Why a Direct Conversion Receiver?
In spite of all the inconveniences DC Receivers have, Hams hold it to their hearts. The main reason is not that its' easy to assemble but the fact that the same oscillator used to receive the signal can be used for transmission also without any fine tuning, thus avoiding complicated extra circuits that keep oscillator frequencies always steady. Commercial transceiver manufacturers too do not ignore this favour of easier 'receive - transmit' change over. In fig. C-5/1 the RX/Tx changeover pattern is shown.
Product Detectors
Every time we feed two different frequencies to an active unit, what we get in the out put are 1) fundamental frequencies, 2) a signal at the frequency of difference, 3) a signal at the frequency of added values and 4) a series of harmonics belonging to all these and their images. The best of them is certainly the signal at the added value. But in all the DC and most BC receivers what we utilise are the difference frequency signal. Suppose what we get in the front end of a DC receiver is a 7 MHz signal modulated with a 2.5 KHz audio, according to this theory, if mixed with a 7 MHz signal the difference (modulated intelligence at 2.5 KHz) is made available at the mixer output. Similar mixers are called Product Detectors.
Simple VFO
Fig C-5/2 is the circuit if a simple VFO (Variable Frequency Oscillator). This VFO is enough to support the DC receiver shown in C-5/3. Note that this DC Receiver VFO circuit has only just one FET (Field Effect Transistor) stage. If we use this stage for transmitter also, an additional buffer stage also is required. It is always better to fix the VFO in a strong body aluminium box (not compulsory while used only for receiving). This circuit is designed for operation at 12 V. Since it should be a regulated voltage, better feed a 15V-18V DC voltage through IC 7812 for a steady 12 V DC.
Receiver Incremental Tuning
Every time we feed two different frequencies to an active unit, what we get in the out put are 1) fundamental frequencies, 2) a signal at the frequency of difference, 3) a signal at the frequency of added values and 4) a series of harmonics belonging to all these and their images. The best of them is certainly the signal at the added value. But in all the DC and most BC receivers what we utilise are the difference frequency signal. Suppose what we get in the front end of a DC receiver is a 7 MHz signal modulated with a 2.5 KHz audio, according to this theory, if mixed with a 7 MHz signal the difference (modulated intelligence at 2.5 KHz) is made available at the mixer output. Similar mixers are called Product Detectors.
Simple VFO
Fig C-5/2 is the circuit if a simple VFO (Variable Frequency Oscillator). This VFO is enough to support the DC receiver shown in C-5/3. Note that this DC Receiver VFO circuit has only just one FET (Field Effect Transistor) stage. If we use this stage for transmitter also, an additional buffer stage also is required. It is always better to fix the VFO in a strong body aluminium box (not compulsory while used only for receiving). This circuit is designed for operation at 12 V. Since it should be a regulated voltage, better feed a 15V-18V DC voltage through IC 7812 for a steady 12 V DC.
Receiver Incremental Tuning
At the same time, if what the receiver gets is a CW signal at 7 MHz, since the oscillation frequency and the receiving frequency are the same, no difference is produced. That is, at CW and SSB signals a slight tuning is necessary at the VFO. It is RIT (Receiver Incremental Tuning) circuits that make corresponding frequency changes during transmission and receiving.
Direct Conversion Receiver Circuits
C-5/3 is the circuit of a very simple DC Receiver. To avoid interferences of out of band signals, DC receivers follow two rules. The first is using tuned antennas and the next is usage of bandpass filers that match with the low gain antenna impedance. Replacing T1, L1 and T2 here will enable the circuit to work at other frequencies too. Even though T1, L1 and T2 are advised to be assembled using ordinary 1 cm IFTs rewound, T 12 (HF-A) toroid, 1 cm IFT type Amidon Iron Powder formers and Philips Shortwave antenna coil formers (IFT Type plastic can) will give you better results in sensitivity and selectivity.
Q1 here functions like feedback broadband amplifier (just to bring all signals more or less same in strength). Q2 is for mixing and detection. Q3 and IC 1 are audio amplifiers.
While tuning the front end coils T1, L1 and T2 choose a position as away as possible from BC stations. Before switching on the full unit confirm if VFO Oscillation is in Ham band itself. Without switching off power supply (better to remove the cord from the socket) soldering IC s may result in damaging Q2 and IC-1.
In C-5/4 a little more improved version of a DC Rx is given. General instructions with regard to front end coils are applicable here too. In this circuit a dual gate MOSFET is used for mixing and detection. The audio received in the output is let through a processing circuit. The first portion of this IC functions as lowpass filter. A clipper circuit which controls heavy power differences between incoming signals also is attached in this portion of the IC. Since the second portion of the MOSFET works as an audio preamplifier with automatic gain control, low power amplifier ICs like LM 386 is enough in the audio output stage. It will be better if the audio amplifier portion is done on a separate board.
In either case, unless the audio output does not match the speaker impedance, the IC is likely to oscillate in audio frequency. In some DC receivers a separate arrangement to control the strength of the VFO signal also are found. In that case use a 22 PF trimmer in series with the coupling capacitor C-10 connected to the Mixer stage.
