Chelmsford Amateur Radio Society Advanced Course 4 Receivers Part1 Receiver Parameters - PowerPoint PPT Presentation

1 / 14
About This Presentation
Title:

Chelmsford Amateur Radio Society Advanced Course 4 Receivers Part1 Receiver Parameters

Description:

Chelmsford Amateur Radio Society. Advanced Licence Course. Anthony Martin M1FDE ... VHF FM typically 7.5 or 15kHz. Usually 3dB BW specified, but not always! ... – PowerPoint PPT presentation

Number of Views:133
Avg rating:3.0/5.0
Slides: 15
Provided by: antho94
Category:

less

Transcript and Presenter's Notes

Title: Chelmsford Amateur Radio Society Advanced Course 4 Receivers Part1 Receiver Parameters


1
Chelmsford Amateur Radio Society Advanced
Course(4) ReceiversPart-1 Receiver Parameters
2
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

3
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.

4
Selectivity
  • Selectivity is the ability to separate the wanted
    signal from nearby unwanted signals (other
    stations)

5
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

6
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!

7
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.

8
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

9
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
    doesnt 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

10
Dynamic Range
  • 1dB compression point
  • Power level where amplifier gain drops by 1dB

11
Noise Figures
  • 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

12
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

13
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.

14
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
Write a Comment
User Comments (0)
About PowerShow.com