radio telescope receivers

Question 1

by a total power telescope, we mean

Answer 1

something that turns the received radio power from a source into a proportional voltage that can be easily measured and recorded

Question 2

components - antenna

Answer 2

turns the EM wave into a proportional voltage

this noise voltage has an approximately white spectrum with a power per Hz of KbTa

Question 3

components - Low-noise preamplifier

Answer 3

boosts voltage by around a factor of 1000 so the microvolt signal from the antenna is now millivolts and strong enough not to be degraded or lost in further processing

Question 4

components - filter

Answer 4

restricts the range in frequencies, defining the bandwidth

also helps to cut out interfering signals

Question 5

components - mixer

Answer 5

shifts the band to a lower frequency by mixing (multiplying)the signal with a sinusoidal local oscillator

the signal can be shifted to baseband, around 0Hz

Question 6

components - square law detector

Answer 6

we are interested in the power of the signal, not the wave voltage

power prop to v^2 so need to sqaure v

a suitable diode can do this or it can be done digitally

Question 7

components - integrator (low-pass filter)

Answer 7

this averages the fluctuating ouput of the detector to determine its mean level accurately and hence improve the SNR

this can be done in analog electronics, with a low-pass filter or digitally

Question 8

in practice what else is involved in total power radio telescope?

Answer 8

several filtering and mixing stages

the LNA is often cooled to reduce noise

Question 9

drift scan

Answer 9

letting a source drift in and out of the beam of a fixed antenna

Question 10

in radio astronomy, bandwidths are usually

Answer 10

narrow
delta v /v around 0.01

Question 11

the output of the filter is

Answer 11

band-limited nosie

Question 12

all the adjacent frequency components in the band beat together to give

Answer 12

a quasi-sinusoidal underlying waveform

an envelope fluctuating randomly

Question 13

if we mix to baseband, the output of the detector is

Answer 13

the fluctuating envelope squared

Question 14

self noise

Answer 14

uncertainty in the mean level due to the naturally fluctuating nature of the signal

Question 15

what is the solution to self-noise

Answer 15

average the signal over time with the integrator

Question 16

what is needed for averaging the signal over time with the integrator

Answer 16

samples must be statistically independent, so there is no advantage in a sampling period shorter than 1/ delta v

Question 17

if the observation lasts for t, the number of independent measurements is

Answer 17

N= t/coherence time
=approx delta v t

Question 18

the uncertainty in a measurement drops as

Answer 18

sqrt N

Question 19

SNR=sqrt N = sqrt (delta v t) is only correct if

Answer 19

there is no other source of noise

Question 20

in a perfect system, with only source noise present, the mean output is proportional to

Answer 20

the antenna temp from the source alone

Question 21

in practical systems, additional factors contribute to the noise. EG

Answer 21

the LNA adds extra noise, making antenna temp appear as T_lna when no source is present

Question 22

T_A from the antenna may itself contain many components

Answer 22

Ta= Tsource + Tbackground + Tatmosphere +Tground

Question 23

we can distinguish Tsource form the rest by

Answer 23

chopping (beam-switching) on and off the source

Question 24

if nothing else changes while switching then

Answer 24

Tsource+Ton - Toff

Question 25

Tsystem

Answer 25

the effective antenna temp of the telescope as if all the noise was coming in via the antenna
(everything added together)

Question 26

at the higher observing frequencies, what is the relationship between Tsys and TA

Answer 26

Tsys»TA ie almost all noise present is from the electronics

Question 27

for nearly all sources at all frequencies, what is the relationship between Tsource and Tsys

Answer 27

Tsource &laquo_space;Tsys

Question 28

what is Smin (after taking the limiting sensitivity at SNR=1)

Answer 28

the minimum detectable flux density detectable in time t

(radiometer equation)