Exercise 1: RF Stage, Mixer, and IF Filter

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1 SSB Reception Analog Communications Exercise 1: RF Stage, Mixer, and IF Filter EXERCISE OBJECTIVE DISCUSSION On the circuit board, you will set up the SSB transmitter to transmit a 1000 khz SSB signal to the SSB The transmitted SSB signal is sent by a direct connection between the transmitter and receiver on the circuit board. A 1 M 244 FACET by Lab-Volt

2 Analog Communications SSB Reception The mixer is a balanced modulator that converts the 1000 khz SSB to a 455 khz SSB IF signal. On your circuit board, you tune variable inductor L4 for the transmitted 1000 khz SSB signal. FACET by Lab-Volt 245

3 SSB Reception Analog Communications matching impedance between the inductor (L4) tap and ground for the receiving antenna. On your circuit board, there is no receiving antenna; however, the signal at the L4 tap represents a signal received by an antenna. a. b. It passes the desired SSB signal and matches the antenna impedance. c. It detects the message signal. 246 FACET by Lab-Volt

4 Analog Communications SSB Reception The emitters of Q2 and Q3 connect to the collector of Q4, which functions as a constant-current source. The Q3 collector connects to an RLC network, which contains a variable inductor (L5). FACET by Lab-Volt 247

5 SSB Reception Analog Communications The Q3 collector is coupled to the base of Q5, which functions as an emitter-follower buffer. The RF I a. high b. low The mixer joins the RF stage to the IF stage. This balanced mixer is a down-converter, because it reduces the SSB frequency from 1000 khz to 455 khz for the IF stage. The balanced mixer combines the 1000 khz SSB signal from the RF stage with a 1455 khz LO signal to produce a 455 khz difference signal to the IF stage. Are frequencies other than 455 khz also present in the mixer s output? a. yes b. no 248 FACET by Lab-Volt

6 Analog Communications SSB Reception range are greatly attenuated. B = khz (Recall Value 1) FACET by Lab-Volt 249

7 SSB Reception Analog Communications PROCEDURE The following procedure is divided into four sections. Connect the SSB Transmitter RF Filter: Tune in the SSB Signal Mixer and IF Filter: Produce a 455 khz SSB Each section starts with an explanation of the SSB signals that you will observe and the parameters that you will measure and calculate. Connect the SSB Transmitter In this PROCEDURE section, you will connect and adjust the SSB transmitter and use the transmitter s output signal as the receiver s input signal. If you must restart this section at a later time, setup the circuit by completing the steps in the following sections in the Resources of this manual. Make sure to complete these sections in the order in which they are listed: Adjust Transmitter Circuit for a 455 khz SSB to Mixer Adjust SSB Transmitter Output to 1000 khz SSB Circuit setup guides and the switch function guide are also located in the Resources. To generate a 1000 khz SSB signal from the TRANSMITTER to the receiver s RF STAGE, complete the steps in the following sections in the Resources of this manual. Make sure to complete these sections in the order in which they are listed: Adjust the Transmitter Circuit for a 455 khz SSB to Mixer Adjust SSB Transmitter Output to 1000 khz SSB With a two-post connector, connect the TRANSMITTER to the 1 M SSB RECEIVER circuit block. 250 FACET by Lab-Volt

8 Analog Communications SSB Reception RF Filter: Tune in the SSB Signal frequency of the transmitted signal. If you must restart this section at a later time, setup the circuit by completing the steps in the following sections in the Resources of this manual. Make sure to complete these sections in the order in which they are listed: Adjust Transmitter Circuit for a 455 khz SSB to Mixer Adjust SSB Transmitter Output to 1000 khz SSB Circuit setup guides and the switch function guide are also located in the Resources. The resonant frequency (f r ) occurs when the inductive and capacitive reactances are equal (X L = X C ). At f r FACET by Lab-Volt 251

