The Hickok 380 Series Frequency Counters are compact, state-of-the-art autoranging instruments designed for precision frequency measurements. The series includes four models: 380, 380X, 385, and 385X, each with different capabilities.
Function Description:
These counters measure frequencies from 1Hz up to 512MHz, depending on the model. They feature automatic gate time selection (1 second or 0.1 second) to provide the most accurate display. The decimal point on the display is automatically positioned as the gate time changes. A front-panel push-on-push-off switch allows manual selection of the 0.1-second gate time for rapid display updates (5 updates per second).
Model-Specific Features:
- Model 380: Measures 1Hz to 80MHz. Signal input is via a front-panel BNC connector at 1 megohm impedance. It includes provision for an external 1MHz time base frequency standard via a rear-panel BNC connector, with selection between internal and external time base by a rear-panel slide switch. Switching to external time base without a connected standard can freeze the display. The time base accuracy is 10ppm, determined by an ECL-driven crystal oscillator.
- Model 380X: Measures 1Hz to 80MHz. Similar to Model 380 in operation, but with a temperature-compensated crystal oscillator (TCXO) for a higher time base accuracy of 1ppm. It also provides a buffered 1MHz TTL level signal output from a rear-panel BNC connector, capable of driving up to four additional Hickok 380 series counters through up to 20 feet of coaxial cable, allowing other units to achieve 1ppm stability.
- Model 385: Measures 1Hz to 512MHz. Features two front-panel BNC connectors for signal input: one for 1Hz to 80MHz (1 megohm impedance) and another for 10MHz to 512MHz (50 ohms impedance). A push-on-push-off switch selects between these inputs. It includes provision for an external 1MHz time base frequency standard, similar to Model 380. The time base accuracy is 10ppm, determined by an ECL-driven crystal oscillator.
- Model 385X: Measures 1Hz to 80MHz or 10MHz to 512MHz. Combines the input capabilities of Model 385 with the high-stability TCXO of Model 380X, offering 1ppm time base accuracy. It also provides a buffered 1MHz TTL level signal output for driving other counters, similar to Model 380X.
Important Technical Specifications:
- Frequency Range:
- 380, 380X: 1Hz to 80MHz guaranteed.
- 385, 385X: 1Hz to 512MHz guaranteed.
- Accuracy:
- 380, 385: ±1 count + 10ppm time base uncertainty.
- 380X, 385X: ±1 count + 1ppm time base uncertainty.
- Display: 7 LED digits, 0.3" high, with auto-positioned decimal for kHz, 2-digit MHz, and (for 385, 385X) 3-digit MHz range indication.
- Operating Modes: Switch selectable AUTO (autoranging) or SPEED READ (fast update).
- Gate Times: 1 second or 0.1 second (automatically selected in AUTO mode), 0.1 second (fixed) in SPEED READ mode.
- Resolution:
- 1Hz up to 10MHz (1-second gate), 10Hz 10MHz to 80MHz (0.1-second gate). In SPEED READ mode, 10Hz at all frequencies (0.1-second gate).
- For 385, 385X 10MHz to 512MHz input: 10Hz from 10MHz to 100MHz (1-second gate), 100Hz from 100MHz to 512MHz (0.1-second gate). In SPEED READ mode, 100Hz at all frequencies (0.1-second gate).
- Display Update:
- AUTO mode: 1.1 seconds below 10MHz, 0.2 seconds 10MHz to 80MHz.
- SPEED READ mode: 0.2 seconds.
- For 385, 385X 10MHz to 512MHz input: 1.1 seconds below 100MHz, 0.2 seconds 100MHz to 512MHz.
- Input Characteristics:
- 1Hz-80MHz Input: 1MΩ, 25pF impedance.
- 10MHz-512MHz Input (385, 385X): 50Ω impedance.
- Sensitivity (Guaranteed 0-50°C):
- 1Hz-80MHz Input: 25mVrms to 10MHz, 50mVrms to 50MHz, 80mVrms to 80MHz.
