1 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters Chelmsford Amateur Radio Society Advanced Course (4) Receivers Part-1 – Receiver Parameters

2 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters Selectivity Selectivity is the ability to separate the wanted signal from nearby unwanted signals (other stations) Frequency MHz Amplitude Wanted signal (weak) Unwanted signal (strong) Filter response

5 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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 Frequency MHz Amplitude 0dB - Filter response -60dB - 60dB bandwidth

6 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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! Frequency MHz Amplitude 0dB - Filter response -3dB - 3dB bandwidth

7 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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 Usually expressed in dB

9 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters Dynamic Range 1dB compression point –Power level where amplifier gain drops by 1dB Input level Output level 1dB Amplifier linearity curve Input compression point Output compression point Noise floor

11 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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 Expressed in dB

12 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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-amplifierReceiver

13 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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. Down- converter Receiver

14 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 9: v1.0, 24-Aug-2004 (4) Receivers-1 - Parameters 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 Down- converter Up- converter Trans- ceiver Power amp