Variable RF attenuator with PIN diode

Variable RF attenuators are often used to control the level of a radio frequency signal using a control voltage in RF design. These variable RF attenuators can even be used in programmable RF attenuators. Here the known voltage generated by a computer for example can be applied to the circuit and in this way create a programmable RF attenuator.

Often when designing or using variable or programmable RF attenuators, it is necessary to ensure that the RF attenuator retains a constant impedance over its operating range to ensure the correct operation of the interfacing circuitry. This RF attenuator circuit shown below provides a good match to 50 ohms over its operating range.

RF attenuator circuit description
The PIN diode variable attenuator is used to give attenuation over a range of about 20 dB and can be used in 50 ohm systems. The inductor L1 along with the capacitors C4 and C5 are included to prevent signal leakage from D1 to D2 that would impair the performance of the circuit.

The maximum attenuation is achieved when Vin is at a minimum. At this point current from the supply V+ turns the diodes D1 and D2 on effectively shorting the signal to ground. D3 is then reverse biased. When Vin is increased the diodes D1 and D2 become reverse biased, and D3 becomes forward biased, allowing the signal to pass through the circuit.

Typical values for the variable RF attenuator circuit might be: +V : 5 volts; Vin : 0 - 6 volts; D1 to D3 HP5082-3080 PIN diodes; R1 2k2; R2 : 1k; R3 2k7; L1 is self resonant above the operating frequency, but sufficient to give isolation between the diodes D1 and D2.

These values are only a starting point for an experimental design, and are only provided as such. The circuit may not be suitable in all instances.

Choice of PIN diode
Although in theory any diode could be used in variable RF attenuators, PIN diodes have a number of advantages. In the first place they are more linear than ordinary PN junction diodes. This means that in their action as a radio frequency switch they do not create as many spurious products and additionally as an attenuator they have a more useful curve. Secondly when reverse biased and switched off, the depletion layer is wider than with an ordinary diode and this provides for greater isolation when switching or providing higher levels of attenuation.

Source: PIN diode variable RF attenuator circuit

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Power Supply with Short Circuit Protection 13.8V 30-40A

Here's a safe power supply with short circuit protection. The power supply is build around the LM723 controller and four BUZ24 (or IRF150) power N-Channel FET transistors. FET transistors are used because of the simplicity of controlling the current through these transistors, it's simply voltage controlled, and because of the low power consumption of the controller board.

 

Project schematics, PCB layouts and data sheets

• Schematic of the 13.8V 30-40A Power Supply
• PCB layout of the Power Supply controller board
• PCB layout of the Power Supply controller board - scaled 1:1
• PCB layout of the Power Supply controller board - mirror
• PCB layout of the Power Supply controller board - mirror - scaled 1:1
• The internal connections of the Power Supply
• Data sheet of the IRF150 N-Channel HEXFET
• Data sheet of the BUZ24 N-Channel SIPMOS FET

Source: 13.8V 30-40A Power Supply

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RF Power Meter Reading by Digital Voltmeter

The RF Power Meter circuit is based on the AD8313 Log Detector manufactured by Analog Devices. In GSM phones AD8313 is used as a Log Detector, part of the Power Control Loop circuit. Generally could be easy identified near the Power Amplifier module.

AD8313 is a Logarithmic Detector which can accurately convert an RF signal at its input to an equivalent decibel-scaled value at its DC output. The DC output is “linear in dB” with a basic slope of 20mV/dB. The slope can be adjusted in a range from 18mV/dB to 30mV/dB. The linear input range of AD8313 is between -60dBm and 0dBm, which corresponds to a DC output between 0.6V to 1.6V (pin 8).

The operational amplifiers LM324 are translating the DC output range of AD8313 (0.6V to 1.6V on Pin nr 8) to a scaled range read by the Voltmeter (-6V to 0V). The scaled range has a resolution of 100mV/dB.

