Series Overview
This series can be broadly divided into two categories:

HP 8647A: Standalone high-end model.

HP 8648A/B/C: A sub-series with progressively enhanced functionality, with the 8648C being the most comprehensive flagship model.

Their common core advantage lies in the use of a main oscillator circuit based on a YIG (Yttrium Iron Garnet) oscillator. The YIG oscillator provides extremely low phase noise (typically better than -126 dBc/Hz at 1 kHz offset), which is a key performance indicator for signal generators.

Detailed Introduction and Comparison of Models
1. HP 8647A
Positioning: High-performance base model.

Frequency Range: 10 MHz to 512 MHz (expandable to 1 GHz or 2 GHz with options, but requires an external mixer).

Core Features:

Excellent spectral purity: Extremely low phase noise and spurious signals.

Standard Configuration: AM, FM, and φM (phase modulation) functions are typically standard.

Optional Features: Pulse modulation requires an optional add-on.

Applications: Ideal for applications requiring high signal purity but not extremely high frequencies or complex modulation (such as IQ modulation), such as filter testing, receiver sensitivity testing, and local oscillator replacement.

2. HP 8648A
Positioning: Entry-level model of the 8648 series, with a wider frequency range than the 8647A.

Frequency Range: 250 kHz to 1000 MHz (1 GHz).

Core Features:

Inherits the high signal purity of the 8647A.

Frequency range covers lower bands (down to 250 kHz), making it more versatile.

Modulation functions (AM/FM/φM) are typically standard.

Pulse modulation also requires an optional add-on.

3. HP 8648B
Positioning: Enhanced version of the 8648A, adding key digital modulation capabilities.

Frequency Range: 250 kHz to 1000 MHz (1 GHz) (same as the 8648A).

Core Features (Major Upgrades Compared to the 8648A):

Built-in IQ modulator: This is the most critical difference between the 8648B and the 8648A. With the IQ modulator, it can generate complex digitally modulated signals such as QPSK, QAM, MSK, GMSK, etc., which was crucial for testing digital communication systems of the time (e.g., GSM).

Pulse modulation is typically standard or more easily configurable as an option.

Applications: Became the ideal choice for testing analog and early digital communication systems.

4. HP 8648C
Positioning: Flagship model of the entire series, with the most comprehensive functionality.

Frequency Range: 250 kHz to 2000 MHz (2 GHz). This is its most obvious hardware advantage.

Core Features:

Wider frequency range: Directly covers up to 2 GHz without external expansion, suitable for testing devices in higher frequency bands such as PCS and DCS.

Enhanced IQ modulation performance: Typically offers higher modulation bandwidth and better modulation accuracy.

More powerful internal baseband generator: Capable of storing and generating more complex modulation waveforms.

Comprehensive standard configuration: Many features that were optional on the A/B models (such as pulse modulation) may be standard or more advanced versions on the 8648C.

Applications: Was the top-tier tool for the development and certification of high-performance communication equipment (e.g., base stations, radios) at the time.
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