Transcript Document

Digital Techniques
for Radio
What is digital?
Digital normally means binary
Digital can mean:
Digital techniques for analogue modes
e.g. SSB AM FM (Overview in this talk)
Or it can mean:
Digital techniques for digital modes
e.g. PSK31, GSM, DAB
(a later talk)
When should one use a digital system?
This is a mixed audience with mixed views.
Amateur radio is a hobby so its up to you!
Some things people may not allow in their hobby:
• Equipment built in a factory
• Mobile phones
• Repeaters (May as well use a mobile phone?)
• Voice (only Morse)
• Transistors (only Valves)
• Valves not made oneself
• ICs (Only transistors)
• Digital (only Analogue)
• PCs (only logic ICs)
• Software (only non programmable Logic)
• Hardware description languages (only Software)
Some reasons to use a digital system
• When it is cheaper
• When you want versatility
• When analogue would be less practical
• When analogue would be impossible!
• When you want to be nearer to the leading edge
• When you have seen this talk 
What do we mean by doing things digitally?
•
•
•
•
We could solder together logic gates
We could use a PC
We could use a microcontroller / DSP
We could use an FPGA
Note that use of software is not necessarily required
(but it can make things a lot easier)
SoftRock Receiver
SoftRock Receiver
I
LO
Inphase and Quadrature
Generator
Q
Mixers
RF Filter
(Balanced Output omitted)
IF Filters
Stereo Output
BITX20 bidirectional SSB transceiver
BITX20 bidirectional SSB transceiver
IF Filter
RF Filter
Mixer
Mixer
Antenna
LO
BFO
Speaker
Receive direction shown
Filter
Compare the SoftRock Receiver
I
LO
Inphase and Quadrature
Generator
Q
Mixers
Rf Filter
(Balanced Output omitted)
IF Filters
Stereo Output
The missing digital parts we need:
Tuning (The SoftRock has a Fixed Xtal LO)
The BFO
The last mixer stage (for BFO)
The audio filter
The audio output
Block diagram of the missing digital parts
Inphase and Quadrature
Generator
I
Q
Mono Output
LO/BFO
L
Sample
& Hold
A -> D
Filter
+/-
R
Sample
& Hold
A -> D
Upper /
Mixers Lower SB
D -> A
How do we do these digitally?
Get our analogue signals into digital form?
Generate a tuneable LO/BFO sine wave?
Create In-phase and Quadrature signals?
Produce the mixer stage?
Filter signals?
Produce the audio output?
We have seen many of these before.
• Generate a tuneable LO/BFO sine wave?
We did this in our simulation spread sheet!
• Create In-phase and Quadrature signals?
Our simulated Current and Voltage were I/Q!
• Produce the mixer stage?
We did this in our ideal mixer talk (it’s a multiply)
• Filter signals?
We already simulated an analogue RC filter
A tuneable digital oscillator
In our simulation talk we simulated an RC oscillator in time
steps.
As we said we could have done this on a sheet of paper, one
row per time step.
Using a computer saves a lot of effort with a pencil but is
perhaps less clear.
We used a spreadsheet rather than a sheet of paper as a
compromise.
Our simulated tuned circuit
+25V
L
Switch
C
0V
R
Spreadsheet for a tuned circuit
(See TunedA.xls for details)
Time
Step
Ohms
Farads
0.05
microfarads
millifarads
Henries
LC Period
50000
50
5000
99.35
Inductor V
Volts Change
Current Change
1
100
Seconds
Capacitor V
0
25
0.0000
0.0
25.0
0.000
0.0050
1
25.0
0.0050
5.0
24.5
0.100
0.0049
2
24.9
0.0099
9.9
23.9
0.198
0.0048
3
24.7
0.0147
14.7
23.2
0.294
0.0046
4
24.4
0.0193
19.3
22.5
0.387
0.0045
5
24.0
0.0238
23.8
21.6
0.476
0.0043
6
23.5
0.0282
28.2
20.7
0.563
0.0041
Amps
milliamps
In-phase and Quadrature generator
based on the spreadsheet (R=0)
Set Frequency (Binary)
Multiplier
Adder
Latch
I
Multiplier
Step
(Clock)
- Subtractor +
Latch
Q
What are these building blocks?
A latch stores a binary number. It replaces it with a new
number when it gets a clock. (Our time step in the
spreadsheet)
As expected the adders and multipliers can add and multiply
binary numbers. We will look at this another day.
The circuit produces 2 sine waves (Just like the simulated
Current and Voltage in our spreadsheet)
These are our I and Q signals to feed to the digital mixers.
The ideal mixer
Previously we looked at the electronics of mixers.
An ideal mixer multiplies rather than adds waveforms.
If you feed two sine waves at frequencies F and G into
a multiplier you just get sine waves at frequencies
F+G and F-G and no harmonics.
Rather than prove this using maths we just looked at
the waveforms and listened.
The inputs to the ideal mixer
1
0.5
2000Hz
0.002
0.004
0.006
0.008
0.01
0.002
0.004
0.006
0.008
0.01
-0.5
-1
1
0.5
2200Hz
-0.5
-1
The output from the ideal mixer
1
200Hz
0.5
and
0.002
-0.5
4200Hz
-1
0.004
0.006
0.008
0.01
The components of the mixer output
1
0.5
200Hz
0.002
0.004
0.006
0.008
0.01
-0.5
-1
1
0.5
4200Hz
0.002
-0.5
-1
0.004
0.006
0.008
0.01
What else do we need?
To get our analogue signals into digital form and to produce
the audio output.
There are many types of Analogue to Digital (A/D) and
Digital to Analogue (D/A) converters. We will cover these
another day.
To filter signals one could simulate the equivalent analogue
filter (as mentioned earlier). However there are much better
ways. Such a simulation would be an approximation to what
is called an IIR (infinite impulse response) filter.
Our next example is an FIR filter.
The Finite Impulse Response (FIR) filter
(Diagram From an Altera FPGA application note)
The basics of an FIR filter
An FIR filter works in the “Time domain”
It represents the filter one wants in terms of its time response
‘h()’ to an “impulse” (A very short pulse)
The input waveform ‘x(n)’ (being digital) naturally can be
regarded as a series of such pulses of varying height.
The output waveform ‘y(n)’ is calculated by the FIR filter by
adding up the response to each input “pulse”
Fortunately there are standard design tools for FIR filters.
Conclusion?
We did not use any software. As stated its not necessary but can
make things a lot easier.
In a later talk we will look at how logic gates and larger
building blocks work. We will also look at CPU design.
We will look at software techniques and development tools
for PCs and microcontroller / DSP chips.
We can build complex digital things without software if we
use an FPGA. However we then need Hardware languages!
Some more real world examples follow
(These next examples are from product briefs on the Broadcom
public website).
MULTIFUNCTION MONOLITHIC IC: AGPS,
BLUETOOTH® 2.1 + EDR, AND INTEGRATED FM
TRANSCEIVER: BROADCOM BCM2075
BCM6368-based VDSL2/ADSL2+ Multimode Residential
Gateway
Questions?