9 SSB Reception Analog Communications yet). No connections are necessary in the RF FILTER and RF AMPLIFIER section of the receiver circuit, except for the two-post connector that connects the transmitter circuit. Refer to the RF FILTER and RF AMPLIFIER schematic below. Due to the delicate nature of the variable inductors, it is suggested that once a student makes a proper adjustment not to change this adjustment. Later when the board is reused, the new student need only tweak the variable inductor. This approach will extend the life of the variable components. 252 FACET by Lab-Volt

10 Analog Communications SSB Reception Connect the channel 1 oscilloscope probe to the RF AMPLIFIER output. Adjust inductor L5, which is in the RF AMPLIFIER collector circuit, to about the midpoint so that the 1000 khz SSB signal appears on channel 1. While observing the signal on channel 1, adjust inductor L4 for the maximum peak-to-peak signal at the RF AMPLIFIER output. Adjusting L4, which is the RF FILTER inductor, tunes in the 1000 khz transmitted frequency. a. The inductive and capacitive reactances are equal (X L = X C ). b. c. matching impedance between the inductor (L4) tap and ground. d. All of the above output power. FACET by Lab-Volt 253

11 SSB Reception Analog Communications If you must restart this section at a later time, setup the circuit by completing the steps in the following sections in the Resources of this manual. Make sure to complete these sections in the order in which they are listed: Adjust Transmitter Circuit for a 455 khz SSB to Mixer Adjust SSB Transmitter Output to 1000 khz SSB Circuit setup guides and the switch function guide are also located in the Resources. about 71 db (power gain of over 12,000,000). You will adjust inductor L5 so that the collector RLC network is tuned for 1000 khz to give maximum gain (do not adjust L5 yet). When the collector RLC network is tuned for 1000 khz, the inductive and capacitive reactances cancel, and the circuit is a purely resistive impedance load. 254 FACET by Lab-Volt

Analog Communications SSB Reception Power levels in and out of an RF STAGE and other components of an SSB receiver are usually in terms of decibels related to a reference

The use of dbm is convenient in dealing with a number of stages. The difference between the dbm level at the stage output and stage input is the power gain in db.

12 Analog Communications SSB Reception Power levels in and out of an RF STAGE and other components of an SSB receiver are usually in terms of decibels related to a reference power level (dbm). A commonly used reference is 1 milliwatt (mw). The dbm = 10 x [log 10 A dbm is an actual amount of power, whereas a db represents a ratio of power. The use of dbm is convenient in dealing with a number of stages. The difference between the dbm level at the stage output and stage input is the power gain in db. While observing the signal on channel 1, adjust variable inductor L5 in the RF AMPLIFIER collector circuit for the maximum peak-to-peak carrier signal at the RF AMPLIFIER output. FACET by Lab-Volt 255

13 SSB Reception Analog Communications On channel 1, measure the peak-to-peak voltage of the SSB signal at the RF AMPLIFIER output. V RF(o) = mv pk-pk (Recall Value 1) Calculate the SSB signal s rms power at the RF AMPLIFIER output. The RF AMPLIFIER output impedance is 2 k. V RF(o) = [(Step 2, Recall Value 1 rms P RF(o) = V RF 2 = W (Recall Value 2) Calculate the output power in decibels with reference to 1 mw (dbm). P RF(o) = W (Step 3, Recall Value 2) dbm RF(o) = 10 x [log 10 (P RF(o) dbm (Recall Value 3) 256 FACET by Lab-Volt

