Electronics > RF, Microwave, Ham Radio

Simple VNA

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ealex:
Hello

I've been playing with RF for some time and i'm starting to feel the need for some basic tools: a way to graph filter, amplifier response, etc.
I have a good signal generator but it's very slow and i can't connect it to a PC.
Checking a few points are ok - dial in frequency, look at VRMS on a scope then convert to dBm.

I've seen some AD8302 ( http://www.analog.com/media/en/technical-documentation/data-sheets/AD8302.pdf ) modules that output gain and phase difference.

I also need an oscillator that can cover ~ 1-100MHz.
The first idea is to use a DDS chip - but they are quite expensive.

After looking around the net ( and at my other signal generator schematics ) I've decided to use the following architecture, with some changes - see the description below.



- a very good DAC - 0-15V will cover 195 - 315MHz  -> 16 bits will have 27.5kHz steps, 24 bits will lead to 107Hz steps
- DAC amplifier - extend DAC range to 0-15V
- VCO: V200ML01 200-300MHz, http://pdf.datasheetcatalog.com/datasheet/Zcommunications/mXrwuxs.pdf
 -10dBm pad, as specified in http://www.zcomm.com/wp-content/uploads/2014/06/AN-102.pdf
band pass filter - 200-300MHz ? is it required ?
- should I add an amplifier as well ? I can't test a diy 200-300MHz amplifier, and if I add a MMIC I'll have to reduce the output power again so I do not overload the mixer
- 200MHz TTL oscillator - square wave
- 200MHz low pass or a narrow band pass filter, so I will not mix higher harmonics
- a pad or buffer to get the signal to ~7dBm for the mixer LO drive
- high pass / low-pass diplexer, at 100MHz to properly terminate the mixer
- attenuator and power splitter, to split the signal 2 ways
- 2 amplifiers, one for the normal output and one for the reference output
- a AD8302 module
- a decent ADC - 12 bits should be enough

For the software - something PC based or get a small A20 board and a compatible LCD to make a stand-alone unit.

Is there anything that I forgot ?

uncle_bob:
Hi

A basic DDS chip is a sub $30 item as a raw chip and under $50 as a "kit board" from a lot of different places. I believe your synthesizer will ultimately cost more and be much less stable than a DDS. Without a PLL involved, the 100 to 200 MHz VCO is going to be wandering all over the place ....

Bob

orin:
You might want to look at the following first:

http://sdr-kits.net/VNWA3_Description.html

and/or:

http://n2pk.com

BTW, if you put < 1dBm (after the BPF) into the RF port of a mixer with +7dBm into the LO port, you are going to get < -5dBm out of the IF port - see the datasheets on the Mini-Circuits ASK-1+ or SAM-1+ for example.  It is unclear from the diagram what you expect the IF level to be.

uncle_bob:
Hi

Some basic "let's guess" math:

16 bit DAC into 100 MHz range VCO: 1.5 KHz steps.

Likely stability of a wide range VCO ~ 10 to 100 ppm: 2 to 20 KHz "wander" on the steps over 1 to 100 seconds.

Bob

Neganur:
how about this, done before and proven.

phase detector:
http://www.scottyspectrumanalyzer.us/slim_PDM.html

DDS:
http://www.scottyspectrumanalyzer.us/slim_DDS_107.html

ADC:
http://www.scottyspectrumanalyzer.us/slim_ADC_16.html

Log detector:
http://www.scottyspectrumanalyzer.us/slim_ld_8306.html

And while you're at it, grab a few other ideas from the individual modules and you'll be on your way to not only understand but also use exactly the measurements needed for what you're trying to achieve:

http://www.scottyspectrumanalyzer.us/msaslim.html (for complete reference)



EDIT 08/2024:(updated the urls) the original .com domain is no longer active and moved to .us - the project is still active on https://groups.io/g/spectrumanalyzer

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