In C-5/4 a little more improved version of a DC Rx is given. General instructions with regard to front end coils are applicable here too. In this circuit a dual gate MOSFET is used for mixing and detection. The audio received in the output is let through a processing circuit. The first portion of this IC functions as lowpass filter. A clipper circuit which controls heavy power differences between incoming signals also is attached in this portion of the IC. Since the second portion of the MOSFET works as an audio preamplifier with automatic gain control, low power amplifier ICs like LM 386 is enough in the audio output stage. It will be better if the audio amplifier portion is done on a separate board.
In either case, unless the audio output does not match the speaker impedance, the IC is likely to oscillate in audio frequency. In some DC receivers a separate arrangement to control the strength of the VFO signal also are found. In that case use a 22 PF trimmer in series with the coupling capacitor C-10 connected to the Mixer stage.
Active and Passive Components
We know that inductance resistance and capacitance are the three properties of electricity. In a circuit, a component with one of these qualities is called an element. Since none of these has the ability to amplify a given power, they are also called passive components. Since both the leads of these elements do not vary anyway, these components are Bilateral too. Active components like diodes, transistors and valves are different by their capacity to amplify a signal. Unlike passive components, they some times cause change of frequency too.
We know that inductance resistance and capacitance are the three properties of electricity. In a circuit, a component with one of these qualities is called an element. Since none of these has the ability to amplify a given power, they are also called passive components. Since both the leads of these elements do not vary anyway, these components are Bilateral too. Active components like diodes, transistors and valves are different by their capacity to amplify a signal. Unlike passive components, they some times cause change of frequency too.
Be it a passive component or an active component, a RF signal that passes through it might generate multiple signals called harmonics. A harmonic of a frequency is a wave with a frequency that is a positive integer multiple of the frequency of the original wave, known as the fundamental frequency. The rank of a harmonic is set as if the second harmonic of a 7 MHz signal is 14 MHz and third is 21 MHz. In a mixer stage where two frequencies come face to face, the result would not be limited to just a new signal at the frequency of difference. There would be a signal at the added frequency, the fundamentals, the harmonics of everything and thus many signals of varying strength and frequencies. Even though the IF stages reject all these signals except the difference between mixed signals, those signals with frequency close to IF, called Image Frequencies, might pose severe threats.
Even elements cannot be used in RF stages as literally free as they are in dc circuits. See pic: C-4/1A to see the change when a capacitor is used in RF circuit. The behaviour of an inductor in an RF circuit will be like that in pic: C-4/1B Radio Circuits
At high frequencies, even Composition Carbon (CC) resistors behave like active mini circuit packs. We need to consider all these factors every time we think of working on a radio circuit. While assembling a radio gadget, even if the concerned home brewer observes all the available instructions exactly as they are meant, the result need not be satisfactory. This is because the components used vary in quality, though same in the given values. In a receiver, even passive components fall prey to environmental influences like heat. We are to be careful in fixing heat sensitive components away from heat developing components and corners. The frequency, the voltage and the heat either applied or developed inside the transistor should always be kept within the specifications of the manufacturer.
Transistors - ‘ft’ and (β)
Transistors that need different power circuits and power supplies at input and out put fall under bipolar components. A bipolar transistor basically consists of three sections called Emitter (E), Base (B) and Collector (C) and the depletion regions inside it fundamentally keeps these sections apart. A depletion region is more or less like the electrolyte in a capacitor. The maximum frequency at which a transistor works might be given in ‘ft’. 'ft' is defined as the unity gain frequency of a transistor’s short circuit current gain. It is always better to have a ‘ft’ of 7-10 times more than the actual frequency range in which a transistor is working.
Transistors that need different power circuits and power supplies at input and out put fall under bipolar components. A bipolar transistor basically consists of three sections called Emitter (E), Base (B) and Collector (C) and the depletion regions inside it fundamentally keeps these sections apart. A depletion region is more or less like the electrolyte in a capacitor. The maximum frequency at which a transistor works might be given in ‘ft’. 'ft' is defined as the unity gain frequency of a transistor’s short circuit current gain. It is always better to have a ‘ft’ of 7-10 times more than the actual frequency range in which a transistor is working.
The Beta (β) of a transistor means how much more in times the output current of a transistor is with regard to the input current of the same transistor. The manufacturer records beta in units of ‘hfe’. ‘hfe’ of a transistor is the current gain or amplification factor of a transistor. Each transistor will be different in its ‘hfe’ features. ‘hfe’ meters are available now. If we keep Emitter and Base in forward bias at a definite voltage, we can measure the collector current and thus compare the ‘hfe’ between transistors. Remember that a transistor is most efficient only when used at a voltage nearer to the specified voltage of that transistor. Even though distortion and instability are sure when bipolar transistors are used in a different voltage spectrum, they can be found to be comfortably working in higher frequencies.
Slightly different are Field Effect Transistors (FET). Source (S), where a carrier begins and Drain (D) where a carrier ends are the two important points in a FET. There would be a depletion region close to the carrier channel. It functions exactly like a common gate. A voltage applied at this gate results in the expansion or contraction of the depletion region, which changes the intensity of the carrier current accordingly. This is the working pattern of all FETs. Compared to a bipolar transistor, current development in a FET is faster. The efficiency of FETs to handle comparatively higher current and wider range of frequencies make FETs more acceptable than bipolar transistors. Else reasons for considering FETs in RF circuits are their low noise factor, high stability and high out put impedance factors.
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