Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Aug Tue 21, 2018 6:15 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 23 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Spectrum analyzers DC blocking devices connected to input..
PostPosted: Feb Fri 02, 2018 8:56 pm 
Member

Joined: Dec Mon 17, 2012 5:09 pm
Posts: 61
Location: Quebec City, Canada G2E5S1
I have a HP 8594A Spectrum Analyzer that I would like to protect
from any incoming DC voltage to its 50 ohms input.
I saw some DC blocking BNC to BNC devices around and I was wondering
what is the internal circuitry of such devices..
Maybe only a series capacitor around 0.1uf between both input and output???
Any clues ??


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Fri 02, 2018 9:39 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1168
Location: Tucson AZ
http://literature.cdn.keysight.com/litweb/pdf/5989-8791EN.pdf


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sat 03, 2018 4:15 am 
Member

Joined: Oct Sun 15, 2006 12:57 pm
Posts: 5719
Location: Liberty, Missorui
Unless I'm looking at a tracking generator, as you would when doing filter work, I always have an attenuater in the line.

_________________
Pics, Bold & underlined text, are usually links.

https://mikeharrison.smugmug.com/

Mike


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sat 03, 2018 5:10 pm 
Member

Joined: Feb Wed 25, 2009 4:06 pm
Posts: 1295
Location: Morristown, N.J.
A series cap is cheap and will do the job of blocking DC well; a 0.01uF being the largest value to use.

I too use a sacrificial attenuator on the input to my spectrum analyzer as well as one on the output of my RF synthesizer. Its cheap insurance. Unless you are working with microwaves you don't need HP attenuators; there are plenty of cheap ones out there.

_________________
Pete AI2V

"It's always darkest before it's pitch black"


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sat 03, 2018 9:44 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 410
Location: Green Valley, AZ, USA
Would measuring the DC resistance of SA input with a Fluke DVM cause a problem? If so what is it that a low DC current destroys that AC can't?
I've never seen a good explanation.
Thanks
Hank WD5JFR


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sun 04, 2018 1:42 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1168
Location: Tucson AZ
wd5jfr wrote:
Would measuring the DC resistance of SA input with a Fluke DVM cause a problem? If so what is it that a low DC current destroys that AC can't?
I've never seen a good explanation.
Thanks
Hank WD5JFR


Measuring the DC resistance....... Depends on the model of spectrum analyzer. My HP 8591E will tolerate a max of 30vdc.


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sun 04, 2018 2:24 am 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7820
Location: Long Island
Like a lot of things in electronics, it's not as simple as it looks. At its most basic level, a DC block is nothing more than a series capacitor. But then it starts getting complicated. First, we have to remember that any capacitor placed in series with a load (the input of the spectrum analyzer) is going to make a high pass filter. The cap has to be big enough to put the roll-off or cut-off frequency well below the lowest frequency of interest. But all practical caps have some self inductance, and the bigger the cap, the greater it is. At microwave and UHF frequencies, a few pF's will do. At audio frequencies you'd need 60-100 uF. Frequencies between those extremes would call for caps of intermediate size.

Quote:
Would measuring the DC resistance of SA input with a Fluke DVM cause a problem? If so what is it that a low DC current destroys that AC can't?
I've never seen a good explanation.


The issue is, the input of a spectrum analyzer is a very specialized sensitive mixing diode which cannot tolerate much DC in either direction without suffering a loss of sensitivity. Yes there is an attenuator ahead of it in most modern spectrum analyzers but it's still best to keep DC off the input as much as possible. If you really need to know the impedance of a SA input for some reason, the best way to test it is with a signal generator having a 50-ohm calibrated, attenuated output. Set the generator up on some frequency and output level within the range of the spectrum analyzer but leave the output cable disconnected. Note the reading on the signal generator's output meter. Connect the spectrum analyzer input to the signal generator, which should drop the output by half. This can be confirmed by increasing the attenuator until the meter shows the original level again. If the difference between the terminated and unterminated reading is 6 dB, then you know the spectrum analyzer and the signal generator have the same impedance.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sun 04, 2018 3:48 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 410
Location: Green Valley, AZ, USA
Hey Chris
Thanks for the comments on the SA DC input. I bought a Tek 496 that's partially well mostly deaf and will post the symptoms in another thread. Had no luck on Yahoo Tek scopes or EEVBlog.
73
Hank


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sun 04, 2018 7:54 pm 
Member

Joined: Dec Mon 17, 2012 5:09 pm
Posts: 61
Location: Quebec City, Canada G2E5S1
Possible choice:
Back to back diodes in parallel with the DC blocker's final output to the SA 50R input.
Since my SA has a max of +30dbm input tolerance (10vpp)
I'm thinking of using enough back to back schotky diodes to do a +/- 5v clipping protection.. with a 0.01uf cap in series between both BNC center
conductors.
Again I'm wondering the freq flatness around 500kHz to 1600khz
using a 0.01uf @ 50R input imp. as a DC blocker..??
What do you think ?


