Tech Manifesto

The following is a proof of concept abstract on the iTrip amplifier using the schematic. Its purpose is to increase the signal produced by the Griffin iTrip. Use of the finished device is prohibited (in the US and Canada) as it violates Part 15 of FCC laws. Remember this is a proof of concept abstract. It is a “theoretical exercise” not a definitive solution for medium-long range portable itrip broadcasting. I (nor anyone else) can or will be held responsible to any damage you inflict upon your itrip, ipod, person, other persons or anything else. This is an electrical circuit, which means there will be live electrical points. Be careful. You do all of this at your own risk! The following is a detailed write-up from Binary Elysium.

(Click for larger images.)

About amplifying

One of the basic problems with boosting the signals of these mini FM transmitters is that they were never intended to produce strong signals in the first place. Thus, their output tends to be relatively “dirty” — producing harmonics of the broadcast frequency that can cause interference to other stations. This aspect is generally not a problem with the stock mini-transmitter, since harmonics are usually much lower in power than the broadcast frequency, which is already ultra-low.

But when you start boosting the output of these mini-transmitters, you definitely boost the interference, too.

Apparently griffin did not include anything to attenuate the output (at least to the transmitters technical specs[link]), therefore the iTrip is effectively limited by the size/efficiency of the antenna. There is a bandpass filter at the output to reduce the output harmonics. It looks like a small yellow cube next to the antenna.(Thanks to tan for this information). The real concern is the “splatter” of interference that can screw up other radio stations on the dial in the vicinity of the transmitter. This is exactly what we’re interested in.

Opening the iTrip

The first step in building the itrip_amp is extracting the itrip circuitry from the protective white case (which will void any warranty). The case is sealed with some form of adhesive and is designed to not be reopened once sealed. Nevertheless opening the case is quite simple. In the end we want to have the itrip circuitry out, but the case sill in decent repairable condition as a hole will be drilled for the antenna the circuitry replaced, and the case glued. One method described [link] on Bob’s Blog wedges it apart, and will most likely break the case along the seam as well as off. The use of a razor blade along the seam proves to be much more effective.

To do this you must first remove the four rubber stoppers from both ends. A small flathead screwdriver does this easily. Peel them up then pull them up with your fingers. Now to separate the case take a razor blade and put the blade along the seam then gently tap with a hammer and the case should split evenly along the seam. Turn the itrip over and repeat the process on the other side. Continue along all parts of the itrip until you can easily pull the pieces apart. Do NOT apply more pressure to the razor blade than necessary. You do not want to damage the case or the circuitry inside (or your finger).
Construct the amplifier

Construction of the circuit will largely remain up to you, though some basic guidelines and tips are provided. Note: In the example circuit a 240 ohm resistor was used instead of a 220 only because the supplier delivered the wrong goods, and an exchange could not be bothered. It works fine in the circuit shown. The only side effect is a loss of a few milliamperes in the power coming in, which should be negligible.

Important: It should be noted that the schematic says “0.001 uF transistor (ceramic disk).” This has been recognized as a typo (as far as was able to be determined) and instead should be a .001 capacitor.

Grab a small 2″ x 2″ circuit board, to solder the pieces to. A tip is to leave all connection points and solder points on one side of the board. If you look at the example images, you can see the lead from the resistor where the positive (+ red) wire is attached and itrip antenna wire is on the opposite side than the rest of the connecting points (ground and antenna). This reduces portability.

The Choke

If unable to find a suitable choke, making one is quite easy. The purpose of the choke is to generate a small magnetic field which produces a phase shift (if you want a further why ask the nearest electrical engineer). It is an inductor. Magnet wire is copper or aluminum wire with a thin insulation to prevent short circuits. It is single-strand wire insulated with enamel, varnish, cotton, glass, asbestos or some combination of these. It is common and can be found almost any place that sells wire. Take about a foot of it and wrap 15-17 turns around a pencil. You should wrap it tightly, but loose enough so that you can slide it off. Make sure you leave a length of wire on both ends equal to the length of the leads (the wires sticking out both ends) of the 1k ohm resistor. Take a razor blade or knife and scrape of the very thin insulation around the wire on both ends (where you will solder it). Put the resistor inside the coil (make sure it is not touching). Solder each end of the magnet wire to opposite ends of the leads. The final result should be a resister inside a coil of magnet wire, and the only place the two touch is where the solder is.

The 12V car adapter

The adapter used in the proof was a modified car adapter for a Motorola cell phone. The end was snipped off, and the wires were stripped to allow attachment via alligator clip (as shown) or soldering. The adapter used in the proof had to be modified, as there was a circuit inside the large black part that reduced amperes (or something - but it essentially changed the current that ended up being outputted through the wires at the end). It would be a good idea to test (using a volt meter) the output and continuity of the adapter you use. A good easy fix that should work on all adapters is to just solder one (or two) wires (for + and -) from the back to the front of the circuit (of one exists) to completely bypass the circuit. Of course don’t just do this, you must look at yours and determine what needs to be done.

Hooking it together

In the example circuit, the antenna, itrip, and + and - wires were connected to the amplifier using alligator clips. This is a simple way to connect everything together, if you do not wish the itrip to remain permanently attached to the amplifier. The power wires were not soldered because once that is done the amplifier can only be used in a car. Plans are in development to power the circuit with a battery making it truly portable.

The Antenna

Telescoping antennas can be found and purchased at radio shack for less than 5 dollars. A less elegant solution is take about 30 inches of 12 gauge wire and strip off the insulation. Since Wavelength = Speed of sound / Frequency it can be determined that the ideal length of antenna is 9.1 - 11.2 feet long (to cover 88.1 - 107.9). Lacking experts on antennas and radio technology the given length of 85 cm was used. Note: It would be interesting to see how the amplifier operates with the correct length of antenna.

Afterward

Obviously the alligator clips make it a hassle to haul around as the circuit might be damaged. A simple solution to this would be to put the circuit in a plastic box of some kind and run wires to the connecting point out of holes drilled in the box (don’t forget to label). Even this though is dangerous as the wires can be pulled out. A better idea would be to buy plugs and attachments. But that is beyond the scope of this abstract.

If you want a quick little (portable) mod to boost your itrips power to get rid of some of that static in the car, run the itrip’s antenna through one of the rubber stopper holes and point it up. Alternatively drill a small hole in the top of the itrip and run the antenna up through it.

Field Tests

A few initial tests show that the circuit does in fact increase the antenna signal, but not enough to effectively overpower a real radio station’s signal (say because you like listening to your ipod on 99.9 - Pirating radio is not condoned nor encouraged). It does however increase range and signal, it has been tested successfully to 55 feet - line of sight. Within 22 feet no static whatsoever. Beyond 22 feet static increases in a manner directly related to an increase in distant from the transmitting unit. Enjoy!