Review of Important AM Receiver Concepts

1. Receiver Sensitivity: PSB,MIN(dBm) - NEFF(dBm) > SNRMIN(dB)

PSENS(dBm) = PSB,MIN(dBm) + 10log10(2/m2 + 1)

NEFF(dBm) = kT0B(dBm) + NF(dB) + 3 dB (if no image rejection after first amp)

PSENS(dBm) = kT0B(dBm) + NF(dB) + SNRMIN(dB) + 10log10(2/m2 + 1) + 3 dB (?)

• Required detector power for undistorted demodulation:

(1-m)EC,DET > VD

2

, 10 2

2

210log

1

2

DET MIN DR

DDR

DC

mP dBm P dBm

m

VP W

R

PSB,MIN

Review of Important AM Receiver Concepts

3. Total Gain Required (RF and IF)

GMIN(dB) = PDET,MIN(dBm) – PSENS(dBm)

4. Image Rejection - Preselectors

fIM = fCH + 2fIF (…depending upon high/low injection)

Worst Case – Highest channel frequency for Low Side InjectionLowest channel frequency for High Side

Injection

5. Adjacent Channel Rejection – IF Filters

Channel spacing includes guard bandCenter Frequency is IF FrequencyBandwith (B) extends to guard band edgesRequired rejection must be exhibited at edge of adjacent

channel guard band

Build a Radio!!1. Frequency Range 161 – 174 Mhz (Land Mobile Band)2. Channel Spacing: 30 kHz, Guard Band: 5 kHz3. IF filter: crystal filter with Qu = 20,000 (want 40 dB adjacent channel rejection)4. Pre-selectors: LC filters with Qu = 180 (want 40 dB image rejection)5. 0.1 dB ripple permitted in filter passbands6. Detector: VD = 0.2 v, RDC = 500 ohms7. Mixer: Conversion loss = 6 dB8. Want sensitivity < 1.5 uV for ma = 0.7 (SNR = 12 dB)

Design Tradeoff: The higher the IF frequency, the easier it is to get image rejection, but the harder it is to achieve adjacent channel rejection.

Tasks1.Determine Pre-selector characteristics2.Determine IF Frequency and filter characteristics3.Determine Required Gain

1. Determine PSENS, noise figure/loss requirements2. Determine PDET,MIN

IF/Preselector Considerations

Use two identical preselectors.

For low-side injection, the worst case image frequency will be 174 MHz - 2fIF. This should be well below 161 MHz.

Preselectors should exhibit no more than 1 dB attenuation at 161 and 174 Mhz, therefore, each preselector should have 21 dB attenuation at 174 MHz - 2fIF.

Approximate center frequency is 167.5 Mhz. A two pole, 0.1 dB ripple exhibits 1 dB attenuation at SF = 0.79, and 21 dB attenuation at SF = 3.25

1 33

21 Im Im

Im Im

22Im Im

Im

13 MHz 13 167.50.79 16.5 MHz 10.15

0.79 16.5

3.25 167.5 MHz0.32

10.15 167.5 MHz

0.32 167.5 MHz 167.5 MHz 0

142.83 MHz 174 MHz 2

dB dB LdB

dB c

L c

IF I

SF BW QBW

SF f f f

Q f f f

f f

f f f

15.6 MHzF

So, choose fIF = 16 Mhz. IF filter BW3 dB = 20 kHZ, QL = 800. Rejection BW40 dB = 40 kHz, therefore, the IF filter should exhibit 40 dB attenuation at SF = 2, requiring 5 poles, 0.1 dB ripple.

Filter Insertion Losses

Preselector:10.15

.056 0.5 dB/pole 1 dB per filter180

L

u

Q

Q

IF Filter: 800.04 0.3 dB/pole 1.5 dB total

20000L

u

Q

Q

20 10

26

23 4 110 10

10 2

( ) ( ) ( ) 10log 2 1

1.5 x 1010log 10log 1.38 x 10 J/K 290 K 2 x 10 sec

50

2 ( ) 12 10log 1

0.7

133.5 161 ( ) 12 7

SENSP dBW kT B dBw NF dB SNR dB m

NF dB dB

dBw dBw NF dB dB d

( ) 8.5B NF dB dB

Receiver Sensitivity

Required Gain

2 22 2

2 2

0.2 1 0.7 21 2(min) 0.08 mw 13.83 1.1 mw 30.4

5001 1 0.7D

DETL

V mP dBw

R m

, ,( ) 30.4 135.5 105.1P Min Det Min SensA dB P dBW P dBW dBw dBw dB