What makes a DX receiver great?
Understanding receiver specs
John Eisenberg K6YP
Agenda
•
•
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•
•
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•
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Introduction
Receiver fundamentals
Sensitivity
Linearity
Dynamic Range and AGC Function
Selectivity
Stability
Conclusion
Introduction
• If you can’t hear him, you can’t work him!
• Hearing him depends on:
–
–
–
–
–
Is he on?
Is there decent propagation?
Do you have enough antenna?
How much QRM/QRN is present?
The performance of your receiver.
• Today’s talk will focus on receiver performance.
What are you up against?
• Weak signals (CW or SSB or a digital mode)
• Atmospheric and man made noise (QRN)
• Interfering signals (QRM)
– Strong signals adjacent to your frequency
– Strong signals far removed in frequency
• Fast or slow fading (QSB)
What are your weapons
• Key receiver performance factors
– Sensitivity (Weak signal reception)
– Selectivity (Bandwidth matched to signal, Ability to
reject adjacent QRM)
– Optimum detector for desired signal modulation type
– Linearity (Spurious free dynamic range)
– Blocking dynamic range (From strong adjacent signals)
– Stability (Keep the signal in the pass band)
What this talk will address
• Key Receiver Specifications
– What are they?
– Why each is important?
• How to read a QST product review.
• I will not address the pros and cons of
specific receiver architectures.
Receiver fundamentals
• What must a receiver do?
– Amplify a weak signal delivered to the receiver by
the antenna.
– Filter out undesired interfering signals and noise .
– Detect the desired signal, extract its intelligence
and present the content in a useful format.
Receiver fundamentals
• What must a receiver not do?
– Add additional excess noise to the received
signal (Degrade SNR)
– Generate additional spurious signals or mixer
images which corrupt the detection process
– Drift off the desired signal frequency
Simple super hetrodyne receiver
Antenna
PreSelect
Filter
AGC Line
AGC
System
Mixer
RF
Amp
Image Reject
Filter
IF Roofing
Filter
Local Oscillator
IF
Amp
IF Pre- Detect
Signal Filter
Detector
BB
Amp
dB’s and dBm’s
• Power ratio in dB = 10log(P2/P1)
• Gain in dB = 10log(Pout/Pin)
–
–
–
–
3 dB is a factor of 2, 6 dB is a factor of 4
10 dB is a factor of 10, 20 dB is a factor of 100
39 dB is a factor of 2x2x2x10x10x10 = 8000
39 dB is 3+3+3+10+10+10 dB
• 0 dBm is 1 milliwatt
– Thus +13 dBm is 20 mW, -9 dBm is 1/8 mW
Noise power
• Noise is distributed over frequency.
• Noise Power is measured “per unit bandwidth”
• Example: A noise signal has a uniform power
spectral density of -120 dBm/Hz.
1 Hz
Bandwidth
1 Hz
10 Hz
100 Hz
1 MHz
Total Noise Power
-120 dBm
-110 dBm
-100 dBm
-60 dBm
PSD
dBm/Hz
Uniform
Noise PSD
Freq
• Noise power increases by 10log(Bandwidth in Hz)
Receiver sensitivity
Signal/Noise ratio at RX Input
• Noise Figure =
Signal/Noise ratio at RX Output
Output S/N = 30 dB
Input S/N = 40 dB
Device with
NF = 10 dB
• Noise figure = Input S/N (dB) - Output S/N (dB)
Best possible receiver sensitivity
• The noise power from a resistor at 25°C (or a
matched antenna in signal free environment) is
kTB (Boltzmann’s Constant (k) x Temp (°K) x
Bandwidth (Hz).
• kTB = -174dBm/Hz This is the noise floor of a
noise free receiver at 27 °C .
-21
• kTB = 3.98 x 10 watts/Hz at 27°C or about
0.01 V in a 500 Hz bandwidth.
Minimum detectable signal
• Noise Floor = KTB + NF + 10log(BW in Hz)
• MDS = KTB + NF + 10log(BW in Hz) + 8 dB
• Maybe for OH2BH, MDS = Noise Floor + 5 dB
(The 8 dB factor is subjective !)
