A micro-receiver UHF

In our last issue we promised you a description of a micro-UHF, companion of the micro-transmitter controlled by voice. It is this micro-receiver that you find in these lines. It is based on the module RX-FM audio Aurel society.

The receiver that we propose in this paper allows the monitoring frequency 433.75 MHz. It is therefore entirely appropriate to receive broadcasts from a transmitter tuned to the same audio frequency, which is the case of micro-UHF transmitter controlled by voice.
Its circuit diagram is given in Figure 1 and we will immediately analyze.

The study schema
The radioactive element is the receiver module U1, the RX-FM audio.
This is a hybrid module CMS society Aurel containing a superheterodyne receiver with a quartz tuning circuit, tuned to 433.75 MHz, a quadrature demodulator and FM-pin input and output enabling insert preemphasis networks.
As the transmitter is not equipped with a network of pre-emphasis, it is theoretically not necessary to place a de-emphasis network at the receiver.
However, the filter is internal to the module U1 and it can not be deleted. Thus, the signal from the pin 18 is, to some extent, filtered above the practical limit of the audio tape, over 17 to 18 kHz.
The capacitor C3, placed at the outgoing part, completes the low-pass filter signal and ensures a clean enough, not only by reducing the noise typical of radio reception but also disturbances induced by the microwave transmitter.
In this arrangement, it is noted that the squelch is fixed at a level determined by resistor R2. Practically, it is disconnected and the receiver still works. Also for this reason that we do not use the CMOS switch included in the hybrid module.
The entire unit is powered by the 3.3 volts supplied by the zener diode DZ 1 and the resistance R1.
The demodulated audio signal is filtered by the hybrid module and sent through C9 and R3, the potentiometer P1.
The signal was picked up on the cursor P1 to be sent to the input of a second integrated circuit (U2), an LM386, which is an amplifier. The signal is amplified and directed to the "OUT" which may be connected to either a small speaker, a mini-helmet.
The LM386 is a small integrated amplifier from National Semiconductor, which can provide up to a 1 watt speaker 8 ohm impedance and can therefore play a very strong and clear. Of course, the quality will be even better if you prefer to connect a headset.
If using a speaker phone, it is necessary to short circuit the resistor R5. By cons, if it's a standard headset (impedance 8-32 ohm) which is connected to the output, R5 can protect it in case the volume is too high.
Still, if the resistance protects the headset, it does not protect your ears! Caution!
The receiver is powered by a 9 volt battery, thereby making it fully independent and thus able to take with you, near the home, office or premises under surveillance.

Figure 1 : Wiring diagram of the micro-receiver UHF.

Figure 2 : Installation diagram of components of the micro-receiver UHF.

Figure 3 : Photo of the prototype receiver.
Note the position of the module RX-FM Audio.

Figure 4 : Design of printed circuit scale 1.

Component List
R1 = 220 Ω
R2 = 22 kW
R3 = 270 Ω
R4 = 10 Ω
R5 = 4.7 Ω
R6 = 100 Ω
C1 = 470 uF 16 V electrolytic
C2 = 100 uF 16 V electrolytic
C3 = 2.2 nF ceramic
C4 = 100 nF multilayer
C5 = 1 nF ceramic
C6 = 10 uF 25 V electrolytic
C7 = 100 nF multilayer
C8 = 100 uF 16 V electrolytic
C9 = 220 nF multilayer
D1 = 1N4007 diode
Zener diode DZ 1 = 3.3 V 1 / 2 W
P1 = Pot. 4.7 kΩ with Inter.
U1 = Module Aurel RX-FM Audio
U2 = LM386N Integrated
ANT = Antenna granted (34 cm)

Miscellaneous:
A 3-pole
1 Terminal 2 poles
1 Support 2 x 4-pin
A 3.5 mm audio jack frame
1 Knob for potentiometer
1 Headset 8 / 32 Ω
1 PCB ref. S208
All resistors are 1 / 4 watt.


Superheterodyne receiver module quartz, produced by the company Aurel, in our prototype is used to having a quadrature FM demodulator, capable of guaranteeing a high fidelity listening, with an especially wide bandwidth and high signal / noise ratio. The module RX-FM Audio is mounted on a plate SIL standard 20-pin.
The module to operate, requires a voltage of 3 volts is applied to pin 1, pin 2, 7, 11, 16 and 20 are connected to ground.
The antenna input is on pin 3, the AF output corresponds to pin 10 while pin 15 is connected to a detector signal level FI (Field-Strength) to 10.7 MHz, which controls the squelch .
In our case, the squelch to a minimum because the resistance R2, which sets the threshold is as low as possible.

The receiver in practice
As the size of the receiver are not critical, we chose, contrary to the transmitter, the traditional components.
Get or make the circuit given the scale 1, in Figure 3.
Insert, first, components having the lowest height, resistors, diodes, as well as support for the LM386, then mount capacitors, making sure the polarity of the electrolyte.
Now you can install the hybrid RX-FM audio in the holes intended for him. Due to the configuration of its legs, it can be mounted in the right direction (Figure 2).
When you've found the right direction, remove the module circuit board and bend his legs at 90 degrees, from back to front. To do this, you can put the legs on a flat surface and raise the module holding the legs firmly pressed against the surface.
Replace the unit and folded into wrestling as much as possible to the circuit board so as to obtain a rather compact assembly. If in doubt, get inspired by pictures of the item.
Install the LM386 in its support, ensuring that its mark-keyed coincides with the mark of the support.
Perform a final inspection and solder the small screw terminals for printed circuit pitch of 5.08 mm, which will help you make connections with 3.5mm stereo audio jack, connected with the two internal contacts together, for headset or speakerphone.
Solder the battery socket on each side of the diode D1. In Figure 2, the left is more and less right.
The receiver circuit may be installed in a small plastic case provided with a housing for a 9 volt battery. It will make two holes.
One for the audio jack and one for the axis of the potentiometer P1. P1 may be a potentiometer with a switch which will avoid an extra hole.
The entire assembly is completed, the receiver is ready.
The simplicity of the electronic circuit and the use of a hybrid module factory set, makes any unnecessary development, apart from adjusting the volume, you adjust to your taste as you listen.

Test run
Put the battery in place, insert the headphone jack into the audio jack on the receiver, turn the potentiometer P1 to turn the receiver and continue to turn to for a low level. Feed the microwave transmitter, which will be used with this receiver, do not hold in hand but put it on a table and ask someone to talk to.
Adjust the receiver volume to your liking and make sure that whatever is said about the issuer is of course in the helmet.
For the tests, listening on speakerphone is recommended. Indeed, the high sensitivity of the micro-stage microphone transmitter may cause unpleasant and feedback that, although a certain distance is maintained between transmitter and receiver.