Vector signal generator differential I and Q outputs not understood  - Page 1
Forum section: electronics / repair

author: 240RS
author_rank: Regular Contributor
timestamp: July 05, 2023, 06:50:27 am
content: I have this signal generator with 512MSamples baseband IQ memory and 160MHz maximum sampling clock. Where I normally use its function to upconvert this with its LO, I am also interested in getting the IQ signals out in baseband.For this there is a 15 pin connector on the back with differential I and Q pairs and the rest GND. Baseband. I have attached the description of those outputs. I want to use the I output single ended to generate broadband 0 - 30MHz signal (first 2MHz and amateur bands notched). But I do not know what connector to make for it. Mostly since I do not understand this “standard” of differential IQ with controllable DC bias. Any explanation helpful.

author: wasedadoc
author_rank: Super Contributor
timestamp: July 05, 2023, 03:50:16 pm
content: Why are you posting in the Repair section?You just need to know which pins of the connector are I and Ground. Set the offsets to zero if you want the signal to be AC (equal excursions above and below Ground).

author: 240RS
author_rank: Regular Contributor
timestamp: July 05, 2023, 03:59:49 pm
content: Mistake on my side posting it here.So just I and ground, combined with zero offset on the software side.What are these outputs types normally used for?

author: rfclown
author_rank: Frequent Contributor
timestamp: July 05, 2023, 04:27:35 pm
content: I don't believe that there is any "standard" for this. It is what the manufacturer tells you it is, and you deal with it as you please. IQ baseband output with 160 MHz clock is pretty impressive. I usually deal with HP/Agilent/Keysight units with max clocks of <= 100 MHz which output their baseband IQ using single ended connectors (like BNC). Whatever the case, you have to interface whatever they give you to what you want. If you want single ended, and they give you differential, then you have to use a differential to single ended converter which will usually be an op-amp circuit. When I've done this in the past it has been some RFIC that has differential I/Q outputs that I wanted to convert to single ended, and I dead bugged an op-amp circuit on a piece of scrap PCB to do the job.

author: 240RS
author_rank: Regular Contributor
timestamp: July 05, 2023, 04:40:12 pm
content: I want to use it to test a 130MSPS 16bit DAC. If I can do that without an opamp that would be great.

author: rfclown
author_rank: Frequent Contributor
timestamp: July 05, 2023, 04:47:18 pm
content: Quote from: 240RS on July 05, 2023, 04:40:12 pmDo you mean test a ADC (not DAC)? If you look at the ADC's data sheet I would guess that they will show you an example application circuit with an op-amp at the input.

author: 240RS
author_rank: Regular Contributor
timestamp: July 05, 2023, 06:10:38 pm
content: It is an HF zero IF software defined radio (RX-888). The actual ADC in it is differential, but my input to it is 50 Ohm single ended

author: wasedadoc
author_rank: Super Contributor
timestamp: July 05, 2023, 06:43:53 pm
content: What does it say in the part of the spec that you have cut off at the bottom of the image in the first post?I think you can use either I or its inverse by itself as a single ended source.

author: 240RS
author_rank: Regular Contributor
timestamp: July 05, 2023, 07:14:41 pm
content: Quote from: wasedadoc on July 05, 2023, 06:43:53 pmAttached

author: 240RS
author_rank: Regular Contributor
timestamp: July 06, 2023, 10:41:54 am
content: Got it, ran a test and below the results. Taking positive I against GND seems to work fine.Instrument: Anritsu MG3700A Vector Signal GeneratorSoftware to upload IQ files: Anritsu "IQ Producer"Waveform file generated: 1kHz signal between +0.2 and 1.0 for AM modulation. I only, Q=0Normalization by "IQ Producer": -1.0 => -8192, +1.0 => 2^14-1 (8192-1)Modulation: AM with 250kHz carrierBaseband IQ output:I+: ~0.3 to 1.5V (high impedance measurement)I-: ~ -0.3 to -1.5VSee screenshot for modulated signal, positive I output, negative I output.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 06, 2023, 11:34:09 am
content: Using I+ and I- along with a diff amp will help reduce the 2nd and higher even order distortion components. If you are not worried about that, then using just the I+ or I- w.r.t. GND should be just fine. You may want to buffer this signal anyway as the output signals may be coming directly from the baseband DAC and these may not be able to drive high loads.

author: 240RS
author_rank: Regular Contributor
timestamp: July 06, 2023, 11:43:50 am
content: Thanks! Any suggestion for what opamp to use for wideband 2-30MHz?Quote from: EEEnthusiast on July 06, 2023, 11:34:09 amGiven that output impedance is given as 50Ohm, can I conclude there is no need to buffer?

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: July 06, 2023, 12:35:32 pm
content: Quote from: 240RS on July 06, 2023, 11:43:50 amSorry... my bad. For 50 ohms output impedance you do not need any buffering. They are made to drive transmission lines and a 50 ohms load at the end.

author: 240RS
author_rank: Regular Contributor
timestamp: July 06, 2023, 01:01:35 pm
content: Happy to hear. Quite convenient then.

author: 240RS
author_rank: Regular Contributor
timestamp: July 24, 2023, 04:20:48 pm
content: Quote from: EEEnthusiast on July 06, 2023, 12:35:32 pmSorry... my bad. For 50 ohms output impedance you do not need any buffering. They are made to drive transmission lines and a 50 ohms load at the end.All works fine. Have my converter plug made and tested. Two 50Ohm outputs by connecting plus I and Q outputs to BNC’s that are grounded.