Eliminating gang error


+ Reply to Thread
Results 1 to 8 of 8

Thread: Eliminating gang error2442 days old

  1. #1
    wastrix
    Guest wastrix's Avatar

    0 Not allowed!

    Default Eliminating gang error

    Hi all,

    I have to smoothly adjust the amplitude of a wideband (DC-50MHz or so) signal from 0 to full scale, but at the same time know what position my adjustment is at. The simplest solution is to use a good quality dual gang pot like: http://australia.rs-online.com/web/p...eters/7293442/

    One gang attenuates the signal, the other is connected 0-5V and the PIC reads it to determine the amplitude setting.

    However these pots are specified to having a gang error (tracking discrepancy between gangs) of +-3dB over -40dB to 0dB signal attenuation.

    I need a way to reduce this error. I thought about running a DC calibration signal through, then measuring the value of each gang for each position of the pot, building up a big lookup to calibrate out the error.

    I'm not sure whether this would work over time and varying temperature though - does gang error change over temperature or age?

    Does anyone have any better solutions?

Attention

This valuable resource relies upon the very small amount of revenue generated by displaying online advertisements to our visitors.

The advertisements we display are relevant to this web site and your browsing history

Please consider supporting us by disabling your ad blocker.


Note: Some users have reported issues related to ad-blockers rendering parts of this wesite unusable,
where possible we will rectify the issues to enable you to use this resource with adblocking enabled.

If you can, please report issues in the forum area WebSite / Forum Issues


Thank you for your attention.

  • #2
    david
    Guest david's Avatar

    0 Not allowed!

    Default Re: Eliminating gang error

    Hi wastrix,
    I had a look at your link and was not greatly impressed by the specification. Alps do make much better pots but you will likely pay about 10x as much (check out their RK50 series)
    Apart from that I really don't think you will get 50MHz to attenuate in a pot like that. Stray C will almost certainly see the attenuation become unpredictable at mid span unless using very low resistances. Source and load Z will also shift around.

    One obvious option is PIN diode attenuators but this will have a minimum frequency limit of somewhere around 1MHz or perhaps a little lower but not down to audio frequencies.
    Another possibility is variable gain, wideband amplifiers. AD603 is an older type but there are many newer types around.

    There are many good FET switch arrays which could be used in binary sequenced attenuators but watch the OFF capacitance rather than the ON resistance as this is what matter as you approach the 50MHz end.

    Many years back I saw an idea for using a small toroid core for passing your signal through. You then add another small winding and run increasing DC through it to saturate the core and thereby attenuate the signal. Never tried it but it sounded somewhat plausible.
    I would start checking out variable gain amplifiers myself.

    Cheers,
    David

  • Attention

    This valuable resource relies upon the very small amount of revenue generated by displaying online advertisements to our visitors.

    The advertisements we display are relevant to this web site and your browsing history

    Please consider supporting us by disabling your ad blocker.


    Note: Some users have reported issues related to ad-blockers rendering parts of this wesite unusable,
    where possible we will rectify the issues to enable you to use this resource with adblocking enabled.

    If you can, please report issues in the forum area WebSite / Forum Issues


    Thank you for your attention.

  • #3
    Senior Member Stephen Moss's Avatar
    Join Date
    Jan 2006
    Posts
    511
    Thumbs Up
    Received: 31
    Given: 7
    Total Downloaded
    3.72 GB

    0 Not allowed!

    Default Re: Eliminating gang error

    I am not sure what you mean by "gang error" however, I would assume that just means the wiper positions on each pot will not be perfectly aligned and so there will be a constant difference between the values across the entire adjustment range of the pot.
    That being the case I would have though that provided you have measured the difference between them you can just add/subtract the relevant value from the ADC reading to know what the attenuation is. Provided neither track is effected by a manufacturing error you have to assume that all changes in regard to operating temperature, humidity, age and wear would be equal otherwise you would need to recalibrate every time you use it which is impractical.

    Alternatively, I have not used one myself but provided that you can find one with a tolerance and adjustment resolution equal to that you would get doing it by hand then perhaps a digital pot would be the answer and you simply increment/decrement a value to keep track of the attenuation as you adjust the resistance up/down.
    Those with an I2C interface may be better than those with an up/down/clock control as I believe you should be able to read the current value and thus know where your are after a power/brown out situation but it has been a while a few years since I read any data sheets for these devices.

  • Attention

    This valuable resource relies upon the very small amount of revenue generated by displaying online advertisements to our visitors.

    The advertisements we display are relevant to this web site and your browsing history

    Please consider supporting us by disabling your ad blocker.


    Note: Some users have reported issues related to ad-blockers rendering parts of this wesite unusable,
    where possible we will rectify the issues to enable you to use this resource with adblocking enabled.

    If you can, please report issues in the forum area WebSite / Forum Issues


    Thank you for your attention.

  • #4
    Senior Member Ken_K's Avatar
    Join Date
    Aug 2007
    Posts
    434
    Thumbs Up
    Received: 7
    Given: 2
    Total Downloaded
    529.65 MB

    0 Not allowed!

    Default Re: Eliminating gang error

    Hi Wastrix

    I have found using pots for RF attenuation is not as good as one would expect, have never tried using a look up table as you suggest, it would be interesting to see how it goes.

    I guess you have already looked at digital step attenuators, this is the first time I've had a look, quite a few on the market, the DAT-31R5-SP seems to have a frequency response of DC to GHz with .5dB steps, a good search may reveal a much more suitable device.

    http://www.cdiweb.com/datasheets/min...AT-31R5-SP.pdf

    Regards
    Ken

  • Attention

    This valuable resource relies upon the very small amount of revenue generated by displaying online advertisements to our visitors.

