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The Hewlett-Packard 8730 series of PIN modulators are variable RF attenuators designed for precise control of RF signals. These devices maintain a nearly constant input and output impedance across all attenuation levels, making them versatile for various applications. The series covers a broad coaxial frequency range from 0.8 to 12.4 GHz, divided into four overlapping bands, and also includes X-band waveguide units.
The PIN Modulator operates as a high-speed, current-controlled absorption type attenuator. It is a three-port device that accepts RF power at either end-port and provides a modulated RF output at the other end-port, depending on the applied BIAS signal. Essentially, it functions as a variable RF attenuator where the attenuation level is controlled by DC current and voltage.
Each PIN unit incorporates a low-pass filter, two high-pass filters, a number of PIN diodes, and a 50-ohm strip transmission line (or ridged waveguide for higher frequency models). The core component, the PIN diode, is a silicon junction diode with P and N trace regions separated by an intrinsic (I) semiconductor layer. At frequencies above 100 MHz, the PIN diode acts like a resistor whose equivalent resistance is inversely proportional to the amount of stored charge in the I layer. Increasing forward bias current (negative voltage) increases stored charge and decreases resistance, while reverse bias depletes stored charge, maximizing resistance.
When PIN diodes are forward-biased, their equivalent resistance is low (around 30 ohms), causing most RF energy to be absorbed. When reverse-biased, resistance is high (thousands of ohms), allowing microwave signals to pass. This mechanism enables precise control over RF signal attenuation.
The 8730 series offers two main models: "A" models provide at least 35 dB of attenuation range, while "B" models offer at least 80 dB.
Maximum RF Power: The maximum input rating (peak or CW) for all PIN Modulators is 1 watt. Exceeding this limit can cause internal damage.
Maximum Bias Input: DC voltage applied to the BIAS input must not exceed +20 volts or -10 volts to prevent internal damage.
Attenuation vs. Frequency: Attenuation varies with frequency, being minimal at the ends of the frequency band. This variation is more pronounced at higher attenuation levels.
Impedance Match: The PIN Modulator maintains a good impedance match across its frequency range and attenuation levels, minimizing frequency pulling effects.
Temperature Stability: Attenuation varies with temperature. With constant voltage, attenuation rises with increasing temperature; with constant current, it drops. This suggests that an optimal voltage with a series resistor could provide temperature compensation, though this is not built into the PIN Modulator.
The PIN Modulators are controlled by DC current and voltage. Applying +5 Vdc to the BIAS input connector allows RF signals to pass with minimum insertion loss. Applying a current equal to the value stamped on the instrument label (around 0.7 Vdc) provides rated attenuation (35 dB for "A" models, 80 dB for "B" models). Varying the forward bias current between zero and the stamped value allows any attenuation level below maximum to be established.
PIN Modulators can be integrated into a closed-loop leveling system to maintain constant RF power levels. A typical system involves a sampling device, a detector, and a comparison device that provides a bias current to adjust the PIN Modulator's attenuation. Leveling capability is primarily determined by the loop equipment. For RF swept frequency rates equal to or longer than 30 seconds/octave, leveled power can be held constant within ±0.2 dB (plus coupler variation). Faster swept frequency rates can be achieved using directional detectors and leveler amplifiers, maintaining constant power within ±0.2 dB (plus frequency response of directional detector).
The PIN Modulator can amplitude-modulate RF signals with time-varying signals. This is achieved by superimposing a time-varying current onto a DC bias current that sets a specific attenuation level. The modulating signal's peak amplitude must not exceed the base attenuation level to avoid peak clipping. The HP Model 8403A Modulator is designed for AM applications, offering frequency compensation up to 10 MHz. The total percent of modulation is limited to about 96.5% for 35 dB attenuation and nearly 100% for 80 dB attenuation.
Modulation Distortion: AM systems using PIN Modulators can experience envelope distortion, which is a function of peak attenuation and total attenuation range. To minimize distortion, the reference level for the modulating signal should be slightly greater than its peak amplitude. Shaping circuits can be incorporated to limit overall distortion.
The PIN Modulator can also function as a pulsing or switching device for RF power levels. This involves applying rated forward bias to switch the RF off and a +5 Vdc level to switch it on. Rise and fall times typically range from 15 to 40 nanoseconds.
Repair Policy: The PIN Modulator CANNOT be repaired in the field. If a unit is found faulty, it must be returned to the factory through a local field office. Removing the screws holding the PIN diodes can cause contamination and high SWR, leading to permanent damage.
Bias Input Check and Repair: If the bias input assembly is damaged, it can be replaced. It is recommended to replace the entire assembly if any part is faulty. To check, remove the PIN Modulator and measure its resistance with an ohmmeter. The filter resistance should be less than 5 ohms, and resistance between the BNC center conductor and shell should be infinite.
Dessicant Check: The desiccant should be inspected regularly. If it appears pink or red, it should be removed, dried, or replaced. This applies to units with serial prefixes 815- (8731A thru 8735A) and 816- (8731B thru 8735B).
Performance Check Procedure: A detailed procedure is provided to check PIN Modulator performance for incoming inspection, periodic evaluation, and troubleshooting. These tests are performed without accessing the interior of the PIN Modulator. The specifications in Table 1 serve as performance standards.
Test Equipment Required: A list of recommended test equipment is provided, including DC power supplies, voltmeters, ammeters, SWR meters, crystal detectors, resistors, RF sources, RF attenuators, power meters, spectrum analyzers, and microwave amplifiers. Instruments other than those listed may be used if their performance meets or exceeds the critical specifications.
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