• Often other problems such as reciprocal mixing
further degrade MDS
MDS for CW and SSB signals
CW Filter
500 Hz BW (27dB)
CW MDS
-134 dBm
SSB Filter
3 KHz BW (35dB)
Minimum
Detectable
SSB signal
-126 dBm
Minimum
detectable
CW signal
-134 dBm
CW Noise Floor
-142 dBm
Noise Power PSD is
-174dBm/Hz +5 db NF
or -169 dBm/Hz
Noise Floor = -174 dBm/Hz + 5dB NF +10log(BW)
SSB MDS
-126 dBm
SSB Noise Floor
-134 dBm
Receiver
Zin = 50
NF= 10 dB
1 ‘S unit’ = 6 dB
S meter reading
The “standard”S meter
Signal level in V
S9 + 60 dB
S9 + 40 dB
S9 + 20 dB
S9 + 10 dB
S9
S8
S7
S5
S3
S2
S1
MDS (in a 3 KHz SSB BW)
MDS (in a 250 Hz CW BW)
50000
5000
500
158
50
25
12.5
3.13
0.78
0.39
0.20
0.195
0.056
Signal Level in dBm
-13
-33
-53
-63
-73
-79
-85
-97
-109
-115
-121
-121.2
-132.0
LO phase noise & reciprocal mixing
• Imagine that you are copying a weak signal and
all of a sudden a very strong clean carrier pops up
100 KHz from your frequency.
• Nothing happens. It is rejected by your receiver’s
battery of filters. Right????
• No! Your receivers sensitivity may be degraded
by reciprocal mixing with local oscillator (LO)
phase noise.
Im(VLO)
Amplitude
A + Nam(t)
VLO
LO Phase noise
Phase
 +  pn(t)
Re(VLO)
VLO = (A + Nam(t)) sin[LOt +  + pn(t)]
The phase noise term pn(t) usually dominates the AM noise Nam(t)
LO Spectrum with phase noise
10 kHz
Offset
dBc/Hz
Phase noise is often expressed in:
dBc/Hz at some carrier offset
1 Hz
FLO
Reciprocal Mixing Process
Strong
Interferer
Interferer
with LO
Phase noise
IF Filter Bandwidth
Weak
Signal
LO phase noise
on interferer
Receiver
RX RF input signals
RX IF Output
Buried Weak
Signal
Local Oscillator
with
Phase Noise
LO phase
noise on
weak signal
Reciprocal mixing
-20 dBm Interferer after 1st mixer
IF Filter Bandwidth
Desired Signal
100 KHz
RX IF Output
RX NF = 15 dB, Gain to 1st
IF filter after the mixer =10 dB
A -20 dBm strong interferer is
100 KHz from desired signal
LO phase noise = -110 dBc/Hz
at 100 KHz carrier offset
RX noise floor = KTB+NF+G = -174 dBm/Hz +15 +10 dB = -149 dBm/Hz
At 100 KHz away from the -20 dBm interferer phase noise PSD is
-110 dBc/Hz -20 dBm = -130 dBm/Hz
Adding noise powers in a 1 Hz bandwidth yields ~ -130 dBm/Hz. Thus the
Equivalent RX NF with phase noise = 15dB + (-130 +149)dBc/Hz = 34 dB!
Receiver total gain
• The lowest noise receiver still must have enough
gain to bring the input signal strength up to the
level the detector requires to process it.
• Both signals and noise are amplified.
• Hopefully the signal is well above the noise.
• A strong interferer can (and often does) reduce
total gain through saturation or AGC action
Receiver sensitivity summary
• Noise figure, predetection bandwidth and total gain
ideally set receiver sensitivity.
• Predetection bandwidth and the detection process
must be matched to the signal characteristics.
• Spurious signals and mixer images generated in the
receiver must be suppressed
• LO phase noise in the presence of strong interfering
signals can severely degrade receiver sensitivity and
usually sets MDS in real world DX situations.
Receiver linearity
• Why worry about linearity?
• Strong signals close to a weak DX signal can
saturate your receiver’s front end or its IF
amplifiers dramatically reducing total gain.
• Pairs (or multiple) strong interferers can place
unwanted intermodulation products on top of that
all time new one you are trying to pull in.
• These issues compound the previously addressed
reciprocal mixing problem.
Gain compression
Gain (dB)
Linear
region
Input Power
@ 1 dB Gain
Compression
Small Signal Gain
SSG - 1 dB
Nonlinear
region
Saturation
region
Receiver Input Signal Level (dBm)
Intermodulation
• When 2 or more signals are presented to a nonlinear
device, the results are harmonics of each signal and
sum and difference products of the signals and their
harmonics. These sum and difference products are
called intermodulation products.
