Author Topic: Open Source HW RF Signal Generator  (Read 29281 times)

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Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #50 on: July 01, 2018, 08:17:00 pm »
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:
https://youtu.be/RAp4lgueNvI

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.

Offline RoadRunner

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Re: Open Source HW RF Signal Generator
« Reply #51 on: July 18, 2018, 07:40:44 am »
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-adf4351-usb-qt5-linux-3Ghz.html
i also posted a crude qulity youtube video showing sweep and hop
« Last Edit: January 02, 2020, 07:09:56 pm by RoadRunner »
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #52 on: July 23, 2018, 11:36:49 pm »
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 :)

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #53 on: July 24, 2018, 10:25:25 pm »
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, 08:32:24 am »
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 30, 2018, 11:04:48 pm »
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.

Offline edigi

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Re: Open Source HW RF Signal Generator
« Reply #56 on: August 01, 2018, 10:36:27 am »
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: September 30, 2018, 11:11:30 pm »
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.

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #58 on: December 07, 2019, 10:26:32 pm »
Many months later and many days of tinkering with the whole system, including installing everyting in a 19 inch case, I can say the the individual modules work as expected (well, mostly - see below) and that basically 2 big problems remain:

- Signal integrity on the SPI-Bus
- Software (Especially UI and calibration routines to compensate for the frequency dependent losses)

While I can't say much about the software-part, as I haven't really started developing a new UI yet, apart from  a rudimentary interface that allows setting the most important parameters via serial commands, I can say quite a lot about the hardware.

The biggest problem so far has been signal integrity on the SPI-Bus, which has been not only frustrating but also actually drove me away from the project for quite some time.
I haven't identified all the issues, but I'm pretty sure that one of the major culprits is the LCD-Controller, which, according to a few forum posts, does not properly tri-state its output and screws up readbacks from other chips (which cost me at least 2 weeks of head scratching and chasing ghosts). Another issue are the cables. Due to the location of the SMA-Connectors the boards are pretty far apart in the case and connected via 10 to 15cm long cables that are bundled together with zip ties and tape. This causes quite extensive overshoot on the Clock and Data-Lines of the SPI-Bus, as well as reflections (the cables are going out in a star-pattern from the CPU-Board).
While the LCD doesn't care and the AD9910 can work with the overshoot most of the time (not always - another ghost I was chasing), the ADF4351 simple crashes when it is confronted with spikes up to 3.7V on the data-lines.
The solution that finally fixed this problem (I tried different resistor-values for the series termination on the CPU-Board with basically negligible results) was a buffer-board that uses a 74AC14 to buffer the signals and catch the spikes. This board also includes over 2000uF of capacitance to filter the power-supply and catch any remaining traces of the spikes.

After installing that, I finally managed to talk to all the chips and tell them what to do - I couldn't really listen to them, due to the stupid LCD-Controller, but I reliably (well, 90% of the time) got the expected results from the entered commands.

So I finished my drivers for the Chips and continued testing, when I noticed rather nasty traces on my Scope: Everytime there's activity on the SPI-Bus, I see a bit of noise on the waveforms. Btw. I built all the modules in a way that the mounting holes are connected to ground and the case is grounded as well, so there's basically a big ground-plane 1cm beneath each board.

But despite all these problems, I was able to verify the basic concept of the system:
- Using several switched filters works (although using a more highly integrated switch would reduce the losses that are created by these switches) and attenuates the higher harmonics to a sufficient degree
- I can (with some difficulty, depending on the frequency) fine-tune the amplitude with the ALC-Circuit
- Switching signal sources in and out works
- The Mixer-board works (I had to make a few adjustments to the circuit however - using just a resistor doesn't yield satisfying results)
- CPU and Keyboard-Interface works
- Communication between the chips works (after overcoming the difficulties described above)
- A basic prototype of the UI works

Which brings me to what I have been doing for the last 3 weeks: The ARSG - Mark III Mainboard.

I've combined all the functionality into one board and also did some changes:
- Replaced the STM32F103 with a STM32F407
- Optimized the SDRAM-Interface for the FPGA to make the traces shorter (which should allow me to increase the SDRAM-Clock to the maximum of 166 MHz)
- FOUR-LAYER-BOARD - No more 2-Layer crap  ;D
- Replaced the ADF4351 with the ADF5355 (13,6 GHz  :o )
- Replaced the AD9910 with the AD9957 (basically the same chip with large portions of the datasheet copy and pasted, but it also has digital I/Q Modulator capability and it can also work as a simple DAC driven by a data-source, whereas the AD9910 could only use the Output Scale Factor for this)
- Improved the Master Clock and used differential clocks wherever possible to reduce noise.
- Changed the power-supply to a +14 / -14V-Scheme with local regulators for the individual circuit-blocks.

