Transmitter Circuit

Switchable RF Probe-Watt Meter

2009-11-15T08:24:00.000-08:00

Here is one of the most trusted homemade test instruments to have, once you get your FM transmitter up and running. For sustained loads of up to 2 watts. This simple little device is capable of finding your FM transmitters: unloaded RMS output voltage, oaded RMS output voltage, output impedance, and output wattage.

Switchable RF Probe/Watt Meter Construction

First, print out the PCB template to the left. Send the picture to a graphics program, such as Paint, and squeeze and/or stretch to get a printout of the said dimensions. Once that is done, drill your hole for a SPST Sub-mini switch (Radio Shack part # 275-645A). Then mimmick the same routing as to the template. Use a single sided PCB. If you only have a double-sided PCB, etch away the backside.

Once you have etched your PCB, then go ahead and look at the picture below for soldering and wiring all of the components to the PCB.

Once you have completed with all the soldering/wiring of the components to the PCB. You are now ready to understand how this little beauty works ! Do take a look at the picture to the right, it shows my completed RF Probe/Watt Meter. Yours should look similar to that. Check to make sure all copper routings show continuity and that all soldering joints are good. Also make sure your germanium diode is oriented correctly into the circuitry. That is, the cathode (negative) side goes toward the output of the meter.

Begin by hooking up your transmitter, RF Probe/Watt Meter and DVM/Analog Meter as laid out in the picture below.

A DVM (Digital Volt Meter) will always give detailed readings to the nearest hundredths, but it usually isn't that reliable in the VHF arena. The analog meter will have a more reliable measurement...although the reading is taken directly upon where the needle rests on the scale therefore giving a less detailed reading. A DVM seems to always give higher readings then the analog meter, when doing tests with our homemade device. If you have the benefit of having both meters. I would strongly suggest making readings with the analog meter.

Source: The Switchable RF Probe/Watt Meter Project

40W Broadband VHF RF Power Amplifier 88-108 MHz

2009-11-15T06:59:00.000-08:00

This RF power amplifier design is to boost the output power of low power FM broadcast band exciters based on Motorola MRF171A MOSFET with power output 40W. To reduce the harmonics to an acceptable level, Integrated 7 pole Chebyshev low pass harmonic filter (LPF) is added at the final section.

Component Choices
As the input power is only half a watt, standard ceramic capacitors and trimmers were used in the input matching circuit. L1 and L2 (refer to schematic) could have been made much smaller, but were kept big for consistency with the inductors used in the output network. On the output network, mica metal clad capacitors and mica compression trimmers were used to handle the power and keep component losses to a minimum. The wideband choke L3 provides some lossy reactance at lower RF frequencies, C8 takes care of AF (audio frequency) decoupling.

The use of an enhancement mode N-channel MOSFET (a positive voltage biases the device into conduction) means the bias circuitry is simple. A potential divider taps off the required voltage from a low voltage stabilised by a 5.6V zener diode. The second 5.6V zener, D2, is fitted as a precautionary measure to ensure excessive voltage are not applied to the gate of the FET, this would certainly result in the destruction of the device. Purists would temperature stabilise the bias current, but as the bias is not critical in this application, this was not bothered with.

A BNC socket had been used for the RF input, due to the low RF input power. I've used N type for the RF output, I don't use BNC for above about 5W and I don't like UHF style connectors. Personally, I don't recommend using UHF connectors above 30MHz.

RF Power Amplifier Schematic-Printed Circuit Board

Power Amplifier Construction
The Power amplifier was constructed in a small aluminium diecast box. RF input and output connections are made by coaxial sockets. The power supply is routed through a ceramic feedthrough capacitor bolted in the wall of the box. This constructional techniques results in excellent shielding, preventing RF radiation escaping from the amplifier. Without it, significant amounts of RF radiation could be radiated, interfering with other sensitive circuits such as VCOs and audio stages, also significant amounts of harmonic radiation could occur.