Power
F1 F2
F1 F2
Nonlinear
Device
Even
dc
Odd
Even
Odd
2F1
2F2
Freq
Intermodulation
• Odd order products (IM3, IM5 ....) are close to the
original signals and can interfere with another weak
close in signal.
• Even order products (IM2, IM4 ….) can also cause
interference. Usually the receiver’s preselect filter
takes care of even order products. (Unless your
neighbors are W6YX and W6XX.)
Even order intermodulation
Interfering Signal Pair
F2
F1
7.10 MHz 7.14 MHz
W6YX
F2-F1= 2F2-2F1=
0.04 MHz 0.08 MHz
IM2
IM4
W6XX
IM product = mF1 ± nF2
Product order is m+n
(1F1+1F2), m=1 n=1, Order is 2nd
F1+F2=
14.240 MHz
IM2
Receiver IF Passband
A92BR
14.243 MHz
2F2+2F1=
28.48 MHz
IM4
Odd order intermodulation
Interfering Signal Pair
F1 F2
14.1 MHz 14.2 MHz
W6YX
W6XX
2F2-F1=
14.3 MHz
IM3
2F1-F2=
14.0 MHz
IM3
3F1-2F2= 13.9 MHz
IM5
F
F=F2-F1 = 14.2-14.1 =0.1 MHz
IM product = mF1 ± nF2
Product Order is m+n
(3F1-2F2), m=3 n=2, Order is 5th
Receiver IF Passband
DX0K 14.303 MHz
3F2-2F1 =14.4 MHz
IM5
F
F
F
F
Intercept point
IF Output Power
(dBm)
Intercept
Point
Fundamental
Signals
Linear
Region
Slope=1
IM3
Slope=3
IM5
Slope=5
RF Input Power (dBm)
Estimating IM level
Power (dBm)
Intercept
Point (dBm)
Order Order Order Order Order
3
3
5
5
7
+40 dBm
 (dB)
Signal
Level (dBm)
26
20
31
106
52
80
124
21 dB
-13 dBm
(P-1) (dB)
P th Order
IM Level
(dBm)
53
-119 dBm
Frequency (kHz)
126 dB
Is your IP3 good enough?
• Its close in IMD performance that matters.
• A great input intercept point without equally great
roofing and predetection filters is worthless!
• IIP3 at 5 kHz spacing not 20 kHz counts in a pileup
756 ProIII (20M/500Hz/No Preamp) -17/+25dBm
IC7800 (20M/500Hz/No Preamp) +22/+37dBm
Source: Mar. 2005 QST Product Review 756ProIII
• Don’t forget that -30 dBc IM products from a “20
over 9” perfectly clean SSB signal are > S8! So the
problem isn’t always your receiver.
Power
(dBm)
Spurious free dynamic range
Intercept
Point (dBm)
+40 dBm
 =54.66 dB, 3rd Order (P=3)
Signal
Level (dBm)
SFDR
P th Order
IM Level
(dBm)
-14.66 dBm
SFDR = (-124 dBm) - (-14.66 dBm)
=109.33 dB (Noise floor = IM3 level)
(P-1)=109.33 dB, 3rd Order (P=3)
-124 dBm
Noise Floor =-174+15+10log 3000= -124 dBm
Frequency (kHz)
Receiver gain distribution
• Minimize RF gain ahead of the mixer to just
enough to achieve required noise figure. Don’t
overdrive the mixers thus degrading the receiver’s
spurious free dynamic range. Use high IIP3 mixers.
• LO phase noise level not NF usually sets real world
receiver sensitivity.
• Two conversions max! Minimize number of spurs.
• Locate the majority of gain after the roofing filter.
Keep IM products out of the IF and detectors.
AGC function
• AGC reduces the gain of the receiver RF
and IF amplifiers in the proper ratio to
maintain sensitivity and SFDR in the face of
rapidly changing signal levels (QSB).
• The analog or DSP detector suite (one for
each mode) drives the AGC function. The
AGC algorithm should be optimized for
each mode.
AGC function
• AGC rate must adapt to the mode in use and
if possible to the QSB conditions.
– Fast attack to minimize pops and thumps
– Adaptive decay matching signal characteristics
• AGC holds the detector input level
approximately constant as receiver input
signal level varies.
• Modern DSP based AGC systems can offer
vastly improved capability.
Receiver selectivity
• Selectivity is determined by the final IF filter
– The filter must be matched to the signal characteristics.