For example the minimum input value (-60dBm) corresponds to a read voltage value of -6.0V, -59dBm corresponds to -5.9V, -58dBm corresponds to -5.8V, and so on up to 0V that corresponds to 0dBm (as in the table below).

The frequency range of AD8313 is between 100MHz to 2.5GHz, but the range that not requires a dynamic slope adjustment is between 100MHz to 1.4GHz. The resolution of the RF Power Meter is better than +/- 1dB; only near 0dBm power input, the resolution is approximately +/- 2dB. The RF input has an impedance of 50 ohms provided by the 53 ohms resistor in parallel with the internal impedance of the AD8313.


For calibration inject first at the input an 800MHz signal at -60dBm and adjust P2 for -6V reading on the output Voltmeter. After that increase the input level up to 0dBm and adjust P3 for 0V reading on the output Voltmeter. The slope can be adjusted by the P1 semi-resistor.

Careful design of the RF input layout should be done for minimizing parasitics which can produce un-wanted resonances that affects the linearity vs frequency of the log-detector. Tolerance of the resistors is +/-1%.

A calibrated attenuator at the input can be used to increase the maximum input power, without damaging the detector.

Source: RF Power Meter using for reading a standard Digital Voltmeter

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25W Broadband RF Amplifier 88-108 MHz

This RF amplifier for FM 88-108 MHz with no tune (broadband) needed to cover all the FM Band. This RF Power amplifier is equiped with the famous Mosfet transistor the BLF245. Depending on the output power level you are able to provide with your FM synthesizer, you can use or not the 2N3866 driver stage included in this amplifier design.

Impedance matching network file(PDF)

All the impedance networks (Input-Output) of this RF amplifier have been determined by using the softwares: Mimp.EXE and Genesy.

Low pass filter measurements file(PDF)

This RF Amplifier need a 9 elements low pass filter ensures that its harmonic frequency meet at least a 60 dB rejection from the carrier.(RF Simulation with RFSIM99)

Gain and Ros measurements file(PDF)

The RF FM amplifier has a 27 dB gain (with driver stage) and provides 25W with a 58% efficiency.

RF Power Amplifier PCB Layout

Source: 25W MOSFET FM AMPLIFIER

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Another K1 Build - this one will be mine this time.

Various shots going through the RF Part I build.

The Front panel and filter board already built.

The extensive home brew bench.

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FtTuthill80 Build

Kits components as sent.

Following the pdf instructions of Dan Tayloe, N7VE

These are the build images from the kit I just started to build.
Up to the power test (used a 9V battery) and the blue LED lighting.

Blue LED

Diodes

Sorting the 0.1uF caps

0.1uF caps

More caps

And more

Electrolytics

Resistors

More resistors

More resistors

Applying 9V.

The LED lights blue.

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Elecraft T1

Was recently given an Elecraft T1 auto tuner kit and as now working in Belgium thought I'd make it up. First started in hotel room.

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The portable 'homebrew' bench is perfectly adequate. Note I have brought a 12" square of PCB to build on.

Initial components going in.

Relays fitted.

Fitted the underside capacitor.

Fitted the sockets and headers.

Built up the control interface.

First fitting. Looks good so far however, visual check of all the components against the build list and couldn't work out why had extra FT37-43 torroid. Then remembered there is an errata...

Oops... errata clearly state use 2 FT37-43 cores for T2 not 1.

No harm done, removed T2 and added the new T2 fitted easily.

Removed the PIC, added battery, checked the voltage through the 2N3906 when tune/pwr switch depressed and onto the regulator says 6V.

Added PIC and tested the power on, seems to be fine. Will test again once have working TX with me in Belgium.

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USB FM Transmitter for MP3 Player

This MP3 Player FM transmitter can be used to listen to your own music throughout your home. The transmitter circuit use no coils that have to be wound. When this FM transmitter used in the car, there is no need for a separate input to the car stereo to play back the music files from your MP3 player.

