A Modular DC Receiver
Eric Edwards GW8LJJ describes a 3.5MHz to 14.35MHz Modular Direct Conversion Receiver.
This design has three fixed ‘starting’ frequency bands. The lowest is 3.500000MHz with the next one set at 7.000000MHz and the third starts at 14.000000 and ends at 14.350000MHz. There is an RF preselector that covers all frequencies between 3.5MHz and 14.35MHz by turning the preselector control from fully anticlockwise to clockwise. This eliminates the need for switched bandpass filters to cover several bands. The project is configured to simplify construction and allow changes to the design by choosing different ‘parts’ of the circuit that you may already have or prefer to use. The audio amplifier, for example, can be one already built, perhaps using the popular LM386 or similar. You may want to use your own antenna tuning unit as a bandpass filter, although the one in this design works very well on my half-wave 80m ladder-fed dipole, bypassing the shack ATU.
The Circuit
The circuit, Fig. 1, is shown complete with a mix of commercial and home-made units. The antenna is connected to the bandpass filter via a 100nF capacitor and enters the first coil (transformer), which is the first part of the tuned preselector. The two transformers are the same and are equivalents to the once readily available TOKO 10mm KANK 3334R coils. The ones used in this design are 5u3H (5.3wH) types. Across each of these coils is a variable capacitor and trimmer that is housed in a 4-gang polyvaricon capacitor block. The values used are 280wF as the main tuning for both sides and 20pF trimmers. The polyvaricon has connections on both sides and it is one side that is used for the bandpass filter. The PCB allows the correct fitting of this unit. The pins should be bent towards the front (spindle end) as shown in Fig. 2. It can only fit one way as there are four pins on one side and five on the other.
TheVFO
The VFO is built on a PCB and houses an Arduino NANO and a Si5351A clock oscillator. The Arduino has been programmed with the software required for the clock oscillator and the LCD (liquid crystal display). There are three preset bands, each with their starting frequency, and the tuning is also variable, from 3.5MHz right through to 14.35MHz. The bandpass filter can follow the tuning frequencies by adjusting the control from anticlockwise to clockwise. Whichever band is preselected, the tuning is continuous in steps of 10Hz, 100Hz, 1kHz, 10kHz or 100kHz with the indication shown by an arrowhead on the bottom line of the LCD pointing directly under the zeros on the top line.
Band select is by momentarily pressing a button switch and it cycles from 3.5MHz stepping to 7.0MHz and 14MHz and back again to 3.5MHz where the operation is repeated. The output of the VFO is taken to the LO (Local oscillator in) marked on the
in
Mixer module. There are also four pins on the VFO board that connections are to be made to, on mating four pins on the back board fitted onto the LCD.
LCD
The display used in this project is a 1602 type HD44780, 16 x 2 character display. This has two rows. The top row is used for the frequency readout and the bottom row indicates the rate of change in steps from 10Hz to 100kHz. There is a serial backlight board attached to the LCD allowing all information sent to the display by using the I²C serial interface. In my unit the LCD and backboard are housed in a plastic holder, Fig. 3, for easy fitting to the base of this project.
Mixer
The mixer module is a commercial unit employing an AD631 surface mount integrated circuit ready fitted, Fig. 4. There is no insertion loss and it has RF and LO inputs between 0.1MHz and 500MHz with an IF output between 0.1MHz and 200MHz. The power supply for this module is 9V to 11V. As this project uses a power supply of 12V (13.8V), a 9V regulator is used to supply the correct voltage for this mixer module.
Audio Preamp and Filter
The IF output from the mixer module is routed to an audio preamplifier and filter. The NE5532 is a low-noise dual op amp (operational amplifier) and the first part is used as a preamplifier with a gain of 200, which is set by the ratio of the input resistor (1kΩ) connecting between the IF output of the mixer module and pin 2 (inverting input) of the op amp and the 200kΩ resistor connected between pin 2 and the output at pin 1. The non-inverting input at pin 3 is biased at half power supply voltage. Op amps usually work with a split supply (plus and minus voltage with 0V connected to ground) but it is often practical to power op amp circuits from a single polarity supply. The problem is that an op amp is a dual-supply device so some type of biasing, using external components must be used to centre the op amp’s output voltage at mid-supply. This bias voltage is supplied by the two 10kΩ resistors, one connecting to the positive supply rail and the other to ground. This is a potential divider and provides the half voltage at pin 3 producing the bias, which allows the maximum input and output voltage swing for a given supply voltage.
