Handheld radio real-time spectrum analyzer / field strength meter

KC908 is a wideband fast-scan receiver that can measure the radio frequency spectrum and field strength, and demodulate and monitor common signals. For unusual signals, raw data (orthogonal data) can be recorded for later analysis.

As is well known, the instrument is based on software-defined radio (SDR) principles. Unlike common SDR receivers, it integrates a complete digital signal processing unit and user interface and works without relying on a computer. If you want to further analyze the signal, you can use USB3.0 to connect with the computer. The instrument supports common community SDR software such as HDSDR, and can be connected to the open source radio software platform GNUAdio.

As a reconnaissance instrument, you need to sweep quickly to find signals. KC908 is significantly different from the traditional scanning spectrum. It uses a fast Fourier transform (FFT) to obtain a spectrum of a certain width, and then stitches segment by segment to obtain a panoramic spectrum.

In this way, at a frequency accuracy of 25kHz, the KC908 can scan a frequency width of 3GHz per second, which is equivalent to 120,000 channels. If it is used to detect 300-500MHz private network radio, it only takes less than 0.1 seconds. For a miniature handheld device, it is relatively fast.

KC908 has two receiving channels, which can theoretically measure the amplitude difference and phase difference between the signals arriving at the two ports. The instrument also has a signal output function, which can output equal-amplitude or simple modulation signals. In theory, it can also output more complex modulation signals.

KC908 SDR spectrum analyzer

Principle

KC908 SDR spectrum analyzerThe KC908 contains a zero-IF receiver with a coverage range of 0.5-6GHz; by mixing, the frequencies outside the above range are moved to the 0.5-6GHz range. The zero-IF receiver directly samples the baseband signal to obtain a digital IQ signal.

The bandwidth of the digital IQ signal is 40MHz, which can be obtained by performing FFT transformation. Because the baseband signal is generated by an analog mixer, the local oscillator leakage cannot meet the extremely high requirements of spectrum analysis.

In order to solve this problem, the instrument discards a sideband and poor performance part of the FFT result, and only takes the 15MHz width for display. This is the origin of the effective real-time bandwidth (15MHz). If the span is greater than the effective real-time bandwidth, the local oscillator scans the entire span at 15MHz intervals and stitches the results together.

Because the spectrum is FFT stitched, the display effect is significantly different from traditional scanning spectrum analyzers, and some concepts are also different. The analysis bandwidth (RBW) is no longer the bandwidth of the resolution filter, but the equivalent resolution bandwidth of the windowed Fourier transform.

Limited by computing resources, under a certain scan width, the RBW can only be set within a certain range, and cannot be set too small or too large. For time-varying signals that are larger than the real-time bandwidth, since the two sides of the splicing point are samples taken at different times, the amplitude of the splicing point may change suddenly.

The signal shape can also be very different from a scanning spectrum analyzer. For example, the frequency spectrum of an FM signal. What the KC908 sees is the peak value (or average value, depending on the setting) of all frequencies in the real-time bandwidth during the previous display refresh cycle. There is no “left-to-right” scan because of any moment “Artifacts” caused by only one frequency. Therefore, the experience of analyzing signals based on “artifacts” summarized on traditional spectrum analyzers may no longer be effective.

But the real-time spectrum analyzer may not be able to get the “truth”, mainly because there is a limit to the time resolution. In simple terms, even if FFT can be performed 10,000 times per second, no display can refresh 10,000 times per second, and no human eye can see 10,000 times per second.

No matter how much data there is, it can only be combined into tens of frames per second and sent for display. This merge process is also called “detection”. The “detection mode” setting is to set the merging rules, such as taking the maximum value of each frequency point frame by frame, or averaging the results of all frames.

As a result, the result displayed on the screen does not represent the “instantaneous” condition of the signal, so it is no longer the “truth”. In fact, even if you print the entire FFT results one page at a time and look at it for one year, the “truth” may not be obtained.

For changing signals, the truth can only be expressed using time-domain waveforms or mathematical formulas. This is because the FFT needs a lot of sampling points (such as 2048) to achieve higher frequency resolution. Collecting these sampling points takes time, during which time the signal may have changed. But compared to the scanned spectrum, the real-time spectrum is much closer to the “truth.”

Due to the wide frequency coverage, in order to avoid overloading the receiver with excessive total energy of the external signal and improve the anti-interference performance, the KC908’s front end has several preselectors.