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sun 04, 2018 8:52 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1168
Location: Tucson AZ
https://www.fairviewmicrowave.com/inner ... 235-p.aspx


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sun 04, 2018 9:35 pm 
Member

Joined: Dec Mon 17, 2012 5:09 pm
Posts: 61
Location: Quebec City, Canada G2E5S1
Very nice device...


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sun 11, 2018 6:29 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4416
Location: NJ, 07645
This might be a useful video for you. I might make one for my HP 3585.

https://www.youtube.com/watch?v=ETKyKC2Zj-M


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Sun 11, 2018 9:14 pm 
Member

Joined: Dec Mon 17, 2012 5:09 pm
Posts: 61
Location: Quebec City, Canada G2E5S1
I already watched that video..
Mr Carlson states that it is made for a 1meg ohms input impedance
spectrume analyzer..
Min has a 50 ohms input.
But thanks anyway for the tip !

JP


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Mon 12, 2018 2:10 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4416
Location: NJ, 07645
All you have to do is add a 50 ohm termination at the box. He shows this in the following video at about 46 minutes into this video: https://www.youtube.com/watch?v=4kT57s0kCAk


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Mon 12, 2018 1:13 pm 
Member

Joined: Dec Mon 17, 2012 5:09 pm
Posts: 61
Location: Quebec City, Canada G2E5S1
Hi Scott,
Correct me if I'm wrong but the Mr Carlson's 50 ohms modified box
that has an added 50 ohms resistor at the BNC connector
is to be connected to a Spectrum analyzer's tracking gen OUTPUT.
Not intended to be connected at an INPUT of 50 ohms right ?


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Mon 12, 2018 1:45 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4416
Location: NJ, 07645
Yes, but I believe it still matches the impedance regardless of an input or output. Impedance match is impedance match, right?
Maybe Chris108 will chime in and confirm that this is correct even for an SA input. A lot of guys on the Youtube site are asking about their 50 ohm input SA's. I am surprised that Carlson has not addressed this.
If you end up building one, I would be happy to share cost to have a small run of PC boards made.


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Mon 12, 2018 2:16 pm 
Member

Joined: Dec Mon 17, 2012 5:09 pm
Posts: 61
Location: Quebec City, Canada G2E5S1
Scott wrote:
Yes, but I believe it still matches the impedance regardless of an input or output. Impedance match is impedance match, right?

I cannot answer this... (??)

Scott wrote:
Maybe Chris108 will chime in and confirm that this is correct even for an SA input.

Chris108 ??

Scott wrote:
If you end up building one, I would be happy to share cost to have a small run of PC boards made.

Good idea.. I'll think about that..


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Tue 13, 2018 1:54 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4416
Location: NJ, 07645
Thinking about this a little further...your biggest issue with the 50 ohm input is going to be working on high impedance tube circuits. It is going to load the signal down and probably drag it into the noise floor. These SA's are not designed for this type of work. I am thinking that a proper probe for a 50 ohm input would need an active stage, and will not be straightforward. I am guessing that is why Carlson and others have avoided answering this question. It is probably why the HP 3585 is still so in demand as it has the 1 meg input and tracking generator also.


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Tue 13, 2018 3:15 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1168
Location: Tucson AZ
My HP 8591E - 75 ohm input, and my HP 8568B - 50 ohm, both have the jacks for an active probe. That may very be the simplest answer.

Edit to add this:

Google search this "poor mans 1-ghz active probe" and the 1st result will be a PDF of an article discussing the build of a 100khz to 1ghz probe that can be used with a spectrum analyzer.


Top
 Profile  
 
 Post subject: Re: Spectrum analyzers DC blocking devices connected to inpu
PostPosted: Feb Tue 13, 2018 1:13 pm 
Member

Joined: Dec Mon 17, 2012 5:09 pm
Posts: 61
Location: Quebec City, Canada G2E5S1
The poor man's probe seems to be what I need !
Thank you so much for the tip !
JP


Top
 Profile  
 
Post New Topic Post Reply  [ 23 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 15 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB