spectrum analyzers

see also:

• multimeters
• oscilloscopes
• frequency response analyzers
• software defined radio (SDR) receivers
• vector network analyzers (VNAs)
• signal generators

• spectrum analyzers display the electrical signal intensity for each frequency of an input across a range of frequencies
• they are utilized heavily in software defined radio (SDR) receivers to display the strength of an incoming radio wave signal at a given frequency which helps identify signal transmissions as well as signal characteristics such as bandwidth (AM vs FM modulation, etc), repeating beacon signals, etc
• they generally also have a waterfall display option to show past signals

• frequency range
  • a software defined radio (SDR) receivers RTL-SDR device for instance is limited to 500 kHz up to 1.7 GHz
  • Fnirsi DPOS350P only has 200 kHz to 350 MHz (from 350-500MHz there is a lot of attenuation)
• analog-to-digital converter (ADC) resolution
  • impacts the dynamic range and sensitivity of the device
  • basic devices such as RTL-SDR are usually 8-bit
  • some, such as the Fnirsi DPOS350P, have optional “16-bit” by software interpolation
• resolution bandwidth (RBW)
  • determines the frequency resolution and ability to separate closely spaced signals.
  • smaller RBW improves signal selectivity but slows scan speed
  • RBW = sample rate / FFT window size eg. 1GSa/sec (ie 1GHz) at 32K window = ~30kHz resolution
• FFT window size
  • the number of samples taken in the time domain that are used to perform the Fast Fourier Transform (FFT) calculation for frequency domain analysis
  • determines how many discrete frequency “bins” the signal is divided into and affects both the frequency resolution and time resolution of the analysis
  • expressed as a number of samples (e.g., 1024, 2048, 4096, 32K), usually a power of 2 for computational efficiency
  • a larger window size provides better frequency resolution because the frequency bins become narrower, enabling finer distinction between close frequencies but also means poorer time resolution (longer analysis time per window), making the analyzer less responsive to rapid signal changes
  • also defines the duration of time the signal is analyzed; a larger window corresponds to a longer time segment
  • thus it is a critical parameter balancing frequency precision and temporal responsiveness
  • examples:
    • Fnirsi DPOS350P allows selection of 4K to 32K FFT windows
    • for software defined radio (SDR) receivers it is the software that determines this not the USB device
• max. sampling rate
  • how fast it can internally digitize or sample the input signal, often expressed in samples per second (e.g. MSa/s)
  • high sampling rates allow better time resolution and can enable faster data capture for real-time analysis
  • examples:
    • Fnirsi DPOS350P has max. 950MSa/s
    • a software defined radio (SDR) receivers RTL-SDR device has max. 2.4MSa/s
• frequency resolution
  • expressed in bandwidth per bin
  • approx. frequency resolution = Sample Rate / Window Size
  • eg. a 1024-sample window at 44.1 kHz sampling rate divides the spectrum into 512 bins with about 43 Hz frequency resolution and a 23 ms time duration
• sweep speed
  • how fast the analyzer scans across frequencies.
  • faster sweep speeds reduce measurement time and improve transient signal capture
  • usually given in ms/sweep or GHz/s
• dynamic range
  • the difference between the smallest and largest signals the analyzer can accurately measure.
  • crucial for detecting low-level signals in presence of strong signals
• Sensitivity and Noise Floor
  • the minimum input signal level detectable above the noise floor, affecting the analysis of weak signals
• Input Power Range and Protection
  • the allowable signal input power levels to prevent damage and ensure accuracy
• Modulation and Demodulation features
  • support for AM, FM, PM measurements, vector signal analysis, and specific protocol analysis (e.g., WLAN, LTE) for communication testing
• sweep type
  • Fnirsi DPOS350P: FFT-based spectrum analyzer using oscilloscope ADC data
  • Siglent SSA3021X-TG: traditional swept spectrum analyzer with tracking generator
• tracking generator
  • Fnirsi DPOS350P: not available
  • Siglent SSA3021X-TG: included

• Siglent SSA3021X-TG
  • widely used in education, lab, and hobbyist environments
  • 9 kHz to 2.1 GHz; 10 Hz RBW
  • Resolution Bandwidth (RBW): 1 Hz to 1 MHz (in 1-3-10 sequence)
  • Displayed Average Noise Level (DANL): Typically -161 dBm/Hz (normalized to 1 Hz)
  • Phase Noise: Typically -98 dBc/Hz at 10 kHz offset at 1 GHz
  • Total Amplitude Accuracy: ±0.7 dB
  • ~$US2000
• Fnirsi DPOS350P 4-in-1
  • the spectrum analyzer in this model is mainly just a hobbyist fun addition to its main function of being an oscillocope although it does have a reasonable 950MSa/s sampling rate
  • freq range is only 200 kHz to 350 MHz (from 350-500MHz there is a lot of attenuation)
  • allows selection of 4K to 32K FFT windows which can be limiting
  • sweep type is FFT-based spectrum analyzer using oscilloscope ADC data
  • no tracking generator
  • no modulation/demodulation features
  • ~$US300 - bargain budget scope with some fun add ons

• Tiny SA Ultra
  • displays a spectrum as in a SDR with a waterfall and you can choose a frequency to listen to via the 3.5mm headphone port
  • built in 20dB LNA to to 4GHz
  • switchable resolution bandpass filters for both ranges between 200Hz and 850kHz
  • 3.95“ 480×320 px screen; 450 points displayed per scan;
  • 0.1-800MHz or, with Ultra mode enabled, 0.1MHz-6GHz; < 1ppm freq stability;
  • max. input +10dBm
  • 2hr 3000mAh li ion battery; microSD card slot;
  • also has Signal Generator with sine wave output between 0.1-800MHz or square wave or dual tone output up to 4.4GHz when not used as Spectrum Analyzer
  • connected to a PC via USB-C it becomes a PC controlled Spectrum Analyzer or Signal Generator
  • ~$AU220
• TinySA ULTRA Plus ZS407
  • 2025 model; up to 7.3GHz in Ultra mode;
  • 5000mAh battery;
  • ~$AU285