The pre-selector segmentation methods of the two ports are different, and the anti-interference performance in different frequency bands will also be different. The specific use techniques will be explained later. Although calculated by area, a large part of the RF circuit is a preselector, but due to the size of the instrument, only a very preliminary filtering can be done. Therefore, if it is used in a complex interference environment, some external filters can be used.

Digital signal processing is implemented by both FPGA and MCU. When demodulating the signal, the FPGA re-converted the digital IQ signal to perform strict filtering. KC908’s demodulation bandwidth is from 150Hz to 300kHz, which is suitable for most private network signal analysis. If you need a wider demodulation bandwidth, you can use USB3.0 to transfer the original digital IQ signal to the computer and use third-party software (such as GNUAdio) to process it. The IQ signal bandwidth can reach 40MHz.

Basic functions

  • Spectrum display
  • Analog demodulation, all modes
  • Field strength meter
  • Simple signal source

Practical functions

  • The frequency measurement accuracy is better than 25KHz under any span, and the signal frequency can be accurately detected in a single operation
  • Provide a variety of noise suppression methods for comfortable monitoring
  • Automatic statistics of strong signals to facilitate approaching reconnaissance
  • Channel storage, 999CH
  • Waterfall map
  • Record IQ and audio to TF card
  • Level tone, the level is indicated by the tone level, which is convenient for tracking on foot
  • loud, adapt to noisy environment

Hardware support, functions that may be enabled through software options

  • Comparator, amplitude / phase
  • Vector network analysis (requires external bridge)
  • Digital call demodulation
  • Vector signal source, comprehensive tester

Applicable fields

Traditional handheld spectrum analyzers and field-strength analyzers can be used in fields that are suitable for digital, pulse, and unstable signals (such as the output of magnetrons).

  • Professional communication engineering
  • Internet of things project
  • Interference search, emission source tracking
  • Detection of hidden emission sources
  • Electromagnetic radiation measurement
  • Radar and satellite earth stations
  • Radio reconnaissance and monitoring
  • Frequency occupation analysis
  • Industrial microwave engineering

 

Main parameters

project Minimum value Typical value Max Note
Frequency Range 5kHz 15GHz
Real-time bandwidth 1kHz 15MHz
Analysis bandwidth 1Hz 2MHz
Demodulation bandwidth 150Hz 300kHz
100% POI 3 210μs 15MHz = the SPAN , the frequency measurement accuracy better than 25KHz when
Level measurement range + 20dBm
Level uncertainty 1.5dB
Noise floor -130dBm -120dBm @ 12kHzBW
Machine noise figure 13dB Amplifier turned on
Enter third-order intercept point -42dBm REF = -60dBm
46dBm REF = 20dBm
Mirror suppression 60dB ≤ 6GHz
Residual response 4 -110dBm Port floating
-90dBm Whip antenna
Spurious response -60dBc
Life time 4h
Charging time 3h
Curb weight 900g

Note:

    1. Only select the parameters that are often concerned for readers’ reference. See the technical parameter table for detailed parameters.
    2. Except where noted, measured when the right port (Port2) is selected and left floating. The left port is slightly different.
    3. KC defines “Intercept” in POI as accurate measurement.
    4. EMI will be generated when the device is working. If the antenna is too close to the device, it will be interfered by its own EMI. The residual response index is a 0.2m whip antenna, 1 meter away from the host, and it is arranged in the same direction as the host, and measured in a dark room.

 

Display

KC908 uses a 3.5-inch color LCD display with a resolution of 800 × 480 pixels. The display is backlit and can also reflect a small amount of external light so that it can still be read in direct sunlight. The front of the display is protected by a semi-tempered glass with a thickness of 1.5mm, which is more wear-resistant than plastic, but should be avoided from impact.

The display screen is divided into several common areas. The following figure shows the area division of the receiver mode.

KC908 SDR spectrum analyzer

The other modes generally maintain the same style. The following figure shows the area division of the spectrum mode.

KC908 SDR spectrum analyzer

It is not necessary to use a protective film for the screen. When the screen glass is dirty, you should first wipe off the hard sand on the surface, and then wipe it with ordinary tissue paper. It is better to wipe with water or alcohol, but do not use too much liquid to avoid seeping into the gap and keyboard.

Do not use alkaline detergents. If the glass is broken or scratched, it must be returned to the factory in time, and the user cannot replace it by himself. The front cover must be removed during replacement. Never operate it directly on the instrument. After removing the old glass, the remaining adhesive and glass slag should be scraped off completely, and the new glass should be bonded with a UV curing adhesive.

Battery

KC908 uses 4 18650 lithium batteries, the single unit voltage limit is 4.2V, and the total capacity is about 50Wh.

