Intro: All-Band Direct Conversion Receiver
This Instructable describes an an experimental "Direct Conversion" all-band receiver for the reception of single side-band, morse code, and teletype radio signals up to 80MHz. Tuned circuits are not required!
This advanced project builds on my first Instructable https://www.instructables.com/id/Arduino-Frequency-Synthesiser-Using-160MHz-Si5351
The concept for this receiver was first published in 2001: “Product detector and method therefor”, Patent US6230000 B1, May 8, 2001, Daniel Richard Tayloe, http://www.google.com/patents/US6230000
Step 1: Theory
The above circuit shows a switch, resistor, and capacitor connected in series.
AC (alternating current) viewpoint
If we close the switch and apply an AC signal to the input, an AC voltage will appear across the capacitor, the amplitude of which will decrease with increasing frequency due to voltage divider action.
Of particular interest to us is the frequency at which the AC voltage across the capacitor falls to 70% of the input. This frequency, known as the "cutoff frequency", occurs when the reactance Xc of the capacitor is equal to the resistance R. Frequencies above the cutoff frequency are attenuated at a rate of 6dB/octave.
The cutoff frequency for my circuit has been set to 3000Hz which means that there is no AC output for broadcast frequencies and above.
DC (direct current) viewpoint
If we close the switch and apply a DC voltage to the input, the capacitor will start charging to that value. Should we open the switch before the capacitor has fully charged then the voltage across C will stay constant until the switch is again closed.
Receiving a high frequency signal.
Let's now pass a high frequency signal through a switch that is opening and closing such that the same portion of the incoming signal is presented to the RC network described above. Even though the incoming signal is well above the cutoff frequency of 3000Hz, the capacitor is always being presented with the same uni-polar DC waveshape and will charge to the average value of that waveshape.
Should the incoming signal differ slightly from the switching frequency then the capacitor will start to charge and discharge as it encounters different shaped segments of the incoming signal. If the the difference frequency is, say, 1000Hz then we will hear a tone of 1000Hz across the capacitor. The amplitude of this tone will drop off rapidly once the difference frequency exceeds the cutoff frequency (3000Hz) of the RC network.
- The switching frequency determines the receive frequency.
- The RC combination determines the highest audio frequency that can be heard.
- Amplification is required as the input signals are very weak (microvolts)
Step 2: Schematic Diagram
The above circuit has two switched RC (resistor - capacitor) networks. The reason for two networks is that all waveforms have a positive-voltage waveshape and a negative-voltage waveshape.
The first network comprises R5, the switch 2B2, and C8 ... the second network comprises R5, the switch 2B3, and C9.
The differential amplifier IC5 sums the positive and negative outputs from the two networks and passes the audio signal through C15 to the "audio output" terminal of J2.
Design equations for R5,C8 and R5,C9:
XC8=2R5 where XC8 is the capacitive reactance 1/(2*pi*cutoff-freq*C8)
The values of 50 ohms and 0.47uF produce a cutoff frequency of 3000Hz
The reason for the 2*multiplier is that the input signal is only presented to each network for half the time which effectively doubles the time constant.
Design equations for R7,C13
XC13=R7 where XC13 is the capacitive reactance 1/(2*pi*cutoff-freq*C13). The purpose of this network is to further attenuate high frequency signals and noise.
The Audio Amplifier:
The audio gain of the op-amp IC5 is set by the ratio of R7/R5 which equates to a voltage gain of 10000/50 = 200 (46dB). To obtain this gain R5 has been connected to the low impedance output of the RF (radio frequency) amplifier IC1.
The RF Amplifier:
The voltage gain of IC1 is set by the ratio of R4/R3 which equates to 1000/50 = 20 (26dB) giving an overall gain approaching 72dB which is suitable for head-phone listening.
The Logic Circuits:
IC4 acts as a buffer-amplifier between the 3 volt peak-to-peak signal from the synthesis and the 5 volt logic for IC2. The buffer amplifier has a gain of 2 which is set by the ratio of resistors R6/R8.
IC2B is wired as a divide-by-two. This ensures that capacitors C8 and C9 are connected to R5 for equal lengths of time.
Step 3: Printed Circuit Board
Top and bottom views of the circuit board before and after it has been assembled.
A full set of Gerber files are included in the attached zip file. To produce your own PCB simply send this file to a circuit board manufacturer ... get a quote first as prices vary.
Step 4: Local Oscillator
This receiver uses the frequency synthesiser described in https://www.instructables.com/id/Arduino-Frequency-Synthesiser-Using-160MHz-Si5351
The attached file "direct-conversion-receiver.txt" contains the *.ino code for this receiver.
This code is almost identical to the code for the above frequency synthesiser except that the output frequency is twice the display frequency to allow for the divide-by-two circuit on the receiver board.
Original code in .ino format attached.
Step 5: Assembly
The main photo shows how everything is inter-connected.
SMD’s (surface mount devices) were chosen as you do not want long leads when switching at 80MHz. 0805 SMD components were chosen to make hand-soldering easier.
While on the subject of hand-soldering it is important to buy a temperature controlled iron as too much heat will cause the PCB tracks to lift. I used a 30W temperature controlled soldering iron. The secret is to use plenty of gel flux. Increase the soldering temperature until the solder just melts. Now apply solder to one pad, and with the soldering iron still on the pad, slide the 0805 component against the soldering iron using a pair of tweezers. When the component is correctly positioned remove the soldering iron. Now solder the remaining end then clean your work with Isopropyl alchohol which is available from your local chemist.
Step 6: Performance
What can I say ... it works !!
Best performance is obtained using a low-impedance resonant antenna for the band of interest.
Instead of headphones I added a 12 volt audio amplifier and speaker. The audio pre-amplifier had its own inbuilt voltage regulator to reduce the chance of a common-mode feedback loop through the 12 volt battery supply.
The attached audio clips were obtained using an indoor tuned loop of
wire approximately 2 metres in diameter. The centre of of the loop was passed through one hole of a two-hole ferrite core with a 10 turn secondary connected between ground and the receiver input.
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