DC block for 8568b - Page 1
Forum section: electronics / testgear

author: cncjerry
author_rank: Unknown
timestamp: December 07, 2015, 06:56:04 am
content: I need a DC block for my 8568B, both bnc and type N connectors. Recommendations? 10K to 3G would be ideal to use with both my analyzers. Thanks

author: TheSteve
author_rank: Unknown
timestamp: December 07, 2015, 07:15:07 am
content: Probably easiest to get SMA models and then convert - don't see BNC or N versions pop up on ebay very often.

author: alterbaron
author_rank: Contributor
timestamp: December 07, 2015, 07:18:25 am
content: I have one ofthesecheap BNC DC blocks and I'm pretty happy with it so far. But it's effectively no-name so buyer beware.

author: TheSteve
author_rank: Unknown
timestamp: December 07, 2015, 07:32:33 am
content: The price is certainly right on that unit - 10 MHz minimum though is pretty bad.I'd say something like this is probably a good starting point if he wants real performance:http://www.ebay.com/itm/281142298514

author: Electro Fan
author_rank: Super Contributor
timestamp: December 07, 2015, 07:33:42 am
content: Would this work with a BNC to N adapter?http://m.ebay.com/itm/Tektronix-DC-Block-015-0221-00-NEW-/121833742833?nav=SEARCH

author: MadTux
author_rank: Frequent Contributor
timestamp: December 07, 2015, 12:28:45 pm
content: 8568B you say? Look closely and you will see, that everything you need is already there. 8566 is different story, here you absolutely need a DC block, because especially the 2-22GHz mixer is very sensitiveOn 8568, N-type connector on the left is already DC blocked, hence the 50V DC offset rating and 100kHz lower limit because of increasing impedance of capacitor at low freq. BNC on the left is DC coupled, so it goes down to 100Hz. Use that one carefully.DC block on 50Ohm down to kHz or 100Hz range requires big capacitors (which also results in big currents on the SA input, if the capacitor is charged to a DC voltage), so these are unpractical and difficult to manufacture, so you won't find many of these.

author: cncjerry
author_rank: Unknown
timestamp: December 07, 2015, 04:27:53 pm
content: Thanks for the referrals. I have an 85685a RF preselector and both connections have 0VDC max. I noticed the N type on the right of the SA itself is good for up to 50V but I have that attached with a jumper to the preselector. That's why I am looking for both, one on the BNC and the other on the N-Type preselector connections.Thanks again.

author: MadTux
author_rank: Frequent Contributor
timestamp: December 07, 2015, 09:30:52 pm
content: Makes more sense now. BTW is your 85685 one of those "parts only" units that recently went for $300 on ebay?Just measured the low freq performance of the cheap DC block for my 8566B. According to cheap china LC-meter, it has a 110pF cap in it. Official rating 800-2500MHz. With Xc=1/(2Pi*f*C) formula, you get an impedance of 50Ohm at around 30MHz => -6dB; 25Ohm at 60Mhz => -3dB; 5.6Ohm at 260MHz => -1dB; and 1.80 Ohm at 800MHz, which is -0.3dB. Because I was curious, I measured it on my 8568 with 8662A as improvised, non synchronized sweeping source.Thereby that Tek 015-0221-00 DC block with 0.047uF cap has -1dB point at 600kHz and -0.3dB point at 1800kHz (it's mine, hopefully)

author: cncjerry
author_rank: Unknown
timestamp: December 08, 2015, 06:17:02 am
content: MadTux,I saw those $300 units and they sold pretty quickly. I came really close to buying (bidding on) one of those two instead of the one this guy had on craigslist for $500. I bought my 8568b from him as well. Trying to save a little money in this case would have been stupid as this one is near perfect and came with the extra adapter for the H-sweep output. I had already had the proper cable on my 8568B (it had the SMA connector on the interconnect) for the H-sweep so I might unload this adapter. I've been in 'If I haven't touched in 30 yrs, sell it" mode lately.If you don't have the preselector I would recommend you take a hard look at it, especially if you can find one for sub $500. I went back and forth on it quite a bit but I've been building 1Ghz cavity filters lately and I wanted a better look at the ultimate rejection. I really does drop the noise floor when the signal is within one of the tracking filters. I also noticed that my tracking generator has more amplitude range now, must be because of the preamp in the preselector.This combo kicks the butt of my much more modern Advantest analyzer but it weighs a ton, probably 150lbs for the three units plus another 20lbs or so for the tracking generator. I am recovering from a badly broken arm so I had to get my kids to stack it up for me. I have $1,515 in it and I doubt it could be beat within its frequency range. Mine was a late model and the center freq can go to like 1.8G plus it still had a wide bandwidth at that CF. Yes, it's a little slow when the freq. counter is on and yes, it makes a ton of noise, but it is my favorite piece of retro gear.While you are taking a look at the blocks, etc, try doing a tracking scan of a decent scope probe sometime. That was an eye opener. I don't know if it is a valid test, haven't thought about it much, but I took a couple of decent Tektronix probes, hooked them to the analyzer and touched them with ground to the TG. Not nearly as flat as I expected! Sort of like Dolly Parton flat.btw: it is amazingly easy to use from the front panel without sacrificing functionality. I'll bet I can do a measurement nearly twice as fast as my Advantest.

