Chelmsford Amateur Radio Society Advanced Course 4 Receivers

Chelmsford Amateur Radio Society Advanced Course (4) Receivers Part-1 – Receiver Parameters Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M 1 FDE Slide Set 9: v 1. 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 M 1 FDE Slide Set 9: v 1. 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 – 1 ppm error at 28 MHz is 28 Hz. Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M 1 FDE Slide Set 9: v 1. 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 M 1 FDE Slide Set 9: v 1. 0, 24 -Aug-2004 (4) Receivers-1 - Parameters 4

Selectivity Measures • Measures of selectivity – 60 -d. B bandwidth – adjacent channel rejection ratio (VHF, UHF channelised) but how far away is the next channel (12. 5 k. Hz? ) – For SSB, may specify opposite sideband rejection Amplitude 0 d. B - Filter response 60 d. B bandwidth -60 d. B Frequency Chelmsford Amateur Radio Society Advanced Licence Course MHz Anthony Martin M 1 FDE Slide Set 9: v 1. 0, 24 -Aug-2004 (4) Receivers-1 - Parameters 5

Bandwidth • Band of frequencies which the receiver should accept – Eg. CW (A 1 A) morse typically 300 Hz – SSB uses 2. 5 to 3 k. Hz – VHF FM typically 7. 5 or 15 k. Hz – Usually 3 d. B BW specified, but not always! Filter response 0 d. B -3 d. B Amplitude 3 d. B bandwidth Frequency Chelmsford Amateur Radio Society Advanced Licence Course MHz Anthony Martin M 1 FDE Slide Set 9: v 1. 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 M 1 FDE Slide Set 9: v 1. 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 12 d. B SINAD – SNR is Signal-to-noise ratio – SINAD is Signal + Noise + Distortion – Intelligible speech needs about 12 d. B SINAD Usually expressed in d. B Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M 1 FDE Slide Set 9: v 1. 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 1 d. B compression point Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M 1 FDE Slide Set 9: v 1. 0, 24 -Aug-2004 (4) Receivers-1 - Parameters 9

Dynamic Range • 1 d. B compression point – Power level where amplifier gain drops by 1 d. B Output level Output compression point Amplifier linearity curve Noise floor Input level Chelmsford Amateur Radio Society Advanced Licence Course Input compression point Anthony Martin M 1 FDE Slide Set 9: v 1. 0, 24 -Aug-2004 (4) Receivers-1 - Parameters 10

Noise Figures Expressed in d. B • • There is a limit of physics to receiver sensitivity – Even for perfect receivers that add no noise (0 d. B NF) – Real receivers can get within a few d. B of the limit Typical noise figures – HF receiver; 12 to 20 d. B - not as critical as atmospherics dominate – VHF receiver; 6 d. B – Microwave receiver; 2 d. B 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 M 1 FDE Slide Set 9: v 1. 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 M 1 FDE Slide Set 9: v 1. 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 M 1 FDE Slide Set 9: v 1. 0, 24 -Aug-2004 (4) Receivers-1 - Parameters 13

Transverters • Transverters are bi-directional frequency converters • May be used for multimode (SSB J 3 E , CW A 1 A) 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 M 1 FDE Transceiver Slide Set 9: v 1. 0, 24 -Aug-2004 (4) Receivers-1 - Parameters 14