Showing posts with label MHz. Show all posts
Showing posts with label MHz. Show all posts
Wednesday, November 19, 2014
144 MHz Simple RF Detector Circuit
This simple circuit helps you sniff out RF radiation from your transmitter, improper joints, a broken wire or poor equipment with RF shielding. The tester is designed for the radio band amateur 2 meter (144-146 MHz in Europe). The instrument has a reading of 4-step LED and an audible alarm for high voltage radiation. The RF signal is received by an antenna and made to resonate by C1-L1. After rectification by the diode D1, the signal is fed to a two transistor Darlington amplifier HighGain, T2-T3. Assuming a 10-inch telescoping antenna using the RF level scale established for the LEDs is as follows:
When all the LEDs light, the (optional) UM66 sound / melody generator chip (IC1) also operates and provides an audible alarm. By changing the zener diode values of D2, D4, D6 and D8, the step size and duration of the instrument may change as needed. To operate in other bands of ham or PMR, simply change the network-L1 C1 resonance.
For example, a transceiver 5 watt handheld equipped with a telescoping half-wave antenna (G = 3.5 dBd), there is an ERP (Effective Radiated Power) of just 10 watts and an emf of more than 8 volts near the head. Inductor L1 consists of 2.5 turns of 20 SWG (approximately 1 mm in diameter) enameled copper wire. The inner diameter is approximately 7 mm and no core is used.
Trimmer capacitor C1 associates is adjusted for the greatest number of LEDs to light at a relatively low fieldstrength position for a 2 m transceiver 145 MHz transmission. The tester is powered by a 9 V battery and consumes about 15 mA when all LEDs are on. Must be enclosed in a metal box.
When all the LEDs light, the (optional) UM66 sound / melody generator chip (IC1) also operates and provides an audible alarm. By changing the zener diode values of D2, D4, D6 and D8, the step size and duration of the instrument may change as needed. To operate in other bands of ham or PMR, simply change the network-L1 C1 resonance.
For example, a transceiver 5 watt handheld equipped with a telescoping half-wave antenna (G = 3.5 dBd), there is an ERP (Effective Radiated Power) of just 10 watts and an emf of more than 8 volts near the head. Inductor L1 consists of 2.5 turns of 20 SWG (approximately 1 mm in diameter) enameled copper wire. The inner diameter is approximately 7 mm and no core is used.
Trimmer capacitor C1 associates is adjusted for the greatest number of LEDs to light at a relatively low fieldstrength position for a 2 m transceiver 145 MHz transmission. The tester is powered by a 9 V battery and consumes about 15 mA when all LEDs are on. Must be enclosed in a metal box.
Monday, September 22, 2014
VHF FM Receiver TDA7000 88 108 MHz
VHF FM Receiver TDA7000 88 108 MHz
Here is a very simple VHF FM receiver which is little more than a single IC and a "slack handfull" of capacitors. Note that an external amplifier is a really necessity since the unit only delivers about 70mV of AF.See High Power FM Wireless Microphone transmitters, probably because it is so simple too.
The 10K resistor (*) is only required if you want the receiver to mute (squelch) under no-signal conditions. You could add a 100K in series with this resistor to get an adjustable squelch. This circuit will JUST drive a crystal earphone or high impedance headphones directly, but an output isolating capacitor (100nF) is needed for any other device. L1 is 6 turns No 18 SWG enamelled wire on a 5mm former, but you may have to play with the values a bit. I used a coil fabricated on the PCB itself, tuned with a trimmer capacitor.
All the other components are just a bunch of capacitors which are fitted to the board at the other side of the chip, just to make it look a bit prettier.
As you can see, this receiver is VERY sensitive, small and seems to work very well indeed. It is in fact a full superhet receiver with a very low RC tuned IF. The Image signal is rejected by the action of the AFC which functions to push it away. With suitable antennas and terrain, with this receiver you could easily get the full 500 meters from the FM wireless microphone v5. I am offering this receiver in kit form, including solder, antenna wire etc. All you will need to provide is the battery, tools and soldering iron. Here is the kit version fully assembled.
The total size is 45mm x 48mm. If there is any interest in the kit then I may even add an AF amplifier kit so that you can make a full bedside/table radio.
Source: TDA7000 RX
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