Electronics > RF, Microwave, Ham Radio

Idea for improving LTDZ Spectrum Analyzer & Tracking Generator 35MHz-4.4GHz

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Kalvin:
I am interested in the LTDZ 35MHz - 4.4GHz spectrum analyzer & tracking generator containing STM32F103 processor for a hobby use (amateur radio). After watching some videos and reading VMA's satellite blog, it is obvious that there are some problems and shortcomings with the existing design. For example, the default RBW is about 120 kHz, and it cannot be changed dynamically. For a general RF filter measurement and SSB spectrum analysis, this wide RBW makes this otherwise very nice instrument almost useless.

Looking at the schematics, the output of the RX mixer is fed through a 120 kHz low pass filter (read: RBW-filter) into input of a AD8307 LOG amplifier/detector. The output of the LOG detector is then fed to STM32F103's ADC input.

Since the STM32F103 has a 12-bit ADC which can run up to 1Msps, why not drop the AD8307 altogether, feed the 120 kHz filter directly into the ADC input, and make the STM32F103 do the required math and DSP. The ADC would give theoretically 12*6dB = 72dB of dynamic range, which is quite similar to AD8307 anyway. Then the STM32F103 could implement adjustable RBW, perform even FFT if needed. At the same time one can even obtain 1 - 3 bits of conversion gain (6dB - 18dB) with narrower RBW for improved dynamic range (if the noise floor is practically low enough). There are some alternative firmwares available which could be used as a starting point for this modification.

What do you think? Do you see any reasons why this scheme would not work?

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Project at Github: https://github.com/kalvin2021/ltdz-dsp

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Here are some pointers to books related this subject:

Freely available ebook "The Scientist & Engineer's Guide to Digital Signal Processing, 1999" by Analog Devices:
https://www.analog.com/en/education/education-library/scientist_engineers_guide.html

Freely available ebook "Software-Defined Radio for Engineers, 2018" by Analog Devices:
https://www.analog.com/en/education/education-library/software-defined-radio-for-engineers.html

Practical introductory book about digital signal processing without too complex mathematics (pun intended):
Lyons: Understanding Digital Signal Processing, 3rd Edition

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PCB:
[attach=1]

Schematics:
[attach=2]

Edit: Added the books, PCB and schematics.

radiolistener:

--- Quote from: Kalvin on May 10, 2021, 06:30:34 pm ---why not drop the AD8307 altogether, feed the 120 kHz filter directly into the ADC input, and make the STM32F103 do the required math and DSP
--- End quote ---

Using digital filter slows down STM32 and don't provide any benefit. There is no need for configurable bandwidth. It just make sweep very slow and low dynamic range.

And AD8307 is logarithmic amplifier with 92 dB dynamic range, so it allows to increase dynamic range by using logarithmic scale. If you remove it, you will get linear scale and bad dynamic range.

If you're not satisfied with software results, you can try my version NWTSHARP-LTDZ, it supports LTDZ and NWT7 devices and use better calibration algorithm, also you can see RAW ADC charts. Just select COM port, press Connect and then Sweep.

My LTDZ instance dynamic range is 50 dB. And it's limited by RF frontend design. In order to get better, it needs a new PCB. There is already exist exactly the same hardware with better PCB layout and shielding, but it cost much more.

Kalvin:

--- Quote from: radiolistener on May 10, 2021, 08:11:01 pm ---
--- Quote from: Kalvin on May 10, 2021, 06:30:34 pm ---why not drop the AD8307 altogether, feed the 120 kHz filter directly into the ADC input, and make the STM32F103 do the required math and DSP
--- End quote ---

Using digital filter slows down STM32 and don't provide any benefit. There is no need for configurable bandwidth. It just make sweep very slow and low dynamic range.

--- End quote ---

For your satellite spectrum observation purposes the 120 kHz RBW might be ok, but for applications that require more detailed spectral analysis, this 120 kHz RBW is way too wide, and makes the board basically useless.

If one wants to obtain narrower RBW with more spectral details, it is necessary to reduce the RBW from 120 KHz down to 1 KHz or even less, depending of the spectral details one wants to see. Yes, this will slow down the sweep rate, but in applications that requires more spectral details like tuning a narrowband filter or investigating narrow SSB spectrum, this needs to be done either in analog domain (AD8307) or in digital domain (using numerical DSP-algorithms).

