14 MHz Single Side Band Receiver

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Derek Goodman
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1 EPFL - LEG Laboratoires à options 8 ème semestre MHz Single Side Band Receiver. Objectives. The objective of this work is to calculate and adjust the key elements of an Upper Side Band Receiver in the 0m range.. Devices to study.. - Architecture of a double heterodyne receiver. - Quartz local oscillator. - "Gilbert" cell mixer. - Quartz selective filter. - Various types of LC filters. - HF amplifier with Automatic Gain Control.. Bibliography. [] Circuits et techniques HF et VHF, Prof. C. Dehollain. [] Traité d électricité volume VIII, Électronique, Prof. R. Dessoulavy et J.-D. Chatelain, PPR. [] Schematics of the LEG board "Emetteur - Récepteur USB" in appendix. [] Data sheets of the components on the intranet.

2 EPFL - LEG Laboratoires à options 8 ème semestre. Bloc diagram... Description. Antenna FLC AGC M FLC M FQ M RF Amp. LF Amp. AGC VFO Osc Osc This is the classic architecture of a super heterodyne receiver. The RF signal from the antenna is filtered and amplified, then translated to a first Intermediate Frequency where a band-pass filter removes the image frequency, then it is translated to a second IF where a sharp selective filter keeps only the wanted channel. The wanted SSB signal at IF is then demodulated by inverse modulation. Finally the LF signal is amplified to give power to the loud speaker... Theoretical forecasts.... Explain the advantages of the dual frequency shift (super heterodyne) in a receiver... Why is it better to have a large frequency difference between the received RF and the first IF?... How is chosen the second IF?... The received signal is in a band from MHz to. MHz. The first IF is. MHz. The selective quartz filter has a bandwidth of 800 Hz precisely centred on 9 MHz. Calculate the frequency of local oscillators Osc and Osc and the range of the VFO (several possible solutions).... Calculate the two first image frequencies...6. Explain the goal of each filter in the circuit...7. Explain the goal of the input RF amplifier and the usefulness of the AGC.

3 EPFL - LEG Laboratoires à options 8 ème semestre. The RF input stage (FLC AMP. RF).. Schematics. 7V CAG In R7 8.kΩ C76 nf C7 L7 00µH C77 Antenne 0Ω In C7 C7 L7.µH C7 L7.µH C7 G G R7.8kΩ T7 BF960 RF Out The input band-pass filter FLC is made of two LC resonators coupled capacitively. An impedance matching is made by a capacitive divider. The RF amplifier is made by a dual gate MOSFET in common source. The DC voltage applied on G controls the quiescent current and thus the gain of the amplifier to allow the AGC function.. Theoretical forecasts.... How are chosen the values of the various coupling and decoupling capacitors and of the load inductor L7?... The 0 m band covering the range from MHz to. MHz, the input filter FLC, should have a bandwidth of MHz centred on. MHz with a near flat response in the pass-band. The input should be matched to an ideal antenna of 0j0 [Ω]. Calculate the values of all the capacitors in the filter...6. Using the typical value of the transconductance of the MOS at V GS = V, estimate the voltage gain and the power gain of the input stage when loaded with 00 Ω... Measurements and tuning.... The measurements will be made with the following stage connected and using a high impedance HF probe to respect the real load in the circuit. The control voltage (AGC) is given by P7 (CAG Ext) (see schematics in appendix).... Measure the Bode plot of the input stage from MHz to 00 MHz with ACG voltage at 0V and adjust the filter's components to have the desired frequency response. Repeat the measurement between MHz and 6 MHz precisely tune the filter.... Making measurements at. MHz, plot the gain of the input stage versus the control voltage (AGC) from - V to V.