Two Filters
There are two filters employed, one for SSB and AM reception and another for resolving CW. There are three pins on the PCB that are used for selecting SSB or CW. A linking plug (same as on PC mother boards) is used to select the mode. The SSB filter is made up on the second part of the first op amp (NE5532) with the main components 470nF and 10nF. This is a lowpass filter and the output is connected to one of the ‘filter select’ pins on the PCB so that it can be routed to the audio amplifier or connected to the second filter, which is a CW bandpass type employing an NE5534. There is a bias arrangement on the second part of the NE5532 to provide the half power supply needed. The CW bandpass filter also has the half supply voltage for the same reason as on the other op amp. The bandwidth of this filter is governed by the two 10nF capacitors and the 220kΩ resistor on pin 2 and pin 6 along with the 10kΩ resistor connected between the two 10nF capacitors and ground. Making this 10kΩ resistor variable the CW band filter can be changed. I found the 10kΩ fixed resistor provided good filtering for CW.
Audio Output
The audio output, Fig. 5, uses a ready-made commercial audio amplifier requiring only a power supply and speaker. The output from the filters is connected to this audio amplifier that has a volume control pre-fitted. The power supply for this amplifier is between 12V and 24V. While this project uses 12V (13.8V) with a good level of audio into the speaker, there will be a significant output level if it is used at 24V and the neighbours will be able to hear the signals as well! The output power at 12V is 10W and at 24V is 30W.
PCBs
There are three single-sided FR4 PCBs.
Fig. 6 shows the antenna bandpass filter and the sockets used are SMA types. Fig. 7 shows the layout of the VFO, which contains the Arduino and clock oscillator modules with the band select and frequency tuning control that also contains the frequency step tuning. The clock oscillator module has an SMA socket pre-fitted for the output. Fig. 8 is the audio preamplifier and filters. The sockets used are SMA for the input and a phono type for the output socket.
In Use
This project can be made up on a base board as shown in the photo, Fig. 9, for demonstration or as a building block for future changes, or the individual modules can be arranged to suit your own requirements. The LCD can be fitted onto a front panel along with the antenna preselector and the
audio amplifier with a suitable speaker.
The only alignment is minimum and is the antenna pre-set calibration. Turn the polyvaricon control anticlockwise and if a signal generator is available, connect a low level 3.5MHz signal into the antenna socket. Set the cores on the coil so that they are flush with the top of the can. Turn on the receiver, allow it to initiate and show 3.5MHz (with all the following zeros) on the LCD. A signal (tone) should be heard in the speaker. Adjust slowly with a plastic (nylon) core-adjusting tool until the signal increases. Adjust both coils, which should be approximately the same level of adjustment. When the loudest signal is heard, turn the signal generator level down so the signal is just audible and adjust the cores again for maximum tone output. Turn the preselector fully clockwise, set the signal generator for 14.35MHz and press the band select button to show 14MHz. Turn the frequency control until 14.35MHz is seen on the LCD. Check the coil for maximum tone output level heard in the speaker. If any adjustment is needed, go back again to the 3.5MHz settings and check for the strongest signal. If a signal generator is not available then plug an antenna into the receiver, tune to an 80m station and with the preselector turned anticlockwise, adjust the cores for maximum signal level. Do the same at 14MHz and it will be set for all the frequencies.
This is a general coverage receiver so other frequency bands can be received. Set any on the preset bands and continue tuning up the band and adjusting the preselector as well for maximum signal. When the filter is set in the SSB position, to resolve an AM signal tune until a centre tone is heard, then with a low setting on the step frequencies adjust until the tone is nulled out. The 10Hz step setting is probably best for this adjustment. The CW setting is very effective for resolving CW stations.
Is there a Kit?
I can supply my usual ‘picking list’ of available parts upon request from me at
Acknowledgement
VFO Arduino programming: Ray G7BHQ. (Please note: This is not available as a download). The Arduino and clock oscillator modules are supplied as a pair.