Review of Important AM Receiver Concepts 1 Receiver

Review of Important AM Receiver Concepts 1. Receiver Sensitivity: PSB, MIN(d. Bm) - NEFF(d. Bm) > SNRMIN(d. B) PSENS(d. Bm) = PSB, MIN(d. Bm) + 10 log 10(2/m 2 + 1) NEFF(d. Bm) = k. T 0 B(d. Bm) + NF(d. B) + 3 d. B (if no image rejection after first amp) PSENS(d. Bm) = k. T 0 B(d. Bm) + NF(d. B) + SNRMIN(d. B) + 10 log 10(2/m 2 + 1) + 3 d. B (? ) PSB, MIN • Required detector power for undistorted demodulation: (1 -m)EC, DET > VD

Review of Important AM Receiver Concepts 3. Total Gain Required (RF and IF) GMIN(d. B) = PDET, MIN(d. Bm) – PSENS(d. Bm) 4. Image Rejection - Preselectors f. IM = f. CH + 2 f. IF (…depending upon high/low injection) Worst Case – Highest channel frequency for Low Side Injection Lowest channel frequency for High Side Injection 5. Adjacent Channel Rejection – IF Filters Channel spacing includes guard band Center Frequency is IF Frequency Bandwith (B) extends to guard band edges Required rejection must be exhibited at edge of adjacent channel guard band

Build a Radio!! 1. 2. 3. 4. 5. 6. 7. 8. Frequency Range 161 – 174 Mhz (Land Mobile Band) Channel Spacing: 30 k. Hz, Guard Band: 5 k. Hz IF filter: crystal filter with Qu = 20, 000 (want 40 d. B adjacent channel rejection) Pre-selectors: LC filters with Qu = 180 (want 40 d. B image rejection) 0. 1 d. B ripple permitted in filter passbands Detector: VD = 0. 2 v, RDC = 500 ohms Mixer: Conversion loss = 6 d. B Want sensitivity < 1. 5 u. V for ma = 0. 7 (SNR = 12 d. B) 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. Tasks 1. Determine Pre-selector characteristics 2. Determine IF Frequency and filter characteristics 3. Determine Required Gain 1. Determine PSENS, noise figure/loss requirements 2. Determine PDET, MIN

IF/Preselector Considerations Use two identical preselectors. For low-side injection, the worst case image frequency will be 174 MHz - 2 f. IF. This should be well below 161 MHz. Preselectors should exhibit no more than 1 d. B attenuation at 161 and 174 Mhz, therefore, each preselector should have 21 d. B attenuation at 174 MHz - 2 f. IF. Approximate center frequency is 167. 5 Mhz. A two pole, 0. 1 d. B ripple exhibits 1 d. B attenuation at SF = 0. 79, and 21 d. B attenuation at SF = 3. 25 So, choose f. IF = 16 Mhz. IF filter BW 3 d. B = 20 k. HZ, QL = 800. Rejection BW 40 d. B = 40 k. Hz, therefore, the IF filter should exhibit 40 d. B attenuation at SF = 2, requiring 5 poles, 0. 1 d. B ripple.

Filter Insertion Losses Preselector: IF Filter: Receiver Sensitivity

Required Gain