The base of the power device sits through a cut-out in the floor of the diecast box and is bolted directly onto a small extruded aluminium heatsink. An alternative would have the base of the power device sitting on the floor of the diecast box. This is not recommended for two reasons, both concerned with providing an effective path to conduct heat from the FET. Firstly the floor of the diecast box is not particularly smooth, which results in a poor thermal path. Secondly, having the floor of the diecast box in the thermal path introduces more mechanical interfaces and hence more thermal resistance. Another advantage of the chosen constructional technique is that it correctly aligns the device leads with the top face of the circuit board.

Using the specified heatsink will require the use of forced air cooling (a fan). If you plan not to use a fan, a much bigger heatsink will be required, and the amplifier should be mounted with the heatsink fins vertical to maximise cooling by natural convection. RF Power Amplifier Part List

Source: A Design for a 40W broadband VHF RF Power Amplifier for FM broadcast

VHF RF Power Meter

2009-11-15T04:57:00.000-08:00

Here's a simple instrument for the homebrewer to measure RF power well through VHF. interests. The reference instrument incorporated a built-in analog meter and provision to connect an external DVM. It also utilized a conversion chart to relate
meter readings to RF power.

The DVM (Digital Volt Meter) also displays the correct polarity sign. There’s neither rocket science nor smoke and mirrors here folks: It’s all done simply with op amps and resistors! The schematic tells all.

In the process of designing these basic of power meter circuit functions, it occurred to me that other simple additions would add a lot to the utility of the instrument. Thus, the project grew “on the fly.” One of these extras is a gain-change option that includes an external OFFSET control for the analog display.

For more information about component parts list, power meter process design, and calibration-operation, please download An Advanced VHF Wattmeter (in Pdf).

Source: RF power meter project by Wes Hayward, W7ZOI and Bob Larkin, K7PUA that appears in QST, June 2001.

USB FM Transmitter for MP3 Player

2009-09-09T02:30:00.000-07:00

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

USB Powered FM Transmitter Circuit

2009-09-09T01:43:00.000-07:00

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

Super J-Pole VHF Antenna Design

2009-09-03T09:26:00.000-07:00

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

General Purpose Wide Band Amplifier 50 Hz-100 MHz

2009-09-03T09:10:00.000-07:00

Here is a general purpose wide band amplifier with a wide bandwidth of 50 Hz to 100 MHz. Typical frequencies are 5 and 10 MHz, but any frequency from 50 Hz to 100 MHz can be used.

Output levels of this wide band amplifier are +35 dB at 100 kHz, +30 dB at 10 MHz, +17 dB at 100 MHz noise smaller than 10 uV across 50 ohms with RF input -30 dBm MAX.

Wide band Amplifier Schematic

AADE Filter Design-Analysis FREE Download

2009-09-02T12:29:00.000-07:00

This Filter Software will design most classical passive ladder Low-pass Filter, High- pass Filter, Band-pass Filter, and Band-Reject filters and analyze them in both the frequency and time domains. You can also enter a schematic and parts list from a book or magazine for analysis. Almost All Digital Electronic.

Download AADE Filter Design

DESIGN

Butterworth, Chebyshev, Elliptic (Caur), Bessel, Legendre and Linear Phase: low-pass, high-pass, band-Pass, and band-reject filters.
Coupled Resonator band-pass filters
Crystal Ladder band-pass filters using identical crystals. Ideal for Amateur construction from surplus or microprocessor crystals.

Download AADE Filter Design

ANALYSIS frequency domain

Power Effective Gain
Power Insertion Gain
Voltage Effective Gain
Voltage Insertion Gain
Current Effective Gain
Current Insertion Gain
Input Impedance
Output Impedance
Group Delay
Phase
Return Loss

ANALYSIS time domain

Impulse Response
Step Response
Pulse Response
Tone Pulse Response (for band-pass filters)

Download AADE Filter Design

UTILITIES

You can enter a schematic and parts list from a book or magazine for analysis or modification.
Insert a filter in front of an existing filter or Append a filter to the end of an existing filter: These allow design of wideband band-pass filters consisting of low-pass/high-pass combinations.

More detail for AADE Filter Design and Analysis - Latest Version AADE Filter Design

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2009-09-01T11:32:00.000-07:00

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