Bypassing the AD8307 LOG-detector altogether, feeding the output of the 120 kHz lowpass-filter directly into STM32F103 ADC input, sampling the signal with up to 1Ms/s, and performing the final RBW-filtering and LOG-detector in MCU with some math/DSP, one can select narrow RBW-filter dynamically, and gain some extra dynamic range due to processing gain.

For more advanced spectrum analysis, one can even implement FFT for computing the spectral components, and one can get very narrow RBW and good spectral resolution without slowing down the sweep rate. That is something that really cannot be done with AD8307 if you reduce the RBW for improved spectral resolution.


--- Quote from: radiolistener on May 10, 2021, 08:11:01 pm ---And AD8307 is logarithmic amplifier with 92 dB dynamic range, so it allows to increase dynamic range by using logarithmic scale. If you remove it, you will get linear scale and bad dynamic range.
<snip>
My LTDZ instance dynamic range is 50 dB. And it's limited by RF frontend design. In order to get better, it needs a new PCB. There is already exist exactly the same hardware with better PCB layout and shielding, but it cost much more.

--- End quote ---

This is partially true. By taking advantage of the processing gain you can compensate the "seemingly lost" dynamic range of 72dB of a 12-bit ADC when using narrower RBW filters. With the original 120 kHz RBW, you can still get extra dynamic range by oversampling and averaging, but this will obviously slow down the scanning rate as more samples are needed.

As you have stated, the actual obtainable dynamic range may be only 50 dB for a given board/design. Even if AD8307 is technically able to provide 92 dB of dynamic range, in practice you will get much less than that due to the noise floor of the board. In that sense the 12-bit ADC with 72dB of dynamic range is sufficient, and the STM31F103 is able to compute the LOG-detector with 72dB dynamic range in software without any problems.
 
Some newer boards are said to be able to give up to 70 dB of dynamic range, so the 12-bit ADC is still sufficient for the LOG-detector, and AD8307 does not provide any real benefits. Of course, the signal level needs to be set correctly so that the full dynamic range of ADC can be utilized. With the processing gain obtainable using the narrower RBW and/or oversampling/averaging, one can compensate lost dynamic range to some extent, even if the signal level is not perfect.

Bicurico:
If you want to use such small RBW you will either need a professional class spectrum analyser like the Siglent SSA3021X/X-P/-RT/SVA series or you might try your luck with SDR alike devices like the HackRF one or Adalm Pluto (using Satsagen).

I just tested the Arinst SSA-R2 TG and it is a much better performing device than the SMA/NWT/D6/LTDZ devices. However, it uses a fixed RBW of 200 kHz (https://vma-satellite.blogspot.com/2021/05/arinst-ssa-r2tg-one-of-themost-frequent.html).

The last cheap option would be a TinySA. At 50 Euro it is very cheap, but in EU you will probably end up getting a fake clone, unless you buy at AliExpress from Zeenko Store and handle customs yourself.

It seems to have a configurable RBW, but it is otherwise limited in terms of frequency range.

At the end of the day, it all depends on what exactly you want to do.

Regards,
Vitor

Kalvin:
I agree that the phase noise may become an issue with smaller RBWs when using AD4351. Unfortunately I do not have any measurement data for the phase noise for these chinese boards. Fortunately there is some measurement data available, giving -60 ... -70 dBc@1 kHz for AD4351: https://www.eevblog.com/forum/rf-microwave/adf4351-simple-you-would-think/msg2514417/#msg2514417

I do have a genuine TinySA and Adalm Pluto, but wanted to take a look at these inexpensive AD4351-based spectrum analyzers / tracking generator boards, and see how their performance could be improved so that these simple and inexpensive boards would be better suited for a typical radio amateur-related measurements (spectrum analysis, filter measurements, signal generator) from the upper HF frequency range, up to UHF frequencies even beyond 1 GHz. Adding a cheap SWR bridge would create a decent and very inexpensive antenna analyzer for frequencies above 35 MHz, for example.

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