– Crystal filters are good but they are expensive and can
suffer from ringing and delay distortion.
– DSP based filters are generated in code and can be
designed for a wide variety of bandwidths, and shape
factors. Thus additional filters are almost free.
– Best of all DSP filters can greatly reduce ringing.
Receiver selectivity
• An excellent receiver has at least 2 crystal
roofing filters wide enough to avoid ringing,
but narrow enough to reject close in
interferers and IM products. For example:
6 - 10 kHz for SSB, 2 - 3 kHz for CW
• These would be followed by a choice of
DSP filters optimum for various conditions.
For example: 3.2, 2.8, 2.4 and 1.8 kHz for
SSB, 500 and 250 Hz for CW
Receiver selectivity
• The set of DSP filters should allow for
various operating conditions such as local
rag chewing and intense contest or DX
situations.
• DSP based filter suites should contain an
adaptive notch filter to reduce CW beat
notes in the IF pass band (Tuner uppers)
• A variable IF band pass filter with
selectable center frequency and bandwidth
can also be very useful.
Blocking dynamic range
• How large can a single CW interferer 20 KHz
away from a weak signal be, before the
desired signal’s detected level drops 1 dB?
• Blocking dynamic range is the difference in
level between the weak and strong signals
• What happens as the interferer moves closer
to the desired signal? How about many close
in intereferers as in a pileup.
Signal -100 dBm
Interferer -29 dBm
IF Po-1 +20 dBm
Gain to
pre-det’n
filter o/p
(100dB)
Blocking
Dynamic
Range =
71 dB
(3 kHz)
Gain to
roofing
filter o/p
(50dB)
Signal
-100dBm
Blocking dynamic range
BDR
Fo-6
kHz
Fo-3
kHz Fo
Offset Total Signal
from Fo Gain Level
(kHz)
(dB) (dBm)
100
?
0
99
?
1
82
Interferer
?
2
50
-29dBm
-1
3
49
-0.7
4
47
-0.3
5
-47
0
7
-59
0
9
-71
0
11
-83
0
13
Fo+3 Fo+6
-95
0
15
kHz kHz
Interferer
Level
(dBm)
AGC
AGC
AGC
+20
+19.3
+17.7
-76
-88
-100
-112
Noise
Receiver frequency stability
• All modern radios employ synthesized LOs.
– Make sure tuning resolution meets your needs
– Verify that the synthesizer reference source is
stable enough for the digital modes
– A 10 ppm TCXO is often a good option to
invest in.
– A 10 MHz reference output is also a useful
feature
• Most important .... How’s the phase noise?
DX superhetrodyne receiver
Two complete receivers with Split/Dual Watch capability
Simple, maximum of 2 conversions
Engineered to minimize IF spurious
Just enough gain ahead of 1st mixer to set noise floor
Take advantage of near perfect DSP linearity
Very high input intercept point
High performance pre-selctor
Multiple high performance matched roofing filters
Stable, low phase noise DDS/DSP LO
Antenna
PreSelect
Filter
Fast IF DSP (MHz), High resolution A/D & D/A
Optimized AGC algorithms for each mode
Several filter choices for each mode
Effective auto notch and dual passband tuning
Adaptive Noise reduction and noise blanker
Separate optimum detectors for each mode
Intuitive, ergonomic user interface, RTTY built in
Straight forward computer interface
AGC Line
AGC
System
Mixer
RF
Amp
Analog
Image Reject
Filter
IF Roofing
Filter
Local Oscillator
IF
Amp
IF Matched
Signal Filter
DSP
Detector
BB
Amp
Conclusion
• My Priorities
1 Close in (5 kHz) phase noise Phase noise usually
sets receiver sensitivity, not noise figure. If you can’t
hear him in the pileup, you can’t work him!
2 Close in (5 kHz) input intercept. You still can’t hear
him if he is wiped out by IM3 from strong stations.
3 Close in (5 kHz) blocking dynamic range. Analysis
has convinced me that long before BDR becomes an
issue, reciprocal mixing has buried the new one I am
trying to hear.
Conclusion
• Rigs with great dual receivers, terrific specs and
good bang for the buck are very important but .......
• Don’t neglect front panel ergonomics, an intuitive
user interface and well thought out menus and
control functions.
• You will most likely using this radio for many
years. Get the rig that is right for you!
• Thanks for coming. See you in the pileups!