I haven't started with the board-layout yet, but I think I might be able to put everything on a 200 by 150mm board.

So, in essence after a rather long hiatus, this project is back on my in progress-list :)

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #59 on: December 09, 2019, 12:45:13 am »
I've attached a very early preview of the layout - Power Supply-Layout is done and will get its own little shielding-cage with holes in it for the Heatsinks (Unless I run out of space. Then I'll put that on a separate board :) ).

The shield-cages will be 3D-printed cages wrapped in aluminium or copper foil and screwed to the board.
For the RF-Cages I plan to use Filament with metal content to create RF-Absorbers. Has anyone done this before and can share some insights?

Online Yansi

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Re: Open Source HW RF Signal Generator
« Reply #60 on: December 09, 2019, 12:58:15 am »
Ah! Finally! A was just wanting to suggest you the AD9957. I have one piece laying somewhere I bought from china some time ago. That'd be a very good opportunity to make a nice  vector signal generator.

Also, why don't you use a servo-loopped ALC for the output? Stepped attenuators are boring.

For frequency of interest up to 3GHz, a cheap nice BAP64Q PIN quad may be used. Not sure what to use further, as I have never worked beyond 3G. (Thats the limit of my instrument setup).  Any RF log-to-lin detector coupled with it will give you a pretty decent ALC building block. Rest is just any DAC with enough resolution and a couple of opamps.
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #61 on: December 09, 2019, 01:20:32 am »
i already have implemented a ALC ciruit to have fine control over the amplitude. And it uses the BAP64Q, which does a pretty good job :)
But since the ADF5355 can offer up to 6.8 GHz, i am planning to use a special IC by Analog Devices as the Attenuator. Namely this one: https://www.analog.com/en/products/hmc346ams8ge.html
Using that chip will also remove the requirement to separately control the Bias-Voltage for the attenuator, which currently requires a rather long calibration process.  ;D

For the control I'm using a quad OpAmp that is regulating the Output amp against the voltage from a 12bit DAC. So the precision should be there to make it possible to remove the step attenuator. :)
I'll have to do some more tests with the current ALC-Circuit to verify this, but its a good idea to just use the ALC-Circuit for this. Thx :)
On my current hardware I'm using LT5505 as RF-Detectors, which work rather well, but only have about 40dB of dynamic range.
The dynamic range of the detector is probably the biggest problem here. Do you know any detector-chip that has a suitable range (about 90dB)?

Online Yansi

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Re: Open Source HW RF Signal Generator
« Reply #62 on: December 09, 2019, 10:05:49 am »
It is very difficult to watch all this thread, as you are way to fast in slapping things together  :)  Sorry if I have missed something.

Have you already verified any RF parts of this huge design?
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #63 on: December 09, 2019, 09:21:04 pm »
What I've verified so far is:
- Filtering of the ADF4351-Output to attenuate the harmonics - Working after adding some attenuators at the input and output of the filter-chain. Since the 1.5 GHz Filter is only 5th order, it probably is a bit weak, but I don't have any test-equipment to check what is going on beyond 3GHz. Could probably use some improvement - LTCC-Type Filter by Minicircuits could offer better performance here as well as significant space reduction (1.5Ghz Microstrip-Filter is quite large, even with 4mil inductors). To design the Microstrip Filter I've been using this tool: http://www.nctdev.nl/page_pg%3d17.htm where I entered the values that this tool http://tonnesoftware.com/elsie.html gave me with pretty good results.

- Automatic Leveling - Working with LT5505 used as the detector, self-made PIN-Attenator based on BAP64Q Application example in the Datasheet, standard quad OpAmp as the controller and 12bit i2c-DAC for the reference voltage. It's a bit tricky to find the correct bias-voltage though as that seems to be changing with frequency. I've replaced the BAP64Q-Attenuator with the HMC346AMS to remove the necessity for actively providing and regulating the bias-voltage for this exact reason.

- Signalpath-switching - Works perfectly with one exception: The input from the AD9910. The Amplifier (BGA616) is generating almost 26 dBm with a 1mH inductor at the input of the RF-Switch that switches between the filtered ADF4351-Signal and the AD9910-Signal. Predictably, this reliably fries the RF-Switch after a short while (I've measured up to 12Vpp on my scope at 20 MHz with the AD9910-Signal source deselected - Absolute Maximum according to Datasheet: 8,5V).
The new design uses OpAmps as the output-amplifiers of the AD9957 that are specified to 1.75 GHz and switches with internal 50 Ohm terminators instead of reflective ones I've been using in the old design.