    The advertisements we display are relevant to this web site and your browsing history

    Please consider supporting us by disabling your ad blocker.


    Note: Some users have reported issues related to ad-blockers rendering parts of this wesite unusable,
    where possible we will rectify the issues to enable you to use this resource with adblocking enabled.

    If you can, please report issues in the forum area WebSite / Forum Issues


    Thank you for your attention.

  • #5
    wastrix
    Guest wastrix's Avatar

    0 Not allowed!

    Default Re: Eliminating gang error

    Thanks for your replies.

    @David, those pots are not great, but they're about as good as I can get my hands on for a reasonable price. I tried them out today and performance was pretty marginal as we expect, especially at the middle of travel. The RK50s though are way out of my price range and even in volume would cost about twice as much as the rest of the unit combined.

    The design actually already has a wideband VGA in it (THS7530), much earlier on in the signal chain. But the relatively low dynamic range made me try to use a pot for main amplitude control.

    I want to keep complexity to a minimum, and low frequency/DC signal components are very important so I am discouraged by the extra circuitry that would be needed to implement an solution that is inherently AC coupled like the PIN diodes or toroid (linearity?).

    @Stephen, yes by gang error I am referring to variations between the gangs, I believe this is mainly due to manufacturing inconsistency in the carbon tracks. Your point about equal aging and tempco is probably right.

    I looked at some digital pots, but as far as I can see they are pretty much all sub MHz bandwidth.

    @Ken, not doing any real RF work myself, I didn't know specifically that those existed (I searched briefly for something like it, but didn't find anything at the time).

    I think what I will do now is try using a discrete attenuator series (either a digital step attenuator or relay/photoMOS chain), and combined with some extra control of the VGA that should allow me to get a far more accurate, linear and better performing amplitude control.

  • Attention

    This valuable resource relies upon the very small amount of revenue generated by displaying online advertisements to our visitors.

    The advertisements we display are relevant to this web site and your browsing history

    Please consider supporting us by disabling your ad blocker.


    Note: Some users have reported issues related to ad-blockers rendering parts of this wesite unusable,
    where possible we will rectify the issues to enable you to use this resource with adblocking enabled.

    If you can, please report issues in the forum area WebSite / Forum Issues


    Thank you for your attention.

  • #6
    david
    Guest david's Avatar

    0 Not allowed!

    Default Re: Eliminating gang error

    Hi,
    I did like the parts Ken linked to - very nice.
    The dual pot idea might be usable if you keep the resistance to the lowest you can tolerate. Probably about 1k for a non wire wound type and this would give a maximum impedance of 250R at mid travel if we assume the source resistance is always quite low. You would need very low stray C on the wiper of the pot to avoid roll off at mid position.

    Cheers,
    David

  • Attention

    This valuable resource relies upon the very small amount of revenue generated by displaying online advertisements to our visitors.

    The advertisements we display are relevant to this web site and your browsing history

    Please consider supporting us by disabling your ad blocker.


    Note: Some users have reported issues related to ad-blockers rendering parts of this wesite unusable,
    where possible we will rectify the issues to enable you to use this resource with adblocking enabled.

    If you can, please report issues in the forum area WebSite / Forum Issues


    Thank you for your attention.

  • #7
    wastrix
    Guest wastrix's Avatar

    0 Not allowed!

    Default Re: Eliminating gang error

    The issue with the dual pot is that for mechanical reasons (length must be greater than 25mm, I spent ages finding a suitable part), the only available type is the 10k type I mentioned.

  • Attention

    This valuable resource relies upon the very small amount of revenue generated by displaying online advertisements to our visitors.

    The advertisements we display are relevant to this web site and your browsing history

    Please consider supporting us by disabling your ad blocker.


    Note: Some users have reported issues related to ad-blockers rendering parts of this wesite unusable,
    where possible we will rectify the issues to enable you to use this resource with adblocking enabled.

    If you can, please report issues in the forum area WebSite / Forum Issues


    Thank you for your attention.

  • #8
    Member Parmin's Avatar
    Join Date
    Jun 2004
    Posts
    112
    Thumbs Up
    Received: 7
    Given: 19
    Total Downloaded
    1.25 GB

    0 Not allowed!

    Default Re: Eliminating gang error

    I am not sure if this will help, but what if you are using say a couple of 1024 steps digital pot, controlled by a rotary switch , will this do?
    Those that say something cannot be done should stop bothering those who are doing it.

  • Attention

    This valuable resource relies upon the very small amount of revenue generated by displaying online advertisements to our visitors.

    The advertisements we display are relevant to this web site and your browsing history

    Please consider supporting us by disabling your ad blocker.


    Note: Some users have reported issues related to ad-blockers rendering parts of this wesite unusable,
    where possible we will rectify the issues to enable you to use this resource with adblocking enabled.

    If you can, please report issues in the forum area WebSite / Forum Issues


    Thank you for your attention.

    Thread Information

    Users Browsing this Thread

    There are currently 1 users browsing this thread. (0 members and 1 guests)

       

    Similar Threads

    1. PIC gang programming
      By [email protected] in forum The Lounge
      Replies: 6
      Last Post: 26th September 2017, 22:30
    2. [SOLVED !] Hundreds of ASM error of the same errors. Error [126]
      By wirecut in forum Proton Plus Compiler v3
      Replies: 2
      Last Post: 28th January 2012, 09:10
    3. Gang programming
      By [email protected] in forum The Lounge
      Replies: 7
      Last Post: 13th February 2009, 16:45

    Members who have read this thread since 5th December 2019, 10:29 : 0

    Actions :  (Set Date)  (Clear Date)

    You do not have permission to view the list of names.

    Tags for this Thread

    Posting Permissions

    • You may not post new threads
    • You may not post replies
    • You may not post attachments
    • You may not edit your posts