Please note
This product reveiw includes comments from SCWIS, as a user of the unit in support of a part 15 broadcast operation, and includes a significant technical discussing contributed by the MWA website.
Part15.us is grateful to MWA, the Medium Wave Alliance, for permission to include the technical discussion in this review.
Please note
This product reveiw includes comments from SCWIS, as a user of the unit in support of a part 15 broadcast operation, and includes a significant technical discussing contributed by the MWA website.
Part15.us is grateful to MWA, the Medium Wave Alliance, for permission to include the technical discussion in this review.
User Report
This is a top quality product, and an amazing value. The PLL circuit is well designed and runs accurately across the dial. This is a “set it and forget it” system with an external power supply for improved reliability. Frequency is rock solid and modulation is controlled at the source.
Gizmo AM Broadcast Transmitter
• Payment via Paypal
• Order placed 6/20
• Item received 7/3
• Well packed, bubble wrap & box
Includes
• Transmitter in plastic case
• Three meter wire antenna
• Mono patch cord
• Twelve page manual
Nice, clean layout inside. PLL circuit selector easy to use, signal rock solid. Note the threaded brass inserts in the case closure corners. Very nice touch. Includes the ubiquitous adhesive rubber feet. Appears to be a hand soldered, single sided PC board, clean and straight. Antenna terminal is a rugged post, dip switches are easy to reach and set.
The Gizmo is a great place for beginners to start, and it’s a simple way to add a new channel to an existing operation.
When you add the Wizard antenna, also available from Vintage Components, broadcasting is simplicity itself. All you do is plug in your power supply, plug in your source, set up the antenna, select your frequency, turn on the power, and fine tune the antenna match. That’s all there is to it, and you’re on the air!
Here’s an example of a simple set up used to broadcast Old Time Radio MP3s to the author’s apartment complex. This set up is running in the corner of a closet, with only the base of the antenna and two ground radials used for transmitting. Range covered 30 units.
This product is entirely too much fun. Many hobbyists might say “Gee, I could probably build that myself” and many of us could – the point is, why bother? For a very reasonable price, and little more than 10 minutes set-up time, you can be on the air RIGHT NOW!!!
Technical notes on the Gizmo low power AM transmitter
What do you get when you marry a classic PLL oscillator circuit to a slightly modified version of the Wenzel transmitter circuit, and build it on a compact and well-laid-out circuit board the size of a baseball card? You get the Gizmo, an affordable, pocket-size AM transmitter.
In early 2003, the Gizmo was the only assembled, PLL-controlled transmitter available for under $90. It has good audio quality and its range with the supplied wire antenna is sufficient to put a strong signal throughout your house, and possibly your immediate neighbor’s house, with weaker reception extending out a block or two.
The Gizmo does have room for improvement in a few areas. First off, it is extremely vulnerable to having a buzz or whine in the signal unless you power it from a super highly filtered, ultra-perfect power supply. With the specimen Gizmo that I borrowed from a friend to write this review, this problem was somewhat relieved when I put my finger on the middle chip in the oscillator section, which suggests that a little extra bypass capacitance somewhere in that neighborhood might help.
I’d like to thank Mr. Phil Bolyn for proofreading the diagrams below, and for offering the following comments about the design of the Gizmo:
“The RF output consists of a differential amplifier (Q3 and Q4). When Q3 is on, Q4 is off, and vice versa. The transistors are driven to saturation during their respective half cycles. The differential pair method produces highly linear modulation and can be driven to 100% modulation (not true for other, simpler schemes). ”
“Q5 modulates the RF by varying the current to the diff pair at audio rate. The resulting current at Q4 collector is a square wave at RF frequency, amplitude modulated by audio from Q5. The output tank circuit filters out the harmonics. C14 and C15 help to reduce ringing at the current peaks in Q2 and Q4. This should improve the output signal, but I am not convinced they are needed based on spice simulations I have done on a similar circuit. ”
“One problem I see with the Gizmo is the method of tuning the output using a fixed inductor and 4 switch-selectable capacitors. The 4-pole dip switch allows selecting 16 different cap values, but the Q of the output tank is high enough to make this a real compromise. To get max output you would need to carefully select a frequency that matches one of the 16 cap values. Since quiet channels are few in populated areas, it would be pure luck if one of the cap values exactly matched a vacant channel frequency. I would suggest an improvement to the circuit.”
“Mouser Electronics sells an inexpensive 100pf variable trimmer cap. I would add one in parallel to the dip switch to fine tune after selecting the nearest value with the switches.”
You can’t help admiring the Gizmo’s elegance and compactness, and it has helped dozens of people to “get their feet wet” in the LPAM waters without having to build something from scratch or purchase a much more expensive assembled transmitter.