- AD9910 Signal - Currently the handover from the AD9910 to the ADF4351 happens at 400 MHz, which is a bit high as I can see a pretty strong image frequency at 600 MHz on the spectrum analyzer, as well as some other spurs that get stronger and stronger above 300 MHz.
Using one more filter at 400 MHz to allow switching in the filtered RF-Source at 250 MHz or 300 MHz should solve that problem, since I can use LowPass-Filters with a lower cutoff-frequency for the AD9957-Output. Using an OpAmp at the output also allows much lower frequencies since the BGA616 needs to be AC-Coupled and becomes almost useless below 1 MHz.

- Mixer-Board - Works without a problem once I fixed the supply of the BGA616 Output-Amp. Output amplitude is high enough to fry the reflective RF-Switches though (see above), so again a switch with internal termination is needed (in case the software routes the signal the wrong way).

- Reference Clock-Generator and distribution-amplifier - Works perfectly after I cut a few traces since I goofed up the footprint of the main oscillator :D

- FPGA and SDRAM - FPGA is working, but I haven't checked the communication with the SDRAM-Chip yet. I'm pretty sure it'll work, but since I used a 2-Layer board there I've got interrupted GND-Return paths all over the place and had to use a short coaxial cable to get the clock from the FPGA to the SDRAM-Chip, I'm guessing that the max clock frequency is rather limited.

Most problems in the RF-Parts I've ironed out with the boards I ordered last year and since then the focus was mostly on the software-side and, as I said, the communication-problems between the CPU and the connected chips. Putting everything onto one board and using short, traces and proper termination should hopefully fix all those issues once and for all.

One more note on the FPGA and controlling the AD9957: I'm currently using a MAX 10 FPGA connected to a 8 MByte SDRAM with 16bit wide databus. I do have a few more pins available to connect a larger chip, but I think 4 Mega Samples (the AD9957 takes 18 bits but 2 of those could be calculated or just be static) is enough for this project.
With a SDRAM-Chip, I estimate a sample rate of about 100 Megasamples/s to be possible. Maybe even more with a prefetcher and some clever use of the banks inside the SDRAM to make the most of the burst-mode transfers.
I've also thought about using DDR-Memory, but laying out a board for DDR-Memory has a lot more pitfalls than normal SDRAM.
If anyone can point me to a good source of guidelines or proven rules of thumb, I might reconsider the choice of memory, however :)

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #64 on: December 10, 2019, 09:48:03 pm »
Here are some of the schematic-blocks of the Mark III Board.

I've tried to stay as close to the application schematics in the datasheets as possible with most of the chips, or used what I verified with my previous boards.

The reason why I'm going for differential clocks is the fact that the clock-lines are rather long and I want to eliminate any additional phase noise sources.

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #65 on: December 11, 2019, 08:28:01 pm »
This is the layout of the FPGA and the AD9957 with all its power supplies.

What I didn't realized when I drew up the schematic was how much power the AD9957 draws on the 1.8V Rail for the digital core: 610mA!!! :o
And on top of that the datasheet is a bit ambiguous - Quote: "DVDD (1.8V) Pin Current Consumption | QDUC mode | 610 mA" - Which could mean that it draws that PER PIN!
I doubt it will draw that much power though, as the next few entries in Table 1 state a power consumption of 1.8 Watts for the entire chip as the maximum in continuous modulation-mode.
As this would mean about 2 watts of waste heat in a linear regulator, I decided to use a switching converter for the digital core while the analog circuits will be supplied by "Ultra Low Noise" RT9193 regulators.

I'll also redesign the Power Supply as it currently takes uses too much space and seeing how large Master Clock Generators and the FPGA+AD9957-Combo is, I fear I'll run out of space pretty soon and I would like to avoid having to put parts on the bottom as much as possible :)

I might put the filter in its own little cage, however.

Btw. I haven't added a Ground-Plane on the top layer yet, as each time I put in a via Circuitmaker takes quite a while to calculate the holes in the planes - even on a Ryzen 5 3600X based gaming machine  :-//

Offline Mechatrommer

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Re: Open Source HW RF Signal Generator
« Reply #66 on: December 12, 2019, 03:57:30 am »
kudos (subscribing...)
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #67 on: December 25, 2019, 08:52:57 pm »
I've got the layout of the board mostly done but I ran into a few problems with designing the filters. Especially the 4 and 6 Ghz-Filters cannot be calculated as the structures have to be too small to achieve the necessary impedance with a 0.1mm 4.05 Ef dieeletric at more than 3 Ghz. And there's also the issue of limited space on the board.

