HMC624ALP4E based variable attenuator for the home lab? - Page 1
Forum section: electronics / rf-microwave

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 10, 2018, 07:44:17 am
content: So, in my wandering over the analog devices pages, I came across the HMC624A. This is a DC-6 GHz (you can get under the 100 MHz lower limit using external capacitors) digital variable attenuator. It has parallel control or SPI input, from as far as I can tell.It goes for about 10 Euros, which seems like a good deal to make a simple home-built variable attenuator in a compact form factor. I think that the parallel input means that you can just use a dip controller to set it, if you desire, which means that the attenuator board would just have power in, and two RF connectors of your choice. Anyone have any experience with these?I'm looking at making a attenuator box using it. Any "traps" I might be missing?Link:http://www.analog.com/en/products/rf-microwave/attenuators/digital-step-attenuators/hmc624a.html?doc=HMC624A.pdf

author: yl3akb
author_rank: Regular Contributor
timestamp: January 10, 2018, 11:33:36 am
content: Hi!I have used it in one of my projects in with frequency range from 1 MHz to 6 GHz with 100nF DC block capacitors with no issues at all. But due to its high price, now I am looking at this one, which is only around 5 EUR, has less external components and 7 bit resolution:http://www.qorvo.com/products/p/RFSA3714

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 10, 2018, 11:50:04 am
content: Waw, thanks for the suggestion! Will definitely look at that one too. Unfortunate that it only goes down to 50 MHz, I was hoping to get something I could also use at lower frequencies since for now I don't have much gear that goes up to high frequencies. Perhaps better to build some simple resistive attenuators for now then instead. Will look at some of qorvo's other products too though, see what they have.

author: henrikf
author_rank: Newbie
timestamp: January 10, 2018, 03:12:36 pm
content: You might want to also check out PE43711. It goes down to very low frequencies. I used this on my VNA and it worked well.

author: Yansi
author_rank: Super Contributor
timestamp: January 11, 2018, 09:23:12 pm
content: Hi,you may find interesting having a look in my other DDS thread here. Currently I am also solving a design problem of a wide band variable attenuator, in fact complete ALC circuit. Some approaches to variable attenuators are evaluated (PIN, FET, stepped relay resistor, even those digital ones).https://www.eevblog.com/forum/rf-microwave/half-decent-diy-rf-generator-utilizing-ad9951-an-idea/100/But my thread does not seem to get much attention, so the progress is kinda slow and I am mostly making things up to move forward.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 13, 2018, 08:23:34 am
content: I have looked at the thread and am keeping an eye on it as it progresses Yansi! Unfortunatly I am still a beginner and really not that familiar with the design of systems and weighing different options. I'll read through the entire thread later today!

author: Yansi
author_rank: Super Contributor
timestamp: January 13, 2018, 10:38:54 am
content: I am no less beginner than you. Feel free to ask, you may sometimes get an answer

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 22, 2018, 08:38:24 am
content: Ok, I've spent some time on this yesterday and I have decided on what I want to do.I'm going to get 3 boards populated, using the PE43711. I'm going to have everything go out to headers such that I can use an external control board to control them.Because I don't trust myself not to screw up I'm going to include DC blocking capacitors on each of the boards. However, since for going down to 9 kHz you need very big capacitors (in terms of RF-capable capacitors at least) I think I'm going to have different versions with different value capacitors. I'll have a version with some bigger capacitors for going down to 9 kHz, and have the other two use proper RF capacitors that have much higher self-resonance frequencies.My current idea is to have them all attached to a 19" rack. I'm thinking of getting small metal shield boxes to put each in separately and mount that to the back of the plate, but I will have to see what is possible here.I'll also have to see what I do regarding control system. My arb is floating, so it might be interesting to keep the modules floating as well and use some form of optoisolation to control the thing. Then again, if I keep them floating that means I have to make sure that the connectors and shields are isolated too, so it might be more hassle than it is worth.Control will be simple - just an arduino and an LCD. Perhaps three sets of up-down buttons (+-0.25 dB, +-3 dB and +- 10 dB?) and a "channel select" option? I'm gonna try to get the RF boards out as fast as possible, so I have them in a few weeks for testing.Will keep you guys updated and am always interested in feedback.

author: Yansi
author_rank: Super Contributor
timestamp: January 22, 2018, 09:12:38 am
content: I will be receiving my boards for the ALC in just a few hours. Waiting the DHL now. Looking forward also to see your design.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 23, 2018, 07:38:39 am
content: I've had a change of plans. Instead of going for one centralized system I think what I will do instead is make a number of attenuator boards that are small, and put them in individual cases. I will then put a controller board inside of my bench where I can hook up the attenuator boxes with a control cable. This way I can put the attenuators where they are most convenient but have a central control point where I can easily set and power everything. Will need a good amount of filtering on the control inputs though to avoid them acting as antennas and coupling in signal from outside the boxes.