- 10MHz-512MHz Input (385, 385X): 15mVrms 50MHz to 100MHz, 25mVrms to 300MHz, 50mVrms to 400MHz and 10MHz to 50MHz, 250mVrms to 512MHz.
- Maximum Input Without Damage:
- 1Hz-80MHz Input: Refer to graph (Figure 1-3).
- 10MHz-512MHz Input (385, 385X): 3.5Vrms (+24dBm).
- Power Requirements: 105-125 Vac/210-250 Vac, 50-400Hz, 14 watts max.
- Dimensions: 8-1/2"w x 4"h x 6"d (not including handle).
- Weight: 4 pounds.
- Handle: Continuously variable handle acts as a tilt stand.
- Time Base (Ref. 25°C after 30 min. warm-up):
- 380, 385: ECL-driven crystal oscillator, 10MHz frequency, ±0.1ppm setability, 0.1ppm change for ±10% line variation, 10ppm change from 0°C to 50°C, 5ppm/year aging rate.
- 380X, 385X: Temperature Compensated Crystal Oscillator (TCXO), 10MHz frequency, ±0.1ppm setability, 0.1ppm change for ±10% line variation, ±1ppm from 0°C to 40°C, 1ppm/year aging rate.
- External Time-Base Input: BNC (rear panel), 1MHz frequency, 1Vrms sensitivity, 3.9kΩ impedance, 25Vrms maximum input, AC coupling.
- High-Stability Time-Base Output (380X, 385X): BNC, 1MHz frequency, TTL compatible level, 200Ω impedance, capable of driving up to four 380 or 385 counters.
Usage Features:
- Simple Operation: Designed for straightforward "hands-off" operation. After initial setup, minimal manipulation is required.
- Autoranging: Automatically selects the optimal gate time (0.1 or 1 second) for resolution.
- Speed Read Mode: Allows manual selection of 0.1-second gate time for rapid display updates, useful for unstable frequencies or quick checks.
- External Time Base: Models 380 and 385 can use an external 1MHz time base for higher accuracy. Models 380X and 385X provide a high-stability 1MHz output to enhance the accuracy of other counters.
- Display Hold: For Models 380 and 385, a frequency reading can be retained indefinitely on the display by switching the rear panel TIMEBASE switch to EXTERNAL (with no external input connected).
- Signal Cable Considerations: Recommendations for using 50-ohm coaxial cables (RG-58) and termination to prevent reflections, especially for high frequencies. A 10:1 probe is an alternative for minimal loading.
- Squarewave/Pulse Train Measurement: Suggests using a damping resistor in series with the signal line to prevent ringing and miscounts, especially for the 80MHz input.
- Radio Transmitter Carrier Frequency Measurement: Provides a basic test setup using a 50-ohm dummy load and a signal pick-off (capacitive or resistive voltage divider) to protect the counter from overload.
- A.M. Modulated Signals: Advises against measuring carrier frequency with amplitude modulation present, as it can lead to inaccurate readings due to signal level fluctuations. Suggests using a demodulator probe or a detector with a low-pass filter for measuring the modulating signal.
- Line Frequency Measurement: Describes how to measure line frequency, noting that the counter's accuracy makes it impractical for precise line checks. Requires a grounded outlet and direct connection of the 80MHz input jack's center conductor to the power line.
Maintenance Features:
- Solid State Design: Uses solid-state components, minimizing "wear-out" issues.
- Fuse Protection: Equipped with an internal 3AG 0.25A Slo-Blo fuse for power supply protection. Instructions are provided for locating and replacing the fuse if the display does not illuminate.
- Calibration Procedures: Detailed instructions for factory calibration, to be performed by qualified service personnel. This includes:
- 80MHz Input Sensitivity Adjustment: Sets the threshold voltage to ensure proper measurement sensitivity and prevent false readings.
- 10ppm TIMEBASE Oscillator Adjustment (Models 380/385): Calibrates the internal master oscillator using a standard frequency signal source.
- 1ppm Time Base Temperature Compensated Crystal Oscillator (TCXO) Adjustment (Models 380X/385X): Calibrates the high-stability TCXO to within 0.1ppm using a highly accurate 1MHz frequency standard.
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