What makes a DX receiver great Understanding receiver specs

1
What makes a DX receiver great?Understanding
receiver specs

• John Eisenberg K6YP

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Agenda

• Introduction
• Receiver fundamentals
• Sensitivity
• Linearity
• Dynamic Range and AGC Function
• Selectivity
• Stability
• Conclusion

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Introduction

• If you cant hear him, you cant 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.
• Todays talk will focus on receiver performance.

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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)

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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)

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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.

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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.

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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

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Simple super hetrodyne receiver
Antenna
Pre- Select Filter
AGC Line
AGC System
Mixer
IF Amp
BB Amp
RF Amp
Detector
Image Reject Filter
IF Roofing Filter
IF Pre- Detect Signal Filter
Local Oscillator
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dBs and dBms

• 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 333101010 dB
• 0 dBm is 1 milliwatt
• Thus 13 dBm is 20 mW, -9 dBm is 1/8 mW

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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.
• Noise power increases by 10log(Bandwidth in Hz)

1 Hz
Uniform Noise PSD
Bandwidth Total Noise Power 1 Hz
-120 dBm 10 Hz
-110 dBm 100 Hz
-100 dBm 1 MHz -60 dBm

PSD dBm/Hz
Freq
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Receiver sensitivity

• Noise Figure
• Noise figure Input S/N (dB) - Output S/N (dB)

Signal/Noise ratio at RX Input
Signal/Noise ratio at RX Output
Output S/N 30 dB
Input S/N 40 dB
Device with NF 10 dB
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Best possible receiver sensitivity

• The noise power from a resistor at 25C (or a
  matched antenna in signal free environment) is
  kTB (Boltzmanns Constant (k) x Temp (K) x
  Bandwidth (Hz).
• kTB -174dBm/Hz This is the noise floor of a
  noise free receiver at 27 C .
• kTB 3.98 x 10 watts/Hz at 27C or about
  0.01 ?V in a 500 Hz bandwidth.

-21
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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

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MDS for CW and SSB signals
SSB Filter 3 KHz BW (35dB)
CW Filter 500 Hz BW (27dB)
Minimum Detectable SSB signal -126 dBm
Minimum detectable CW signal -134 dBm
SSB MDS -126 dBm
CW MDS -134 dBm
SSB Noise Floor -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)
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The standardS meter
Receiver Zin 50? NF 10 dB 1 S unit 6 dB
S meter reading Signal level in ?V Signal
Level in dBm S9 60 dB 50000 -13 S9 40
dB 5000 -33 S9 20 dB 500 -53 S9 10
dB 158 -63 S9 50 -73 S8 25 -79 S7
12.5 -85 S5 3.13 -97 S3 0.78 -109 S2
0.39 -115 S1 0.20 -121 MDS (in a 3 KHz
SSB BW) 0.195 -121.2 MDS (in a 250 Hz CW
BW) 0.056 -132.0
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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
  receivers battery of filters. Right????
• No! Your receivers sensitivity may be degraded by
  reciprocal mixing with local oscillator (LO)
  phase noise.

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LO Phase noise
Im(VLO)
VLO
Amplitude A Nam(t)
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
FLO
1 Hz
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Reciprocal Mixing Process
Interferer with LO Phase noise
Strong Interferer
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

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Reciprocal mixing
-20 dBm Interferer after 1st mixer
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
IF Filter Bandwidth

Desired Signal
RX IF Output
100 KHz
RX noise floor KTBNFG -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!
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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

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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.

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Receiver linearity

• Why worry about linearity?
• Strong signals close to a weak DX signal can
  saturate your receivers 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.

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Gain compression
Gain (dB)
Linear region
Input Power _at_ 1 dB Gain Compression
Small Signal Gain
SSG - 1 dB
Nonlinear region
Saturation region
Receiver Input Signal Level (dBm)
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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
Even
Nonlinear Device
Odd
Odd
Even
2F1 2F2
Freq
dc
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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 receivers preselect
  filter takes care of even order products. (Unless
  your neighbors are W6YX and W6XX.)

