Introduction to the Component
The CBL ASSY BNC JACK RG58 9" from Amphenol Custom Cable (SKU: Q-090090008009i) is a pre-assembled coaxial cable assembly featuring a BNC jack connector on one end and a 9-inch length of RG58 coaxial cable. This component is ideal for a hands-on tutorial because it provides a ready-to-use, shielded interconnect for high-frequency signals up to several hundred MHz, while the RG58 cable offers a controlled 50-ohm characteristic impedance. The BNC jack allows quick, secure connections to test equipment or other BNC-based circuits. Its simplicity and repeatable performance make it perfect for teaching impedance matching, signal integrity, and practical RF design principles without requiring complex cable fabrication.
Design Requirements and Specifications
Consider a practical circuit: a 50-ohm impedance-matched buffer amplifier driving a 50-ohm load over a short cable. The design must ensure minimal signal reflection and loss at frequencies up to 100 MHz. Key specifications: input impedance 50 ohms, output impedance 50 ohms, bandwidth DC to 100 MHz, gain 0 dB (unity), and maximum cable length 9 inches (0.23 meters). The cable assembly itself must have a characteristic impedance of 50 ohms, with a maximum VSWR of 1.2:1 across the band. The BNC connector must provide a low-resistance, shielded connection to maintain signal integrity. The overall circuit must operate from a single 5V supply with less than 50 mA current draw.
Step-by-Step Design Process with Calculations
Start by selecting an appropriate amplifier. A common choice is the AD8009 high-speed op-amp, which offers 1 GHz bandwidth and 50-ohm output drive capability. Configure it as a non-inverting amplifier with gain set by Rf and Rg. For unity gain, set Rf = 0 ohms and leave Rg open. The input impedance is set by a 50-ohm resistor to ground at the input. Calculate the -3dB bandwidth: the AD8009 has a gain-bandwidth product of 1 GHz, so at unity gain, bandwidth is 1 GHz, well above our 100 MHz requirement. The cable assembly’s length (9 inches) introduces a delay of about 1.5 ns (propagation velocity ~0.69c for RG58), which is negligible at 100 MHz (period 10 ns). Ensure the output impedance matches 50 ohms by adding a 50-ohm series resistor at the op-amp output if needed. Power supply decoupling: use 0.1 uF ceramic capacitors close to each supply pin. The total current draw is approximately 15 mA from the op-amp plus 10 mA from the 50-ohm load, well within the 50 mA limit.
Component Selection Rationale for the Complete BOM
The BOM includes: AD8009 op-amp (U1) – chosen for high bandwidth and 50-ohm drive capability; 50-ohm input resistor (R1, 1% metal film) – sets input impedance; 50-ohm output resistor (R2, 1% metal film) – ensures output impedance match; 0.1 uF ceramic capacitors (C1, C2) for supply decoupling; 10 uF electrolytic capacitor (C3) for bulk decoupling; PCB with 50-ohm microstrip traces; and the Amphenol Custom Cable assembly (SKU Q-090090008009i). The cable assembly is selected for its pre-terminated BNC jack and precise 50-ohm impedance, eliminating the need for manual cable assembly. The BNC jack provides a robust, shielded connection to external equipment. All resistors are 0805 size for easy hand soldering.
Simulation Tips and What to Look For
Use SPICE simulation with the AD8009 model. Set up a transient analysis with a 100 MHz sine wave input at 1V peak-to-peak. Observe the output waveform for distortion, overshoot, or ringing – these indicate impedance mismatch or instability. Perform an AC sweep from 10 kHz to 200 MHz to check the -3dB bandwidth; it should be flat within 0.1 dB up to 100 MHz. Plot the input and output impedances using a Z-parameter analysis – both should be close to 50 ohms (real part) and near zero reactance. Simulate the cable as a 50-ohm transmission line with a 9-inch length; its effect should be minimal at 100 MHz (phase shift ~5 degrees). Look for any oscillations in the time domain, especially near the output – add a small series resistor (10 ohms) if needed to dampen ringing.
Prototype Build and Testing Methodology
Build the circuit on a small piece of copper-clad board (dead bug style) or a perfboard with ground plane. Place the op-amp close to the input and output connectors. Solder the 50-ohm input resistor directly at the BNC input jack. Connect the cable assembly’s BNC jack to the input; the other end can be left open or connected to a signal generator. For the output, use a second BNC jack (not provided) or directly solder a 50-ohm load. Power the circuit with a 5V supply. Test with an oscilloscope (50-ohm input) and a signal generator. Drive the input with a 100 MHz sine wave at 0 dBm (0.316 Vrms). Measure the output amplitude; it should be within 0.5 dB of the input. Check for reflections by looking at the input signal with a TDR or by observing any standing waves on the cable.
Performance Verification and Optimization
Measure the frequency response using a network analyzer: S21 should be flat within 0.2 dB from DC to 100 MHz. Measure VSWR at the input; it should be below 1.2:1. If VSWR is high, check the 50-ohm resistor values and solder joints. Use a spectrum analyzer to check for harmonics or spurious signals – they should be at least 40 dB below the fundamental. Optimize by ensuring short, direct traces and minimizing parasitic capacitance at the op-amp inputs. If bandwidth is limited, reduce stray capacitance by lifting the input pin slightly off the board. The 9-inch cable assembly should contribute negligible loss; measure insertion loss at 100 MHz – it should be less than 0.1 dB. For final verification, test with a 100 MHz square wave; the output should show clean edges with no overshoot or ringing. This tutorial demonstrates how a simple, pre-assembled coaxial cable can be integrated into a high-frequency design with predictable performance.
