Author Topic: Open Source HW RF Signal Generator  (Read 19657 times)

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Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #50 on: July 02, 2018, 06:17:00 am »
Some Measurements of the Filters on the analog Board I took with the noise-source and the ADF4351 as Inputs.

I have also made a Video of a Frequency-Sweep:


AGC is Set to max Output (Control-Voltage: 5,96V)
LPF-Select is set to Auto.

There are some curious effects at some frequencies. Suddenly signals appear that are lower than the set fundamental frequency.

The last picture attached here shows a frequency-sweep from 500 MHz to 2.490 GHz measured directly at the output of the ADF4351.
« Last Edit: July 02, 2018, 07:12:58 am by SaabFAN »
 

Offline RoadRunner

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Re: Open Source HW RF Signal Generator
« Reply #51 on: July 18, 2018, 05:40:44 pm »
i have also made my self a crude Signal source based on AD4351. this seems be really nice candidate but goes only down to 35Mhz. it is an USB based hardware with PC application running in QT 5. i am using raw hid packages to transmit register information from. with HID raw you can send new regsiter configuration at 1ms interval.

At the lower frequency from 35 to 500Mhz i have seen signal is not very clean,  there are lot of harmonics . i am not a rf person may be because of high signal output level.

https://www.circuitvalley.com/2018/07/diy-4ghz-rf-signal-generator-ad4351-usb-qt5-linux-3Ghz.html
i also posted a crude qulity youtube video showing sweep and hop
« Last Edit: July 18, 2018, 06:11:48 pm by RoadRunner »
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #52 on: July 24, 2018, 09:36:49 am »
That's an interesting device. Especially the software controlling it via USB seems handy. I haven't done anything with HID so far, but I really should look into it - Can you point me to some easy to use references?

The spectral purity of the ADF4351 isn't great because it puts out square waves. The VCOs inside the chip start at about 2.2 Ghz and frequencies lower than that are generated by dividing the VCO-Output with digital dividers.
That's why I added filters to the signal chain. Thx again to user nctnico for the software to design the microstrip structures.

I found something interesting though - the subharmonic signals visible around 1.2 GHz in the video are caused by the first LC-element of the 3Ghz LPF on the analog board.
If I put my finger on the trace, the subharmonic signal vanishes - see attached picture.
The Signal from the ADF4351 doesn't have any subharmonic signal content.

I'm now wondering how I can modify the filter accordingly to achieve the same result that placing my finger on it has and what's more at play here: Capacitance or resistance? According to my Fluke 87, placing my finger on the trace increases capacitance of the entire filter-structure by ca. 80nF and brings the resistance to about 1 MOhm. But that's measured at sub kHz frequencies obviously.


On the software-side I'm making slow progress. The ADF4351-Driver works now, as well as the drivers for the analog board and the attenuator. I want to include some more features in the setup-menu to make it compatible with builds that use different hardware, but that's secondary.
Next step is to get the AD9910-Driver working.
By the way: Does anyone know of a UI-Designer for Arduinos using the GFX-Library by Adafruit? Coding the UI by hand is quite tedious :)
« Last Edit: July 24, 2018, 09:46:41 am by SaabFAN »
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #53 on: July 25, 2018, 08:25:25 am »
The problem with the subharmonic signals could be resolved together with forum user bastler-xx. It turns out that the output of the ADF4351-Module that has no resistor can't be used. The signal power is higher at that output, but signals can bounce back and forth. My theory is that the PIN-Attenuator reflects a significant portion of the signal , which then bounces back and forth at 1.2 GHz (the Coax cable is about 7 cm long, so wavelength / 4 @ 1,2 Ghz isn't too far away).

Now that that's solved, I installed a preamp for the ADF4351-Module to have more room to play with the PIN-Attenuator. The frequency response from 500 MHz to 2 GHz is reasonably flat, but lower and higher and signal-strength drops quite drastically and there seems to be a limit that the output-amp can't pass. It's within my original design-goal: "- -120 to +13dBm digital Amplitude control (This does not need to be flat all the way up to 3GHz, I'd be ok with some amplitude-drop above 2GHz)", but it remains a bit of an eyesore :)
I'm guessing this is in part caused by the fact that i used two double PIN-Diodes instead of a BAT64Q quad diode in the PIN-Attenuator (I've got the diodes here now, but it's too warm to fire up the hot air station :) ), as well as losses in the FR4-Board, limited precision in the etching of the filters and maybe also probably a limited frequency response of the detectors that give the OpAmp of the PIN-Attenuator feedback on the signal-strength.

Attached are two measurements with my SA:
- The Output of the PreAmp I bought on ebay, with the ADF4351 directly connected - I'm suprised by the frequency response of the thing. And wondering what type of IC is in there. - This is the Amplifier btw.: https://www.ebay.de/itm/5-6000MHz-Gain-20dB-Broadband-RF-VHF-UHF-Signal-Power-Amplifier-Verstärker-Modul/362185905078?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649
- The Output of the whole signal-chain. As you can see, as soon as I start increasing the Bias-Voltage of the PIN-Attenuator, the amplitude below 500 MHz starts to drop, but I can get a reasonably flat (+/- 2dB) frequency response up to about 2 GHz. After that there's a sharp drop.