4 EPFL - LEG Laboratoires à options 8 ème semestre... Measure the harmonic distortion ratio for various input levels and AGC voltage values.... Measure the intermodulation distortion ratio for various input levels and AGC voltage values. 6. The first mixer (M FLC). 6.. Schematics. C8 C87 7V 8 C8 nf IC8 NE60 C86 Osc RF In C88 LOIN 6 L8 C8 C8-0pF FI Out VFO In = LO In C8 This mixer uses the IC NE60, a "Gilbert Cell" mixer. The output is loaded by a parallel LC resonator which behaves as a band-pass filter. 6.. Theoretical forecasts What is the goal of the band-pass filter (FLC)? 6... Knowing the output resistance of the mixer (00 Ω) and the input resistance of the next stage M (00 Ω), calculate L8 and C8 to have a pass band at - db of MHz centred on,7 MHz. 6.. Measurements and tuning. 6.. The measurements will be made with the following stage connected and using a high impedance HF probe to respect the real load in the circuit At LO In, apply a signal at 9 MHz of 00 mveff. At the input of the receiver, apply a signal at, MHz of 00 µveff. Adjust the gain control voltage to V. measure the spectrum of the signal at Fl Out from MHz to 0 MHz and adjust C8 to maximize the signal at IF =, MHz. Check that no parasitic component is in the useful band of khz around IF. 6.. Measure the maximum level at the receiver's input which give an intermodulation distortion ratio of -0 db at Fl Out.

5 EPFL - LEG Laboratoires à options 8 ème semestre 7. The second oscillator (Osc). 7.. Schematics. 7V R 7kΩ C -6pF L C C XT 6.MHz L 0.µH C -pf R 60Ω T BFR9 C P 0kΩ 6 MHz Out This oscillator is built around a bipolar transistor in common emitter. The parallel LC resonator in the collector allows to select the fundamental frequency or an harmonics of the quartz, in this case the fifth harmonics. The inductor L allows to widen the frequency range of the quartz. 7.. Theoretical forecasts What is the maximum peak to peak voltage at the collector of T? 7... The transistor used has a unity current gain transition frequency (f t ) greater than GHz, why such a choice? 7... How is made the positive feedback which allows self-oscillation? Calculate L to have an oscillation at the desired frequency (quartz overtone ). 7.. Measurements and tuning The adjustment of such an oscillator is delicate. Make this tuning with Out connected to LO In of M, because the parasitic capacitance, mainly due to the coax cable, influences the oscillator. With C in the middle position, adjust C to have an oscillation of maximum amplitude and a frequency near the desired value. Then push the frequency to the precise value by adjusting C. If not successful, readjust C then C Adjust the output amplitude to 0 mv peak-to-peak. Readjust the oscillation frequency if necessary Measure the spectrum of the output signal from MHz to 00 MHz and measure the relative level of the harmonics.

6 EPFL - LEG Laboratoires à options 8 ème semestre 6 8. The third mixer (M) Description. C9 C9 7V 8 C9 nf IC9 NE60 C96 FI In C97 LOIN 6 C9 R9.kΩ FI Out LO In C9 This mixer also uses the IC NE60, a "Gilbert Cell" mixer. The resistor R9 reduces the output resistance from 00 Ω for the IC to 000 Ω for the complete stage, which is the ideal source resistance for the quartz filter which will follow. 8.. Theoretical forecasts. 8.. What should ideally be the spectrum at FI Out?. 8.. Measurements and tuning. 8.. At the input of the receiver apply a signal at, MHz of 0 µveff. Adjust the gain control voltage to V. measure the spectrum of the signal at FI Out from MHz to 0 MHz. 8.. Measure the maximum level at the receiver's input which give an intermodulation distortion ratio of -0 db at FI Out. 8.. Measure the minimum level at the receiver's input to have a signal to noise ration of 0 db at the output of this stage.

7 EPFL - LEG Laboratoires à options 8 ème semestre 7 9. The quartz filter (FL). 9.. Schematics. In C0-0pF L' 6.8µH F0 A09FA L' 6.8µH C0-0pF Out The goal of this filter is to select only one channel and reject the adjacent ones. The required bandwidth is 800 Hz around 9 MHz. Such a selective filtering is only achievable with a quartz network. To obtain a pass-band response with minimum ripple, this filter needs to be driven by a source with precise impedance, in this case the output impedance of stage M, and loaded with a precise impedance, in this case the input impedance of stage M. The reactive part of these impedances are made minimum by adjusting C0 and C0 so that the total capacitance (parasitic adjustable) is cancelled by the inductance L'. 9.. Theoretical forecast Calculate the quality factor required for this band-pass filter. 9.. Measurements and tuning Replace the IC NE60 of mixer M by the small set in the schematics. C9 Point de calibration 00Ω C96 FI In 0Ω R9 C9 FI Out.kΩ Connect the filter between mixers M and M. Connect the source of the network analyser to FI In of M. Connect the input of the network analyser to FI In of M using an active probe with high input impedance. These cautions are taken to avoid to disturb the impedances at the filter's accesses In these conditions, measure the frequency response of the filter and adjust C0 and C0 to minimise ripple in the pass-band and insertion losses Measure precisely bandwidth at db and insertion losses.

