Transcript Receivers - Performance Parameters

Chelmsford Amateur Radio Society
Advanced Course
(4) Receivers
Part-1 – Receiver Parameters
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
1
Receiver Parameters
Important performance measures for receivers
• Frequency stability
• Selectivity
• Bandwidth
• Sensitivity
• Dynamic range
• Signal-to-noise ratio (SNR)
• Effect of RF amplifiers & pre-amps
• Transverters
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
2
Frequency Stability
•
Frequency stability is the same as for transmitters
– Accuracy of tuning to an entered or displayed frequency
– Ability to remain on frequency without drifting off
– Often given in ppm – parts-per-million
– 1ppm error at 28MHz is 28Hz.
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
3
Selectivity
•
Selectivity is the ability to separate the wanted signal from nearby
unwanted signals (other stations)
Amplitude
Unwanted signal
(strong)
Wanted signal
(weak)
Frequency
Chelmsford Amateur Radio Society
Advanced Licence Course
Filter response
MHz
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
4
Selectivity Measures
•
Measures of selectivity
– 60-dB bandwidth
– adjacent channel rejection ratio (VHF, UHF channelised)
but how far away is the next channel (12.5kHz?)
– For SSB, may specify opposite sideband rejection
0dB Amplitude
Filter response
60dB bandwidth
-60dB Frequency
Chelmsford Amateur Radio Society
Advanced Licence Course
MHz
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
5
Bandwidth
•
Band of frequencies which the receiver should accept
– Eg. CW (A1A) morse typically 300Hz
– SSB uses 2.5 to 3kHz
– VHF FM typically 7.5 or 15kHz
– Usually 3dB BW specified, but not always!
Filter response
0dB -3dB Amplitude
3dB bandwidth
Frequency
Chelmsford Amateur Radio Society
Advanced Licence Course
MHz
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
6
Sensitivity
•
Sensitivity defines the limit of detection of weak signals.
•
Receivers must have enough gain to bring weakest signals to
comfortable level.
the gain does not define sensitivity
•
Sensitivity is determined by 2 factors:
– Bandwidth of the receiver
the wider the bandwidth, the more noise power it lets in
– Noise figure of the receiver front-end
a noisy receiver needs more signal to overcome the noise
•
Receivers bandwidth should match the transmitted bandwidth.
so as not to exclude any signal or accept unnecessary noise.
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
7
Sensitivity Definitions
•
Sensitivity is defined as the receiver input signal level for a given
SINAD at the output
– eg. 0.2μV for 12dB SINAD
– SNR is Signal-to-noise ratio
– SINAD is Signal + Noise + Distortion
– Intelligible speech needs about 12dB SINAD
SNR Signal Power
Noise Power
Usually expressed in dB
SINAD  Signal  Noise  Distortion
Noise  Distortion
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
8
Dynamic Range
•
Dynamic range is the range of signal levels between the smallest
and greatest a receiver can handle
– Lower limit set by sensitivity
– Upper limit set by distortion or AGC control range
•
In practice, we are more concerned about dynamic range to
handle unwanted out-of-band signals (AGC doesn’t apply).
– How large an unwanted signal will it reject without affecting
sensitivity to wanted signals
– Overload level may be specified for receiver front-end (RF amplifier,
mixer) as the 1dB compression point
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
9
Dynamic Range
•
1dB compression point
– Power level where amplifier gain drops by 1dB
1dB
Output level
Output compression
point
Amplifier linearity curve
Noise floor
Input level
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Input compression
point
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
10
Noise Figures
Expressed in dB
NF SNR at input
SNR at output
•
•
•
•
There is a limit of physics to receiver sensitivity
– Even for perfect receivers that add no noise (0dB NF)
– Real receivers can get within a few dB of the limit
Typical noise figures
– HF receiver; 12 to 20dB - not as critical as atmospherics dominate
– VHF receiver; 6 dB
– Microwave receiver; 2dB
Raw sensitivity is traded for dynamic range and selectivity in
environments where these are more important
Adjusting RF gain can optimise sensitivity vs. dynamic range
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
11
RF Amplifiers
•
If the RF pre-amp has a similar noise figure to the receiver
– Sensitivity not improved, dynamic range made worse
•
If the RF pre-amp has a better noise figure to the receiver
– Sensitivity improved, dynamic range still worse
•
If there is a feeder loss before the receiver (masthead amp)
– Without preamp, receiver performance degraded by loss
– Amplifier can overcome feeder loss, performance improved
RF pre-amplifier
Chelmsford Amateur Radio Society
Advanced Licence Course
Receiver
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
12
Downconverters
•
Downconverters convert signals from one band to another.
•
This enables reception of signals out of the tuning range of the
receiver.
•
Use of downconverters common for VLF, microwave, and weather
satellite.
•
Operator must mentally add the frequency offset to the frequency
displayed by the receiver.
Downconverter
Chelmsford Amateur Radio Society
Advanced Licence Course
Receiver
Anthony Martin M1FDE
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
13
Transverters
•
Transverters are bi-directional frequency converters
•
May be used for multimode (SSB J3E , CW A1A) operation by
using a HF transceiver on VHF or UHF
•
Displayed frequency must be mentally adjusted
•
Reliable high isolation switching is vital
Power
amp
Upconverter
Downconverter
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Transceiver
Slide Set 9: v1.0, 24-Aug-2004
(4) Receivers-1 - Parameters
14