The battery is located in the middle layer, with circuit boards on both sides, which cannot be replaced by the user. To replace or remove the battery:

Turn off the power, remove the nuts on the RF port, remove the back cover of the instrument, separate all visible data cables, and remove the larger screws on the circuit board (small screws cannot be removed). Use a hooked tool to lift the circuit board from the end away from the RF port, pull the RF port out of the case in the opposite direction to the RF port, and remove the circuit board to see the battery. Pay attention to protect the data cable and connector during operation.

To ensure battery life, the capacity of a single battery should be greater than 3200mAh. You must choose the same type and same batch of batteries. Before installation, confirm that the difference between the 4 battery voltages is less than 50mV.

The battery quality must be excellent, and regular qualified products from well-known manufacturers should be selected. In normal use, the temperature of the instrument can reach 60 ° C; if used at the limit temperature (55 ° C), the temperature of the battery may exceed 80 ° C, and the use of inferior batteries will be dangerous. The 4 batteries should be installed as soon as possible, and should not be prolonged for a long time to avoid imbalance because some batteries are connected first.

After replacing or removing the battery, reinstall the circuit board, replace the screws on the circuit board but do not tighten, replace and tighten the nuts on the RF port, then tighten the screws on the circuit board, connect the data cable, and confirm that the computer is normal , Close the back cover.

Product has not been released, and detailed indicators have not been announced.

 

General parameters

project Smallest typical maximum Remarks / conditions
DC withstand voltage 15V RF port
External DC power supply voltage range 10.5V 26V 5.5 / 2.5 round mouth
4.9V 20V TYPE-C port
External DC power supply current 2A 5.5 / 2.5 round mouth
1.5A 4A TYPE-C port
battery voltage 6.5V 8.5V
Power consumption

( Volume 10% , display brightness 50%)

10W 12W 15W Battery powered, running
14W 16W 20W 12V power input, battery charging, operation
4W Battery powered, stopped
Power off 500 μ W 1mW Battery powered
Live storage time 1a 2a The initial cell voltage 7.5V when
Speaker power 2W 4W
Audio delivery power 1W 1.5W At 4 Ω
MIC input sensitivity 50mV Impedance of 600 [Omega
Internal barometric pressure measuring range 300hPa 1100hPa
Internal atmospheric pressure gauge uncertainty 0.5hPa Core temperature 40 ℃
Internal inclinometer uncertainty 1 ° After calibration
Internal magnetic compass uncertainty 5 ° After calibration
Ambient temperature 0 ℃ 40 ℃ normal range
-40 ℃ 50 ℃ Allowable range
-40 ℃ 70 ℃ Short-term storage
0 ℃ 35 ℃ Long-term storage
The core temperature of the device must not be higher than the upper limit of the battery’s allowable temperature range; the low temperature limit within the above temperature range depends on the lowest available temperature of the battery.
Relative humidity 0% 95% Work, short-term storage
Waterproof performance Level 0 Not waterproof
Resistant to fall ( electrical functions are intact and allow appearance damage ) 30cm Without sheath
1.2m With sheath
Anti-seismic 20Hz , 5G , 30min in any direction
volume 187 × 102 × 33 Main unit only, with protruding parts
177 × 102 × 32 Does not include protruding parts
Net quality 800g Main unit only, with battery
Factory packing quality 3kg Use original safety box

 

Difference between KC908 and KC901

The design intent is different: KC908 is a real-time spectrum analyzer, and KC901 is a network analyzer.

KC908 is suitable for space radio wave measurement; KC901 is mainly used for measuring various antennas and filters.

KC908 has full-mode analog modulation and demodulation functions, including AM, FM, SSB, CW, etc. KC901 has no demodulation function.

Although the KC901 also has spectrum functions, the KC908’s spectrum functions are much more powerful.

The volume is smaller than KC901, and it takes a lot of effort to reduce the volume.

Can i connect a computer

IQ data can be sent to a computer via USB 3.0 and connected to GURARADIO, similar to the usage of B210. However, the KC908 is mainly designed for handheld mobile use, and it is wasteful to use it to connect to a computer.

There is a TF card slot, which can record low-speed IQ signals to obtain WAV files and transfer them to a computer for later analysis. You can also record audio.

Can I connect an external display

No. There are already more than one display interface chip in it.

KC908 comes with a screen resolution of 800 × 480px, which is similar to common digital oscilloscopes.

The size of the built-in display is 3.5 inches. It is recommended that senior friends consider using the following products

Source: Science and Technology Research Institute (c) 2020

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