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Even order intermodulation
Interfering Signal Pair
IM product mF1 nF2 Product order is mn
(1F11F2), m1 n1, Order is 2nd
F2
F1
7.10 MHz 7.14 MHz
W6YX
W6XX
F1F2 14.240 MHz IM2

Receiver IF Passband
F2-F1 0.04 MHz IM2
2F2-2F1 0.08 MHz IM4
2F22F1 28.48 MHz IM4
A92BR 14.243 MHz
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Odd order intermodulation
Interfering Signal Pair
?FF2-F1 14.2-14.1 0.1 MHz IM product mF1
nF2 Product Order is mn (3F1-2F2), m3 n2,
Order is 5th
F1
F2
14.1 MHz 14.2 MHz
W6YX
W6XX
2F2-F1 14.3 MHz IM3
2F1-F2 14.0 MHz IM3
Receiver IF Passband
DX0K 14.303 MHz
3F1-2F2 13.9 MHz IM5
3F2-2F1 14.4 MHz IM5
?F
?F
?F
?F
?F
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Intercept point
IF Output Power (dBm)
Intercept Point
Fundamental Signals
Linear Region Slope1
IM3 Slope3
IM5 Slope5
RF Input Power (dBm)
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Estimating IM level
Power (dBm)
Order Order Order Order Order 3
3 5 5 7
40 dBm
Intercept Point (dBm)
53 26 20 31 21 dB
? (dB)
-13 dBm
Signal Level (dBm)
106 52 80 124 126 dB
(P-1)? (dB)
th
P Order IM Level (dBm)
-119 dBm
Frequency (kHz)
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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
• Dont forget that -30 dBc IM products from a 20
  over 9 perfectly clean SSB signal are gt S8! So
  the problem isnt always your receiver.

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Spurious free dynamic range
Power (dBm)
40 dBm
Intercept Point (dBm)
? 54.66 dB, 3rd Order (P3)

-14.66 dBm
Signal Level (dBm)
SFDR (-124 dBm) - (-14.66 dBm) 109.33 dB
(Noise floor IM3 level)
SFDR
(P-1)?109.33 dB, 3rd Order (P3)
th
P Order IM Level (dBm)
-124 dBm
Noise Floor -1741510log 3000 -124 dBm
Frequency (kHz)
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Receiver gain distribution

• Minimize RF gain ahead of the mixer to just
  enough to achieve required noise figure. Dont
  overdrive the mixers thus degrading the
  receivers 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.

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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.

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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.

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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.

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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

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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.

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Blocking dynamic range

• How large can a single CW interferer 20 KHz away
  from a weak signal be, before the desired
  signals 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.

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Blocking dynamic range
Signal -100 dBm Interferer -29 dBm IF Po-1
20 dBm
Gain to pre-detn filter o/p (100dB)
Offset Total Signal Interferer from
Fo Gain Level Level (kHz)
(dB) (dBm) (dBm)
Blocking Dynamic Range 71 dB (3 kHz)
100 99 82 50 49 47 -47 -59 -71 -83 -95
? ? ? -1 -0.7 -0.3 0 0 0 0 0
0 1 2 3 4 5 7 9 11 13 15
AGC AGC AGC 20 19.3 17.7 -76 -88 -100 -112 Nois
e
Interferer -29dBm
Gain to roofing filter o/p (50dB)
BDR
Signal -100dBm
Fo-6 kHz
Fo3 kHz
Fo-3 kHz
Fo6 kHz
Fo
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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 .... Hows the phase noise?

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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
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
Antenna
Pre- Select Filter
AGC Line
AGC System
Mixer
IF Amp
BB Amp
RF Amp
Detector
Image Reject Filter
IF Roofing Filter
IF Matched Signal Filter
Analog
DSP
Local Oscillator
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Conclusion

• My Priorities
• Close in (5 kHz) phase noise Phase noise usually
  sets receiver sensitivity, not noise figure. If
  you cant hear him in the pileup, you cant work
  him!
• Close in (5 kHz) input intercept. You still cant
  hear him if he is wiped out by IM3 from strong
  stations.
• 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.

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Conclusion

• Rigs with great dual receivers, terrific specs
  and good bang for the buck are very important but
  .......
• Dont 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!