14 Analog Communications SSB Reception The input SSB signal power ( 85 dbm is a typical value for the circuit conditions) to the RF stage and the output power that you calculated are shown. From the input and output power in dbm, calculate the power gain of the RF stage in decibels (db). dbm RF(i) = 85 dbm dbm RF(o) = dbm (Step 4, Recall Value 3) Ap (RF) = dbm RF(o) dbm RF(i) = db (Recall Value 4) Mixer and IF Filter: Produce a 455 khz SSB If you must restart this section at a later time, setup the circuit by completing the steps in the following sections in the Resources of this manual. Make sure to complete these sections in the order in which they are listed: Adjust Transmitter Circuit for a 455 khz SSB to Mixer Adjust SSB Transmitter Output to 1000 khz SSB Circuit setup guides and the switch function guide are also located in the Resources. The mixer that performs a down-conversion is a balanced modulator. There are two inputs to the mixer. FACET by Lab-Volt 257

15 SSB Reception Analog Communications The local oscillator input (C) is the 1455 khz signal from the VCO-HI circuit block. The local oscillator frequency used for the down-conversion (1000 khz to 455 khz) must have the same 1455 khz frequency that was used for the up-conversion of the transmitter s SSB from 455 khz to 1000 khz. If they are not exactly the same frequencies (1455 khz), the message signal from the product detector will be distorted. frequency (455 khz) and suppresses the 1455 khz local oscillator signal. The mixer output signal contains the sum (2455 khz) and difference (455 khz) frequencies; the 1000 khz input frequency is very weak in the output. 258 FACET by Lab-Volt

Analog Communications SSB Reception bandwidth. Connect the output of the 1455 khz VCO-HI circuit block, which also connects to the transmitter s mixer, to the local oscillator input (C) of the mixer.

16 Analog Communications SSB Reception bandwidth. Connect the output of the 1455 khz VCO-HI circuit block, which also connects to the transmitter s mixer, to the local oscillator input (C) of the mixer. Connect the MIXER to the IF FILTER with a two-post connector. Connect the oscilloscope channel 1 probe to the MIXER s M input, and connect the channel 2 probe to the MIXER s output. FACET by Lab-Volt 259

17 SSB Reception Analog Communications Adjust the MIXER s null potentiometer so that a double sideband (DSB) signal appears at the mixer s output, as shown below. What frequencies are present in the DSB from the MIXER? a. 455 khz and 2455 khz b. 455 khz, 1455 khz, and 2455 khz DETECTOR. 260 FACET by Lab-Volt

18 Analog Communications SSB Reception Measure the period (T) between peaks of the SSB waveform at the IF FILTER output. Each vertical division line is 1 s. T = s (Recall Value 1) From the period (T), calculate the frequency of the SSB waveform. T = s (Step 7, Recall Value 1) khz (Recall Value 2) CONCLUSION A balanced mixer combines an SSB signal and an LO signal to form a DSB signal that contains a 455 khz difference signal to the IF stage. the mixer. FACET by Lab-Volt 261

19 SSB Reception Analog Communications REVIEW QUESTIONS 1. resonant frequency (f r ) of 1000 khz? a. The USB and LSB frequencies are equal. b. The inductive reactance (X L ) equals the resistor impedance. c. d. The inductive reactance (X L ) equals the capacitive reactance (X C ). 2. The SSB signal power at the RF stage input is 75 dbm and the power gain is 67 db. What is the RF stage output power? a. 8 dbm b. 8 dbm c. 142 dbm d. 16 dbm 262 FACET by Lab-Volt

20 Analog Communications SSB Reception 3. To prevent distortion of the message signal, the LO signal to the receiver s mixer must be similar to the LO signal to the transmitter s mixer in what respect? a. equal peak-to-peak voltages b. equal frequencies c. in phase with each other d. All of the above 4. This mixer s output signal is a 455 khz LSB signal. The frequency of the local oscillator signal to the mixer is MHz (10,455 khz). What is the frequency of the SSB to the mixer? a MHz b MHz c MHz d MHz 5. a. less than 449 khz b. 451 khz to 459 khz c. 453 khz to 457 khz d. greater than 461 khz FACET by Lab-Volt 263

Exercise 1: Amplitude Modulation

AM Transmission Analog Communications Exercise 1: Amplitude Modulation EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe the generation of amplitudemodulated signals