _at_ 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 lt10.5MHz
• 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 dont 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