Modern direct mixing receiver for 80 m

Many beginning radio amateurs are faced with the same question – how to get at least a simple receiver to receive amateur radio signals? Sometimes you can get an older RX for an ad (pioneer, Odra, R4 a pod.), You can also buy a new RX from renowned companies for heavy money (Yaesu, Icom, Daiwa and others).

Quality direct mixing receiver

Those who are more technically proficient can try to design the receiver by hand. The simplest does not mean the worst. High-quality direct-mixing RX is characterized in particular by the natural color of the received signal and good sensitivity. The disadvantage is, that the received signals appear twice in the NF spectrum during retuning.

Use of MOSFET transistors in the receiver

I designed a quality direct mixing receiver for the band 80 m, which is most suitable for a beginner, because it can also capture DX stations locally. The signal from the antenna passes through a two-circuit bandpass filter to a mixer with a two-base MOSFET. A signal from a tunable oscillator with a FET transistor is fed to G2 of this transistor (VFO). The resulting NF signal is amplified in a single-stage preamplifier also equipped with a MOSFET and fed to a selective NF headphone amplifier with an operational amplifier..

With the use of quality components and proper handling of field-controlled transistors, the receiver will work on the first connection. The HF generator will be suitable for proper tuning (can be a colleague with a QRP TRX). How about that?

Construction of the receiver

Input bandpass

It is advantageous to start with a two-circuit input bandpass. We install L1 – L4, C1, C2 a C3. It is advantageous to consist of two pieces of capacitors – solid capacitor and trimmer. If we have a HF generator and a HF millivoltmeter, we can set it immediately. Connect a load resistor to the filter output (50 ohm) a VF millivoltmeter. RF generator input. We need to set the permeable band of the filter to the range 3,5 to 3,8 MHz – that is, within this range, the deflection of the millivoltmeter hand must be maximum. If the filter is tuned above, the value of capacitors C1 and C3 must be reduced. If below then vice versa. Capacitor C2 sets the bandwidth. With such a bandwidth, it is also possible to detect a certain filter ripple in the passband. The resistance of the receiver to unwanted out-of-band signals will depend on the quality of the bandpass design..

Unless we harvest the toroidal nuclei N05, we can use others suitable for this frequency (N1 and others). It is also possible to use two-hole cores made of balancing members (the bigger ones, Tesla). However, the number of turns and the capacity of the capacitors will have to be determined experimentally, and without measuring technology we will only reach a satisfactory result with luck..

VFO Oscillator

Then we install the oscillator components. When the power supply is connected, the consumption will move around 10 mA. If we have a HF millivoltmeter we can check if the oscillator oscillates. This is also possible with the other receiver, on which we can check the oscillator to fine tune and adjust the bandwidth (3,49 – 3,81 MHz). We can also help ourselves with a reader. The most important part of the oscillator is the coil. It must be wound on a suitable body (best ceramic) VF lankom. The threads must be fixed, for example with wax, or use a frame with pre-cut grooves and fix the end of the winding in a suitable way. All capacitors must be of good quality, either of NP0, ceramic or mica. All of these components affect the stability of the oscillator. It is advantageous to stabilize the supply voltage of the oscillator, however, it is not necessary, if the entire receiver is powered by a stabilized source.

The tuning capacitor CL has a maximum capacity of about 15 pF, it can also be a capacitor from a FM radio receiver. Of course, an air tuning condenser of better quality design would be even better, however, these are difficult to obtain. I do not recommend using varicap, when strong signals penetrate from the mixer, the frequency would be entrained. If we do not have the ability to measure the inductance of the coils, then we will make coil L5 “from the eye”. Let's wrap, for example 40 threads and find out by the counter, auxiliary RX, absorption wavemeter and the like., where the oscillator resonates. If it resonates only slightly above the band, we can use the kernel for precise delivery. If the resonance is below the band, then it is necessary to reduce the number of threads.

NF part of the receiver

Now we will install the LF part with the transistor T2 and the operational amplifier IC. The components in the feedback set the gain and determine the bandwidth (approx. 3 kHz). Output power is maximum 100 mW in the impedance range 200 ohm až 2 kohm, which is enough for listening to headphones. Some may find it a luxury to use on NF preamplifier MOSFET, however, it is characterized by less distortion and higher gain than a bipolar transistor connection. The correct function of this part of the receiver can be verified by placing a finger on the gates T2 – a hum or a strong radio station should be heard in the headphones.

