Author Topic: Open Source HW RF Signal Generator  (Read 7561 times)

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Online chrisl

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Re: Open Source HW RF Signal Generator
« Reply #25 on: September 06, 2017, 06:34:39 PM »
A couple of quick comments:

Looks like all the RF switches  have DC block caps at the GND pin (2).  You want to ground this pin directly to the board GND.

Do not expect that the fab house can control and hold the tight tolerance on the DK and 4 mil trace width - especially the low coast PCB houses.
Simulate your filter design to make sure that your design can tolerate +/- 10 to 20% of variation.
It would be much easier and cleaner to use SMT filters at 1.5 and 3 GHz.

 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #26 on: September 09, 2017, 02:23:18 AM »
A couple of quick comments:

Looks like all the RF switches  have DC block caps at the GND pin (2).  You want to ground this pin directly to the board GND.

Do not expect that the fab house can control and hold the tight tolerance on the DK and 4 mil trace width - especially the low coast PCB houses.
Simulate your filter design to make sure that your design can tolerate +/- 10 to 20% of variation.
It would be much easier and cleaner to use SMT filters at 1.5 and 3 GHz.

Yes, they do have DC Blocks at the GND-Pin. The reason behind this: I've used some attenuators from MaCom in a previous design that required exactly that: DC Blocks at the GND-Pin if driven by only positive voltages.
To avoid having to respin the board if that's the case with these switches too, I've but the DC Blocks in there. If they work without those caps, I'll replace them with 0Ohm Resistors or solder-blobs.

I'm currently using 6mil traces, as that's the smallest ShenZhen2U or EasyEDA can manufacture without massive cost increase.
SMT-Filter available from digikey unfortunately don't really have the features I require. There are filters available that could replace the 3GHz Filter, but a Filter for 1.5GHz isn't really available.

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #27 on: September 10, 2017, 10:23:14 AM »
I've just finished the controller-board. I don't have a block-diagram yet, but it is pretty straight forward:
A NodeMCU-Board with an ESP-12E contains the micro, as well as the 3.3V-Converter and the USB-Connection.
Attached to that are 3 PCF8574. One for the Keyboard (Address 0x21), One for the ChipSelect-Signals for the ADF4351 and AD9910, as well as two Interrupt-Inputs - one for the Keyboard, one for the Touch-Controller (Address 0x22), and One PCF8574 for the Attenuator-Control (Address 0x20).
Because the Attenuator requires 5V TTL-Levels, I've added what I had in my parts-drawers to do the level shifting: 74HCT14 and a 74LS00 (Both have a Positive Threshold of about 2V).
To use the TouchScreen, I've installed the STMPE610 Module from Adafruit and the Display is the 3.5inch 480x320 TouchScreen-Display-Module from Adafruit.

The MCU-Board is socketed by the way, because I had problems uploading a new Firmware when I had it soldered in. Really strange - GPIO-Pins 0 and 2 were showing a 0.5Vpp oscillation at about 2V DC-Offset when the Reset came and the bootloader was supposed to start and accept a new Upload.
Did anyone of you guys had similar problems with that board? Or do you have an idea what could have caused this behavior? Installing a socket made with female 2.54mm headers and plugging the MCU-Board into that solved the problem...

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #28 on: September 22, 2017, 10:22:41 AM »
I've measured the Attenuator. Looks like it is rather flat across the frequency-band I'm interested in.

To measure it, I used one of those noise-sources from ebay that get ridiculously hot after a few minutes :)

The B-Trace is just the noise-source connected via a 1m RG316-Cable with SMA-Connectors.
The A-Trace has the Attenuator inserted. It is powered with +5V and no attenuation is selected. The loss at no attenuation is about 10dB.