Offline edigi

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Re: Open Source HW RF Signal Generator
« Reply #54 on: July 30, 2018, 06:32:24 pm »
I plan to create a similar signal generator using ADF4351 what is discussed here. I've checked the data sheet and it describes the logic compatibility of the chip as 1.8 V. I couldn't find any information if it's 3.3 V tolerant. So far I've used a board that has 1K resistors serial with each controlling pin (CLK, DATA and LE; board is from banggood) and resistor dividers from the controlling board (Arduino nano). However I'd like to switch to ESP32 (that uses 3.3 logic) and avoid the resistor divider to shift logic level (and I have also a card that does not have any resistors between the controlling pin and the headers so the risk is also higher there).

Does anyone have a good source of information if the ADF VCO+PLL family is 3.3V tolerant? Is there any forum where I could ask this and get reliable information?

For this minimalist hobby project I'd like to avoid any extra board, I'd simply just wire the controlling board, the ADFs (2 for mixing) the OLED display and the rotary encoders.

Thank you in advance for any suggestions.
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #55 on: July 31, 2018, 09:04:48 am »
The chips are 3.3 V chips. They are powered by 3.3 V and can receive data at 3.3V-Levels.
You can use 1.8 V, to send data to the chip, but anything up to VCC + 0.3 V is okay.

I am using about 75 Ohm in series as terminators on the SPI-Bus.
1K will limit the communication-speed, as the edges aren't fast enough an logic levels might not be stable long enough for the chip to recognize the data correctly.
 
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Offline edigi

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Re: Open Source HW RF Signal Generator
« Reply #56 on: August 01, 2018, 08:36:27 pm »
That sounds reasonable.
Actually I've connected the ESP32 directly with the ADF4351 board yesterday and LD LED was showing lock and frequency meter could measure the output signal of ADF4351.
I've used only 1 MHz CLK so maybe the 1K resistor is till OK there (I guess the board vendor opted for this dumb proof protection), it could be an issue for higher clock though.
Checking with DSO the clock was already not so nice and pretty.
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #57 on: October 01, 2018, 09:11:30 am »
Some progress has been made and so far several flaws of the analog board have been identified (Fixed in Revision C):
- Reflections from the first Filter cause problems at higher frequencies (3dB Attenuator added)
- Input-Side of PIN-Attenuator has DC-Voltage on it, but no DC-Block (DC-Block between PIN-Attenuator and 3dB-Attenuator added)
- Frequency-Response of the whole signal-chain varies too much. Especially the final Amplifier is a problem here (redesigned signal-chain between filters and output - See below)
- Manufacturing-Process required rather long traces to build inductors (changed manufacturer to JLCPCB to make use of their 4.5mil minimum trace-width on 4-Layer Boards - Changed first filter from 5th order to 7th order Cauer Low Pass-Filter)
- 750 MHz Lowpass-Filter Module from Minicircuits had mirrored Footprint  |O (Mirrored Footprint :) )
- Power-Detectors have terrible frequency response (replaced resistive taps with resistive dividers and replaced diode-detectors with LTC5505)
- Output-Power too low (Added BGA2866 and SBB5089 Amplifiers to the signal-chain after the filters, plus Attenuators to dampen reflections and prevent saturation of amplifiers - Output-Power of signal-chain should be about +13 dBm)
- ADC-Inputs were left floating, which threw off the readings (Added Pin-Headers for the unused ADC-Inputs to ground them or use them to measure voltages)
- Only one Aux-Input, which requires external switching-board to select between a modulated signal or the Signal from the AD9910 (Added a dedicated Input for the AD9910 with amplifier to get Output-Power close to +13dBm - Fine-Adjustment of amplitude will be done in software with the AD9910, no Hardware-ALC for signals from AD9910)
- 5V Regulator reaches over 90°C without heatsink, with heatsink you can barely touch it (Replaced with AZ1117IH-5.0 and changed Layer 3 from +12V to +5V to act as heatsink in order to get temperatures down and account for additional current-draw of the added amplifiers)
- Additional change: Replaced the two Single-Channel DACs with one 4-Channel DAC, to have 2 additional DAC-Channels.

I'll order new boards soon and testing will continue after that.

The next part is the mixer-board, which I did today - It uses a RMS-30+ Mixer (Anyone in Germany who wants one? Minicircuits asks a 20€ shipping fee, so I think I'll order 4 or 5 pieces.) and has a BGA616 Amplifier on board, to come as close as possible to +16dBm output-power as possible. +16dBm is required to get a +13dBm power-level at the output of the analog-board, due to losses in the RF-Switches and the resistive divider.

And in case a DC-Offset is needed: +/-10V of DC-Offset can be added to the signal on the "Final-Board" (current capability: ca. 40mA). Sonnet Lite simulation of the RF-Path on the board looks good.
Added that mostly just for fun :D

Attached are the Top-Views of the boards.


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