8 EPFL - LEG Laboratoires à options 8 ème semestre 8 0. The third oscillator (Osc). 0.. Schematics. C 7V 8 C8 nf U NE60 C6 9pF 7 pf 6 9MHz Cadj -0pF This oscillator is build around a transistor integrated with its biasing circuit in the IC NE60, which is used as fourth mixer for demodulation. 0.. Theoretical forecasts What should be the exact frequency of this oscillator? 0... Where is the point of low impedance in this oscillator? 0.. Measurements and tuning Using a high impedance probe on pin 7, observe the spectrum of the signal and adjust the frequency precisely at the desired value.

9 EPFL - LEG Laboratoires à options 8 ème semestre 9. The fourth mixer (M)... Schematics. C C 7V 8 C8 nf U NE60 C FI In R.kΩ C6 9pF 7 pf 6 C C7 7nF BF Out 9MHz Cadj -0pF This mixer uses the same IC NE60 as mixer and. The resistor R reduces the input resistance from 00 Ω for the IC to 000 Ω for the complete stage, which is the ideal load resistance for the quartz filter which precedes... Theoretical forecasts.... What is the function of C7? Output resistance of the mixer is 00 Ω... Measurements... With the AGC voltage at V, measure the maximum level at the receiver's input which give an intermodulation distortion ratio of -0 db at BF Out... With the AGC voltage at V, measure the minimum level at the receiver's input to have a signal to noise ration of 0 db at the output of this stage.

10 EPFL - LEG Laboratoires à options 8 ème semestre 0. The automatic gain control and the LF amplifier (CAG Amp. BF)... Schematics. CAG U LF6 - C8 R7 kω P kω R R 0kΩ D BAW6 R kω R 0kΩ C6 R6 kω 0pF C U LF6 - C BF In R.kΩ R9 0kΩ - U LF6 BF Out C7 P 0kΩ C 00nF 6 C 0µF U LM86 C9 0µF HP 8Ω The LF amplifier is composed of an inverting amplifier (U) and an integrated power amplifier (U). The automatic gain control is controlling the gain of the input RF amplifier, depending of the amplitude of the LF signal recovered after demodulation. The goal is to have as much as possible a constant level of the signal in each stage for large variations of the level of the RF received signal. To do this, the LF signal is somehow rectified and the mean value is subtracted from a reference given by P. the difference is amplified to give the AGC voltage... Theoretical forecasts.... Why is it preferable that the amplitude of the signal in each stage stays constant when the received signal at the antenna has large variations?... Which elements give the AGC time constant?... Calculate R and C to have a CAG time constant of 0, s... Measurements and adjustment.... Close the receiver's input with a coaxial 0 Ω resistor to have no signal, and adjust P to have a CAG voltage of V in these conditions.... Plot the CAG voltage function of the RF input level.

11 EPFL - LEG Laboratoires à options 8 ème semestre. The complete receiver... Questions.... Explain the problems linked to the compromise between sensitivity and dynamic range, encountered in any receiver... Measurements.... Measure the levels of the signal and the intermodulation products at the output of each stage for various levels at the RF input.... Add a disrupting signal 00 khz apart from a weak (-00 dbm) received signal, and measure the amplitude of the demodulated signal function of the level of the disruption. Explain how this can affect the quality of reception of a channel with strong signals elsewhere in the band? What can be modified in the design of the receiver to avoid this problem?.. Final spec of the receiver. - Frequency range: - Rejection of the adjacent channels: - Rejection of the image frequencies: - Sensitivity for SNR = 0 db at the BF output: - Dynamics for IMD < -0 db at the BF output: - Audio bandwidth at -6 db:

EE12: Laboratory Project (Part-2) AM Transmitter

EE12: Laboratory Project (Part-2) AM Transmitter ECE Department, Tufts University Spring 2008 1 Objective This laboratory exercise is the second part of the EE12 project of building an AM transmitter in