This video provides a comprehensive teardown, reverse engineering, and functional analysis of a vintage Elmer SP-668 UHF antenna filter, likely used in a base station such as RAF Neatishead. The large, pressurized unit features four mechanically tuned LC networks adjusted by a stepper motor with an optical encoder for precise positioning. The control electronics, manufactured in 1979, rely on CMOS logic (counters, flip-flops, 555 timers) to process a 12-bit input, determining the filter's center frequency across high and low ranges. Functional testing demonstrated a tunable range from approximately 211 MHz to 300 MHz with a 100 kHz frequency resolution, though performance varied at range extremes. A notable discovery during reverse engineering was a PCB routing error on a hex inverter that was subsequently corrected.
Summarizing the UHF Antenna Filter Teardown & Analysis
0:00:05 Introduction to the Unit: The video begins with an introduction to a large Elmer SP-668 UHF antenna filter, identified as potentially coming from an RAF base station.
0:00:40 External Features & Pressurization: The filter unit includes a manometer, indicating it operates under pressure (max 2 bars), and features an N connector for input, a specific coaxial connector for output, and a Sub-D 25 for control.
0:01:36 Internal Mechanical Tuning: Inside, four beautiful variable capacitors form LC networks. The tuning involves rod inductors and variable coupling between stages, all precisely adjusted by a central shaft.
0:03:10 Stepper Motor & Optical Position Detection: The tuning mechanism is driven by a stepper motor (initially thought to be DC) and includes an optical detection system with a wheel, apertures, and photodiodes to precisely track its position.
0:03:54 Vintage Control Electronics (1979): The control system comprises two circuit boards from 1979, featuring CMOS ICs such as 4029 counters, 4013 D flip-flops, a 555 timer, and other logic gates for motor control and counting.
0:05:34 Stepper Motor Driver Circuit: The motor driver uses four transistors and diodes in an H-bridge configuration, with thorough decoupling provided by pass-through capacitors and other passive components.
0:06:58 Initial Functional Test: The presenter tests the motor's response using only the four most significant bits (8-11) of the input, demonstrating its movement and range selection capabilities.
0:09:19 RF Testing Setup: An adapter (PL259 to the specific output connector) is built to test the filter's frequency response with a spectrum analyzer.
0:10:00 High Range Frequency Response: In the high range, the filter achieved a minimum of approximately 300 MHz. The center frequency resolution was determined to be 100 kHz (a 10% change in input code shifted the frequency by about 10 MHz).
0:10:50 Low Range Frequency Response: The low range allowed tuning down to approximately 211 MHz, though performance degraded at the extremes. A flatter response was observed around 220 MHz, with a bandwidth of roughly 500 kHz for a 5 MHz span.
0:13:01 Reverse Engineering - Counting Logic: Detailed reverse engineering revealed three decimal counters receive a 12-bit input as a preset value. A divided clock signal drives the stepper motor and counters, with an overflow output stopping the motor. An optical sensor establishes the 'home' position.
0:14:08 Identified PCB Design Flaw: A significant routing error was found on the PCB: pin 13 of a 4049 hex inverter was incorrectly connected to ground, which, while not immediately catastrophic for CMOS, could impact reliability. This was corrected.
0:14:46 Range Selection & Offset Logic: A D flip-flop, along with the range input, controls the motor's stop condition. The high range requires two pulses (binary code + 1000 additional counts) to achieve a 100 MHz offset, while the low range requires a single pulse.
0:16:23 Clock Generation with Ramp Modulation: A 555 timer generates the clock signal for the motor and counters. A crucial ramp modulation, created by an RC network with a 1-second time constant, ensures smooth acceleration/deceleration of the motor to manage torque and inertia.
0:17:28 Stepper Motor Direction Control: Another D flip-flop on a separate board manages the stepper motor's direction, reversing it after the first cycle (detected by the optical sensor) until the final target frequency is reached based on the binary input.
0:18:17 Conclusion: The video concludes by offering the complete schematic (link in description) and summarizing the findings on this unique and well-engineered vintage UHF filter.