Searching for suitable filters, on Mouser and from Minicircuits, I've come across a new product by Minicircuts: A "Reflectionless Filter".
https://www.minicircuits.com/pdfs/XLF-13H+.pdf

They are quite expensive at 12 Dollar per piece in low quantities, but the reflectionless nature of the filter would be a nice property as the reflections from the filters caused the PLL to lock to the wrong frequency or there were excessive spurs when I didn't have a 10dB attenuator between the Filter-Board and the ADF4351-Module (problem solved with the second revision of the Filter-Board).
Anyone here who used those filters before or know of similar components / how to buld those as microstrip? I'm tempted to use them, but I'm a bit hesitant because of the high price.


Online ogden

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Re: Open Source HW RF Signal Generator
« Reply #69 on: December 26, 2019, 12:59:56 am »
Using 12$ filter to solve problems caused by pin diode attenuator do not sound right to me. Why don't you use digital step attenuator which do not have impedance matching problems? 1dB step is good enough for most occasions.
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #70 on: December 26, 2019, 04:58:21 pm »
The Attenuator is used to have a defined level before the signal is being fed into a step attenuator. Using the chip from Analog Devices takes care of impedance matching and all the other things.

What I was wondering was if it would make sense to use a reflectionless filter instead of the cauer filters I've been using previously.
I'll stick with the cauer filters for now, as I know they work (with 10dB fixed attenuator between the PLL and the filters).

Current state of the development:
- Power-Supply scheme redesigned (using TPS54302-Chips)
- All function-groups laid out and most connections done
ToDo:
- Add the filters
- Connect the CPU to the function-groups

Online ogden

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Re: Open Source HW RF Signal Generator
« Reply #71 on: December 27, 2019, 02:50:36 pm »
AFAIK pin diode is kinda nonlinear. I would measure distortions of such "attenuator" min/max/mid range before commitment.
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #72 on: December 27, 2019, 03:37:59 pm »
 According to the datasheet, the HMC346AMS8GE is based on GaAs MOSFETs and it specifically states that the application circuit takes care of impedance matching.

The PIN-Diode Attenuator works pretty good too, but requires careful adjustment of the Bias-Voltage depending on the frequency. That's why I want to use the Chip from Analog Devices - Only one input required and no lookup-table that needs to be filled with calibrated values :)

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #73 on: January 02, 2020, 11:39:11 pm »
Mk. III Board is almost done.

I've laid out the RF-Section with just the filters for 1.5 and 3 GHz still missing.

To make sure that the signal is strong enough at the output and arrives leveled at the Mixer, I've added a second ALC-Stage that sits in front of the RMS-30+ Mixer (Anything above 3.5 GHz tends to get really expensive really fast - Especially amplifiers and I don't have a dealer for Minicircuits-Parts - so I decided to limit the upconverter-stage to 3.5 GHz).

Basically every functional block in the RF-Section has its own linear power-supply that regulates down from a +8V or +/-12V Supply from the Switching Regulators far away on the other side of the board.
In addition to that, I've positioned everything in a way that it can be shielded and the holes to screw the shield in place are 3.4mm diameter so M3 screws can be used.

To make layout a bit easier, I've used Microstrip-Transmission lines instead of Coplanar Waveguides this time. Saves me the time to put a ton of vias next to the RF-Traces and also keeps the bottom side clear of vias so routing of control-signals and power is much easier.
Also based on my experience with the JLCPCB Assembly-Service I've used 0402 components in the RF-Path. To make it less likely to blow the tiny parts off the board when hand soldering the other components, where possible I've kept a rather large distance between the parts. It makes the RF-Path a bit longer, but as I plan to release this as a partially assembled kit in the future, I think ease of assembly in the hobby-room is the more important factor here.
Which is also why I tried to avoid hard to solder packages wherever possible - The ADF5355 and the SP4T RF-Switches are the only QFN-Parts for example.

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #74 on: February 01, 2020, 12:51:43 pm »
Board-Layout is finally done.

I've decided to not implement a 6GHz-Filter, since anything above 6 GHz is already being attenuated quite a bit by the amplifiers that are only specified up to 6 GHz with more than 3 dB drop from their maximum figure already.
Basically anything above 3 GHz is a gimmick and not part of the core-specs (see first post for those :) ).
I'll probably send one board to TheSignalPath though. If he's interested in checking it out and willing to work around the most likely below standard User Interface  ;)

Regarding Keyboard, Display and other things that can be connected to the mainboard, I'll update the github-repository with some specifications about peripherals. Those will contain pinouts and some notes about the communication-protocols.

I've also had to switch IDEs since the Sloeber-IDE (A modified Eclipse Distribution for Arduinos) has been abandoned by the developer and there are now bugs piling up that make it unsuable with the STM32Duino Libraries.
I'm currently undecided whether to stick with Arduino and use Visual Studio as the IDE (with a Plugin), or switch completely to the STM32CubeIDE with which I've programmed a gamecontroller over the last 2 weeks with not too many showstoppers.


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