Input-Power is planned to be about +15dBm, which results in output-power of +5dBm that can be attenuated down to -90dBm.
With the help of the PIN-Attenuator the dynamic range is about +5dBm to -100dBm.
And when I use the AD9910, the amplitude scale factor-setting should allow for even weaker signals, as well as much finer control :)
« Last Edit: September 22, 2017, 06:49:28 PM by SaabFAN »
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #29 on: September 23, 2017, 02:16:08 AM »
Updated RF-Detectors for the RF_LEVEL_SENSE and RF_OUT_SENSE-Signals.

RF_OUT_SENSE is used during calibration to fill the lookup-table with the right correction-factors. RF_LEVEL_SENSE provides the feedback-signal for the AGC.

I've also decided to use Microstrip-Traces instead of CPW, after reading through a PDF by Rogers that showed Microstrip to be less sensitive to changes in the dielectric constant of the material.
Manufacturing will take place at EasyEDA, as the 0.012mm thickness of the dielectric between first and second layer is too small to build the filters.

I'm not entirely sure, but I think the board is done now. Unless there's something wrong that I missed :)

Edit: Forgot one thing: The Soldermask-Cutouts to solder RF-Shields.

Edit 2: I just realized that I made a mistake, calculating the DC-Voltages. They don't go higher but lower once a signal is present at the input of the detector. So I don't need the bias-circuit on the RF_LEVEL_SENSE-Detector.
« Last Edit: September 23, 2017, 04:21:47 AM by SaabFAN »
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #30 on: September 24, 2017, 05:51:44 AM »
I've changed the Supply-Voltage to 3.3V on the RF-Board, which allows me to work without any level shifters on the i2c-Bus. Doing that also meant I had to add some gain to the Output of the DAC that generates the AGC_BIAS. How much gain I need will be determined during testing.

On the PCB I've added pads to solder some kind of shield-can to. I've also moved the components around a bit to improve the layout and cleaned up the mess of the designators, which were sitting all over the place.

Attached you'll find the current gerbers and the schematics for the board.

This is the memory-map of the i2c-NVRAM-Chip on the RF-Board btw.:
 *  Memory-Map:
 *  0x000 - 0x0F0 = CAL-DATA: 12bit DAC-Calibration Data for PWR_SET-DAC is stored for every 25MHz-Step Starting at 0MHz and going up to 3GHz (120 16bit-Words)
 *  0x0F2 - 0x1E2 = CAL-DATA: 12bit DAC-Calibration Data for AGC_ATTEN_BIAS is stored for every 25MHz-Step Starting at 0MHz and going up to 3GHz (120 16bit Words)
 *  0x400 - 0x5FE = User-Config / User-Data
 *  0x600 - 0x7FE = Sweep-Settings
« Last Edit: September 24, 2017, 06:21:31 PM by SaabFAN »
 

Offline ogden

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Re: Open Source HW RF Signal Generator
« Reply #31 on: October 03, 2017, 03:52:36 AM »
Filter banks, unless very narrowband, will not help against square wave harmonics. I would go "traditional RF synth gen" way: downmixing. For example check Agilent E4400 service guide, E4400-90335.pdf and you will see what I mean. Instead of filter banks you need one 2.4GHz(?) CW source (like CC2500 in CW mode), 4.4GHz - capable mixer and 1GHz(?) low pass filter. Then you either use direct output from ADF4351 in 1GHz-4.4Ghz range or downmixed and filtered signal. Benefit of using 2.4GHz mixed with 2.4-4.4GHz of ADF4351 is: it is range of its VCO w/o divider.

« Last Edit: October 03, 2017, 07:11:40 AM by ogden »
 

Offline SaabFAN

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Re: Open Source HW RF Signal Generator
« Reply #32 on: October 09, 2017, 09:47:23 AM »
Thanks for the advice, but I've already ordered the boards and am now comitted to this path  :)

For frequencies up to 400MHz, I'll use the DDS-Module anyway, so not that big of a problem there.
I'll have to see how strong the harmonics are after filtering. If they are too strong, I can always put something between the Synthesizer-Board and the Filter-Board.  ;)


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