This FM transmitter use a chip made by Maxim Integrated Products, the MAX2606. The VCO (Voltage Controlled Oscillator) in this IC uses a Colpitts oscillator circuit. The variable-capacitance (varicap) diode and feedback capacitors for the tuning have also been integrated on this chip, so that you only need an external inductor to fix the central oscillator frequency.

The supply voltage to the IC should be between 2.7 and 5.5 V, the current consumption is between 2 and 4 mA. With values like these it seemed a good idea to supply the circuit with power from a USB port. A common-mode choke is connected in series with the USB connections in order to avoid interference between the circuit and the PC supply.

The stereo signal connected to K1 is combined via R1 and R2 and is then passed via volume control P1 to the Tune input of IC1, where it causes the carrier wave to be frequency modulated. Filter R6/C7 is used to restrict the bandwidth of the audio signal. The setting of the frequency (across the whole VHF FM broadcast band) is done with P2, which is connected to the 5 V supply voltage.

The transmitter PCB designed uses resistors and capacitors with 0805 SMD packaging. The size of the board is only 41.2 x 17.9 mm, which is practically dongle-sized. For the aerial an almost straight copper track has been placed at the edge of the board. In practice we achieved a range of about 6 metres (18 feet) with this. There is also room for a 5-way SIL header on the board. Here we find the inputs to the 3.5 mm jack plug, the input to P1 and the supply voltage. The latter permits the circuit to be powered independently from the mains supply, via for example three AA batteries or a Lithium button cell. Inductor L1 in the prototype is a type made by Murata that has a fairly high Q factor: minimum 60 at 100 MHz.

Take care when you solder filter choke L2, since the connections on both sides are very close together. The supply voltage is connected to this, so make sure that you don’t short out the USB supply! Use a resistance meter to check that there is no short between the two supply connectors before connecting the circuit to a USB port on a computer or to the batteries.

P1 has the opposite effect to what you would expect (clockwise reduces the volume), because this made the board layout much easier. The deviation and audio bandwidth varies with the setting of P1. The maximum sensitivity of the audio input is fairly large. With P1 set to its maximum level, a stereo input of 10 mVrms is sufficient for the sound on the radio to remain clear. This also depends on the setting of the VCO. With a higher tuning voltage the input signal may be almost twice as large (see VCO tuning curve in the data sheet). Above that level some audible distortion becomes apparent. If the attenuation can’t be easily set by P1, you can increase the values of R1 and R2 without any problems.

Measurements with an RF analyzer showed that the third harmonic had a strong presence in the transmitted spectrum (about 10 dB below the fundamental frequency). This should really have been much lower. With a low-impedance source connected to both inputs the bandwidth varies from 13.1 kHz (P1 at maximum) to 57 kHz (with the wiper of P1 set to 1/10).

In this circuit the pre-emphasis of the input is missing. Radios in Europe have a built-in de-emphasis network of 50 μs (75 μs in the US). The sound from the radio will therefore sound noticeably muffled. To correct this, and also to stop a stereo receiver from mistakenly reacting to a 19 kHz component in the audio signal, an enhancement circuit is published elsewhere in this issue (Pre-emphasis for FM Transmitter, also with a PCB). Author: Mathieu Coustans, Elektor Magazine, 2009


MP3 FM Transmitter Parts List
Resistors (all SMD 0805)
R1,R2 = 22kΩ
R3 = 4kΩ7
R4,R5 = 1kΩ
R6 = 270Ω
P1 = 10kΩ preset, SMD (TS53YJ103MR10 Vishay Sfernice, Farnell # 1557933)
P2 = 100kΩ preset, SMD(TS53YJ104MR10 Vishay Sfernice, Farnell # 1557934)
Capacitors (all SMD 0805)
C1,C2,C5 = 4μF7 10V
C3,C8 = 100nF
C4,C7 = 2nF2
C6 = 470nF
Inductors
L1 = 390nF, SMD 1206 (LQH31HNR39K03L Murata, Farnell # 1515418)
L2 = 2200Ω @ 100MHz, SMD, common-mode choke, 1206 type(DLW31SN222SQ2L Murata, Farnell #1515599)
Semiconductors
IC1 = MAX2606EUT+, SMD SOT23-6 (Maxim Integrated Products)
Miscellaneous
K1 = 3.5mm stereo audio jack SMD (SJ1-3513-SMT
CUI Inc, DIGI-Key # CP1-3513SJCT-ND)
K2 = 5-pin header (only required in combination with 090305-I pre-emphasis circuit)
K3 = USB connector type A, SMD (2410 07 Lumberg, Farnell # 1308875)


