Transcript HF Receiver Principles

HF Receiver Principles
Noise and Dynamic Range
Jamie Hall WB4YDL
Reelfoot Amateur Radio Club
April 26, 2012
Receiver Sensitivity Limitations
Define
Ant Noise Factor
Noise Temp
F(x) of graph
Frequency
Ant Noise Factor
Noise Temp.
Overview
Linear except
Daytime
atmosphere
Understanding Logarithms
• A logarithm is simply an exponent.
• Example: given a base of 2 and an exponent of 3
we have 2³ = 8.
• Inversely: given a base of 2 and power 8, (2ⁿ = 8),
then what is the exponent that will produce 8 ?
• That exponent is called the logarithm.
• We say the exponent 3 is the logarithm of 8
with base 2.
• 3 is the exponent to which 2 must be raised to
produce 8.
Understanding Logarithms (con’t)
• Common logarithms use base 10 and are used
in fields such as engineering, physics, chemistry,
and economics.
• Since 10³ = 1000, then log 1000 = 3.
• 3 is the exponent to which 10 must be raised
to produce 1000.
• Using logarithms greatly simplifies talking
about differences in very large and very small
numbers.
• Important logarithm rule:
log (x/y) = log x – log y
How to find logarithms
Calculator – use base 10, not ln (natural)
 Table of logarithms
 Slide rule. What’s that?
 Basics of determining the Characteristic
and mantissa

Using Decibels in Ham Radio
Using dB allows us to talk about very large differences in power or voltage
levels with numbers that are easy to comprehend.
Example: The maximum power output of a transmitter in the USA is 1500
watts and the noise floor of a modern receiver is 0.04 microvolts.
The received power at this voltage level into 50 ohms (E²/R) is
0.000000000000000032 watts or 3.2 x 10-17. This is not easy to deal with !
We refer to power levels as dB above or below 1 miliwatt in a 50 ohm system
and call the result dBm. Thus 1 milliwatt is 0 dBm.
1500 watts in dBm = 10 log (1500 / .001) = +62 dBm
3 x 10-17 watts in dBm = 10 log (3 x 10-17 / .001) = -135 dBm
This is much easier to comprehend and deal with.
Question: How many dBm does a 100 watt transmitter produce ?
Dynamic Range
 In general, DR is the ratio (or difference in dB) between the weakest signal
a system can handle and the strongest signal that same system can handle
simultaneously.
 Example: The normal human ear can detect a 1 kHz sound wave at a level
of 10-12 watts/m² while the upper limit is about 1 watt/m², where pain is felt.
The dynamic range of our ears is thus about 120 dB.
 Example: Our eyes can detect the light from a star in a dark sky when
about 10 photons per second reach the retina, which is about 10-13 watts/m².
The Sun with its 300 watts/m², does not damage our eyes unless we look
straight into it.
 The baseball outfielder who drops a fly ball certainly knows about the
blocking effects of the Sun. He employs an attenuator (sunglasses) to reduce
the interference, but this may in fact put the desired signal (the baseball)
below his noise floor. In radio, the attenuator would be a front-end AGC.
Blocking Dynamic Range
BDR is the difference in dB between minimum discernible signal (MDS)
and an off-channel signal that causes 1 dB of compression in the receiver.
Sensitivity and Blocking
• Blocking happens when a large off channel signal causes the frontend RF amplifier to be driven to its compression point.
• As a result all other signals are lost (blocked).
• This condition is frequently called de-sensing—the sensitivity of
the receiver is reduced.
• Blocking is generally specified as the level of the unwanted signal
at a given offset.
• Original testing used a wide offset—typically 20 kHz.
More
recently, recognizing our crowded band conditions and the narrow
spacing of CW and other digital modes, most testing today is done
with close spacing of 2 kHz.
Wide & Close Dynamic Range
20 kHz Spacing
IMD 20 kHz Away
15 kHz Wide
First IF Filter at 70.455 MHz
2 kHz Spacing
IMD 2 kHz Away
15 kHz Wide
First IF Filter at 70.455 MHz
Nonlinearity and Intermodulation
Distortion
• Nonlinearity in RF and IF circuits leads to two
undesirable outcomes: harmonics and intermodulation
distortion.
• Harmonics in and of themselves are not particularly
troublesome.
• For example, if we are listening to a QSO on
7.230 MHz, the second harmonic, 14.460 MHz is well
outside the RF passband.
• However, when the harmonics mix with each other and
other signals in the circuit, undesirable and troublesome
intermodulation products can occur.
Intermodulation Distortion (IMD)
RCV
INPUT
FILTER
Intermodulation Distortion
Products: An Example
(1)
Fifth-Order
3f1-2f2
7.218
(2)
Third-Order
2f1-f2
7.221
(3)
Signal One
f1
7.224
(4)
Signal Two
f2
7.227
(5)
Third-Order
2f2-f1
7.230
(6)
Fifth-Order
3f2-2f1
7.233
Intermodulation Distortion
Products: An Example
(1)
Fifth-Order
3f1-2f2
7.218
(2)
Third-Order
2f1-f2
7.221
(3)
Signal One
f1
7.224
(4)
Signal Two
f2
7.227
(5)
Third-Order
2f2-f1
7.230
(6)
Fifth-Order
3f2-2f1
7.233
Intermodulation Distortion
Products: An Example
(1)
Fifth-Order
3f1-2f2
7.218
(2)
Third-Order
2f1-f2
7.221
(3)
Signal One
f1
7.224
(4)
Signal Two
f2
7.227
(5)
Third-Order
2f2-f1
7.230
(6)
Fifth-Order
3f2-2f1
7.233
Intermodulation Distortion Products:
An Example
120
f1
f2
100
dB
80
60
40
2f1- f2
2f2- f1
3f1- 2f2
3f2- 2f1
20
0
7.216
7.218
7.220
7.222
7.224
7.226
mHz
7.228
7.230
7.232
7.234
Example: Transmitted IMD
Third Order
Intermodulation Products
• The 3rd order products will be the largest
(loudest) of the intermodulation products.
• As a general rule, the 3rd order products will
increase (grow) 3-times faster than the
fundamental signal (the signal of interest).
ARRL Receiver Test:
Measured Response of the Signal of Interest
ARRL Receiver Test:
Extrapolated Linear Region of the
Measured Response of the Signal of Interest
ARRL Receiver Test:
Measured Response of the IMD Product
ARRL Receiver Test:
Extrapolated Linear Region of the
Measured Response of the IMD Product
The 3rd order intercept point (IP3):
A Measure of Merit
Our graph illustrates that the
3rd order intercept point is
defined by the intersection of
two hypothetical lines. Each
line is an extension of a linear
gain figure: first of the signal
of interest; and second, of
the 3rd order intermodulation
distortion product—from which
IP3 gets its name.
You will note that the larger the value of IP3, the less likely the receiver will be
adversely affected by 3rd order intermodulation products.
ARRL Receiver Test:
Extrapolated Linear Region of the
Measured Response of the IMD Product
IMD Dynamic
Range
Summary
Rx sensitivity limitations
 Antenna noise factor, frequency, noise
temp
 Logs & decibels to manage references
 Dynamic range, BDR, Sensitivity & blocking
 IMD, 3rd order IMD
 Examples of receiver tests : Sensitivity
[(S+N)/N] and MDS measurement and Smeter calibration