If we have an oscillator and input circuits pre-tuned after installing the mixer and connecting the antenna, it is possible to immediately capture a number of amateur radio signals in the evening. It is advantageous to supplement the tuning capacitor with a slow gear, thereby greatly facilitating precise tuning to stations and a scale divided at least by 50 kHz so that it can be quickly oriented in the band.

Alternative setup procedure

For those, who have not been able to set the receiver circuits, the following procedure applies. After all components have been fitted, they are checked first, whether the NF preamplifier and amplifier work. We connect the antenna directly to the G1 of the transistor T1. When retuning the oscillator, we look for signals from the band. When the tuning capacitor is closed, we should have CW stations, when the SSB station is open. The tuning capacitor should have a small reserve from the most extreme CW and SSB stations. This is how we at least roughly tune the oscillator to the band. Unwanted AM stations are known as such, that with slight tuning, a constant tone begins to appear in the received signal. We may be able to tune in to other amateur radio bands by mistake (1,8 or 7 MHz – both can be identified, for example, by bandwidth – are narrower and according to the nature of the signals – on 1,8 MHz is dominated by CW and is a night band, on 7 MHz operation is more DX than evening “rounds” OM and OK stations).

Now we connect the antenna via a bandpass filter. The signal strength is significantly reduced. It's not important yet. We find a stronger SSB station at the upper end of the band and by changing the capacitors of values ​​C1 and C2 we try to reach the maximum volume of this station. Capacitors interact with each other, therefore, it is almost necessary to use capacitive trimmers for tuning. After setting the maximum, tune the CW station at the lower end of the band and set the most necessary bandwidth with the coupling capacitor C2 (when by further increasing the capacity C2 the increase of the volume of the given station is minimal, or it does not change).

Working with field controlled components

Some care must be taken with field-driven components, so that we do not destroy them. This means, in the first place, not to use pistol solder for their soldering, which creates a very strong magnetic field. In addition, they are sensitive to electrostatic fields (should be stored wrapped in foil, which shorts all transistor terminals. I will mention, that foreign equivalents are characterized by better stability (BF) as Tesla products (KF). Why? I do not know it.

Conclusion

The described receiver will certainly bring its designers a lot of joy and pleasant moments spent listening to 80 m. The receiver can be adjusted to other bands by retuning the oscillator and bandpass. In my experience, the oscillator with FET is much more suitable for beginners (it oscillates over a much wider range of circuit component values, cleaner spectrum, it puts less strain on the resonant circuit, thereby increasing stability and higher output voltage). However, the multiplicative mixer is not durable, however, it is more affordable than a diode mixer and simpler than various balanced mixers. The NF part is solved inventively, using a MOSFET and an operational amplifier, which thanks to the feedback also works as a simple NF filter. Good luck with the construction and many valuable DXs!

Parts list

R1, R4, R8, R11, R12, R14 100k
R2 270
R3, R5, R7 100
R6 15k
R9, R10 470
R13 1k
Rd 2,7k * set to best S / N
P1 10k potentiometer
C1 130pF * see text
C2 22pF * see text
C3 130pF * see text
C4 10nF
C5, C9 47uF/16V
C6, C8, C20 10uF/16V
C7, C11, C16 100nF
C10, C17, Q18 1uF/16V
C12 22pF * see text
C13 100pF * see text
C14, C15 220pF * see text
C19 470pF
CL 15pF ladiaci capacitor
T1, T2 BF9xx, KF907, KF910 a pod.
T3 BF245, KP303 a pod.
I 741
L1 5z VF lankom na L2
L2 32with HF cable on toroid N05 diameter 10 mm * see text
L3 32with HF cable on toroid N05 diameter 10 mm *viď text
L4 5z VF lankom na L3
L5 about 20uH * see text

Literature

[1] Jan Hajek OK1-9251: Simple receivers for radio orienteering, AR-A 10/1989
[2] OK1DLP: Connection with two-base field-effect transistors, RZ 2-3/1986
[3] Ing.Vít Kotrba, OK2BWH: Transvertor k tranceiveru M160, AR-A 9/1987
[4] Jan Bocek, Jaroslav Winkler: Construction manuals for radio engineering 2 – Direct mixing receivers

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