Re: Alternative SG504 head- lower cost, better function.

 

I never received the email asking for Gerbers. I've no clue why.

I don't get the 3" extension bit - are you connecting the Rigid coax to the PCB, then passing it through a gland and then have a BNC female on the end? (PS don't you mean BNC male here? BNC Female is what's on the front of most scopes.)

Dave

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: 08 February 2016 03:48
To: TekScopes@...
Subject: [TekScopes] Alternative SG504 head- lower cost, better function.

As we're discussing the SG504:
I broke my Partridge leveling head the other day, because the RG58 input and lemo cable are directly tied to the PCB, they got yanked cuz I was moving a nearby scope and cables tangled and bent/cracked the PCB.

I wrote David about trying to repair the PCB ..(asking for the layout files etc.) But I guess that's not gonna happen...no reply. Permittivity is probably screwed due to the cracks anyway.


So, now I have delved into reverse engineering the head myself with some improvements. The detector diode selection app note from HP (1999) does suggest the HSMS-282 series as optimal for >20dbm AGC signal management under 4Ghz.
Having traced all the signal paths and the method used by Tek to manage the signal strength I have come up with a couple cost & functional improvements. I noted significant fluctuations in the 'leveled' signal when using 3' RG58 patch cables (from the head) to reach a D.U.T. . This defeats the purpose of the remote head. So I moved the D.U.T to the head and used a shorter patch cable. When moving the D.U.T away from the head, the accident happened and here I am with no head.
An expensive accident.

So the plan is:
1) Do away with the lemo cable, remove the lemo bulkhead connector from the faceplate and use a custom bushing holding a simple 2mm, 2 row x 3 pin gold header & cable to connect to the SG504. Same arrangement on the Head.
2) Add a BNC-m bulkhead for the head input & output using a semi rigid
RG142 (3") to BNC Female . The input provides for strain relief and low loss cable upgrades for longer extensions eg. RG400.
3) The semi rigid pigtail output provides great signal integrity/flatness across the 1Ghz range and the Fem. BNC 'end' allows the head to connect directly to a 'scope or any device using a BNC-m connector directly with minimal loss.

Advantages:
1) Cut out the lemo connector costs. Using a 6 pin, keyed M22 gold header in a custom, round faceplate bushing. No alteration of the faceplate hole.
2) This allows for the DC level sig. cable to pull loose (from the
headers) if accidentally yanked (the lack of this killed my last head).
3) Allow simple cable length upgrades (user DIY) for both the RF sig and the DC level feedback sig. This adds bench top flexibility which does not now exist.
4) Improve level signal integrity by directly connecting the head to the D.U.T. (like a scope) with a 3" semi rigid pigtail and no adapter/ extension losses. Probably 1 to 2dBc of flatness benefit here.

As I now have a calibrated HP8753D 3Ghz VNA and a Kirby Microwave professional SOL cal. kit, I can assess the performance of the upgrades and tweak the design as required as per VSWR and transmission tests across the Ghz range.

This link:
http://www.taconic-add.com/pdf/technicaltopics--effects-of-solder-masks-and-conformal-coatings.pdf
shows that at the 1Ghz range, conformal coatings & copper surface finish have minimal effect on the transmission performance.

If anyone is interested in getting such an SG504 head, fully assembled, calibrated, conformal coated and VNA tested, complete with the custom
SG504 faceplate bushing and 6 pin DC, sig. level connector, let me know.
I can consider a group build. I figure the cost should be around $80 complete. A copper shielded custom acrylic housing as an option. You will have to replace the lemo bulkhead on the SG504 with the provided bushing & M22 connector but that is dead easy to do, desolder & solder 5 pins from a ribbon cable.

If you wish, after the upgrade, you can now 'sell' your lemo connectors etc. on EBAY for $35 and offset costs. Or sell your old working head etc. and 'earn' $$.







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Posted by: AncelB <mosaicmerc@...>
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