Notice. The use of a VHF FM transmitter, even a low power device like the one described here, is subject to radio regulations and may not be legal in all countries.

Source: FM Transmitter for MP3 Player Powered from USB

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USB Powered FM Transmitter Circuit

The radio is a very simple media. Just a few electronic components able to transmit on the FM band. Aside of course, make these frequencies is illegal. This FM transmitter, which is powered by USB, recovers output on your computer or your MP3 player to the relay on the tape FM (frequency 108 MHz).

For Assemblying this FM transmitter kit, a beginner will take about 3 hours to tinker the issuer, an electronics hobbyist will have built in about 30 minutes.

FM Transmitter Construction
It is not necessary to drill the transmitter PCB. All components will be soldered to the plate with their legs folded, like this:
The two transistors and the LEDs are polarized:
The transistor has a flat side, the LED a foot longer than the other is the anode (A), the other is the cathode (K). The audio cable (minijack) must be transformed from a stereo cable into a cable

Mono Sound:
Soldering together the white and red cables, leaving aside the yellow cable (mass). The frequency setting will be turning the variable capacitor gently with a screwdriver or thin cardboard but rigid.

FM Transmitter Parts List

• 1 Ohm resistor 510 (green - brown - brown)
• 100 resistor 1 kOhm (brown - black - yellow)
• 1 MOhm resistors (brown - black - green)
• 1 capacitor 0.1 uF (0.1)
• 1 nF capacitor 47 (0.047)
• 1 capacitor 4.7 pF (479)
• 2 pF capacitors 22 (22)
• 1 variable capacitor 1.5 pF ... 15
• 2 transistor BF 246 (F246A)
• 1 red LED
• 1 audio cable (minijack)

Source: Pi-Radio Mini-Shop

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Super J-Pole VHF Antenna Design

Here's the super J-pole antenna design that is a regular J-pole antenna with a 1/2 wave element mounted above with a 1/2 wave phasing harness connecting the element to the bottom part of the J-pole. The antenna can be built for any band.

This J-pole antenna is made with copper pipe. The phasing harness is made from large diameter copper wire. The harness can be made by bending the wire into a square "U" shape, then bending it into a circle by bending it around a large coffee can.

The top element must be insulated from the bottom part of the antenna. A fiberglass rod can be used as an insulator by inserting it into the pipe. A wooden dowel rod can be used also, but it should be coated with epoxy to waterproof it. Be sure to cap off the top element and mounting stub to keep water out. PVC pipe caps can be used.

The feedpoint for the antenna is in the same place as a normal J-pole, between the 3/4 wave element and the 1/4 wave matching stub. Attach the shield braid of the coax to one side and the center conductor to the other side. It doesn't matter which side is attached to which. Move the feedpoint up & down to find the best match with an SWR meter or antenna analyzer. Once a good match is found, secure the coax to the antenna. A good way to do this is use small hose clamps. One method I've found that works well is to solder stiff wire to a mounting hole and the center pin of an SO-239 jack, so cables with PL-259 connectors can be connected to it. BNC jacks can also be used.

The super J-pole built for 2 meters is around 9 feet tall, depending on the mounting stub. If constructed properly, it should give around 3 dB gain over a regular J-pole. This antenna has a surprisingly broad bandwidth, giving a VSWR of 1.5:1 or less from 133 to 175 MHZ according to my analyzer.

Source: Super J-Pole

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