author: cncjerry
author_rank: Unknown
timestamp: December 08, 2015, 06:28:20 am
content: Quote from: TheSteve on December 07, 2015, 07:32:33 amanybody have experience with this one?

author: TheSteve
author_rank: Unknown
timestamp: December 08, 2015, 06:33:45 am
content: Quote from: cncjerry on December 08, 2015, 06:17:02 amWould this really be a valid test due to impedance mismatch? For fun I connected a Rigol probe to my network analyzer and it was actually impressively flat across its rated bandwidth.

author: sorenkir
author_rank: Regular Contributor
timestamp: December 08, 2015, 12:42:12 pm
content: I have this one:http://www.minicircuits.com/pdfs/BLK-6+.pdfI think they go for less than $30, deliveredMichel.

author: MadTux
author_rank: Frequent Contributor
timestamp: December 08, 2015, 05:52:51 pm
content: I grabbed on of these $300 units.Should arrive here in central europe in a few days/week or so. Hopefully, the former owners were carefully with it, but so far those parts units from that guy were quite nice and mostly complete, so I went and grabbed it.That 85662A/8568B/85685A combo should be quite nice. My 8568B (late 1989/1990 unit with grey HP logo, old one is blue) already goes down to -140dBm @ 10Hz RBW. 85685A gives an additional 10-15 dB, so I hopefully get close to -150-155dBm, which is around the noise floor of new Keysight 200k+$ units. Maybe I get a 8444A tracking gen for it some day.BTW, do I need any additional cables for the 85685A? I already have that 8568 to 85662A cable with SMA output. Anything else needed, except the AC coupled 85685 to 8568B cable?Concerning probes, I recently added a 4pin LEMO 0S connector to my 8568B to use cheap Tek 6201/6202 probes. Bought these for as low as 40$ for my Tek 7000 scopes. These probes use discrete FET transistors in the tip, unlike newer and much more expensive (but higher bandwidth) Agilent ones, which mostly use crazy GaAs ASICs. Thereby these are not user repairable and probably more to sensitive to electrical/mechanical abuse than old P6201/6202. Since I rarely see high impedance signals above 500MHz on PCBs, I consider the official 500MHz rating of P6202A as sufficient for most of my applications.Funny thing is the difference in bandwidth between the old P6202A from 1981 and the newer one from 1990. 1981 one has -3dB point at around 600MHz, 1990 one at 800MHz. However I don't know whether that is caused by change in hardware or by someone who "recalibrated" the P6202A. Frequency response graph in the pics. Put the probes directly into the hole of female BNC connector of a T-BNC adapter with 50-Ohm terminator on other end. Trace on top is BNC (without terminator) connected directly to 8568 and with +20dB on 8662A to compensate for the 10x voltage divider (100x power divider) on the tip of P6202A.

author: cncjerry
author_rank: Unknown
timestamp: December 08, 2015, 11:00:53 pm
content: MadTux,You need:1) the power cord;2) SMA to BNC with a 90 degree end on the SMA for h-sweep;3) Type N jumper for the front;4) GPIB cable; and5) BNC to BNC that goes from the SA sweep output to the 85685 horizontal input.I haven't looked into why that last cable is needed when you have the h-sweep cable but it is in the instructions as well. I can kick myself because one of those custom Type N jumpers was on ebay for $39 a few weeks ago. Also, the 85685 talks to the SA thru the gpib so that is not optional.I bought an 8444a tracking generator for 165 with both the 500Mhz secondary oscillator and option 59. Works well and I see them for even less sometimes, sometimes more. My 8568B and 85685 have the chrome/black logos on them as well. I think you will get a kick out of the 85685. The SA is quite a piece of work by itself but with the preselector you really would need to spend a lot of 'new' money to get anywhere close. I get a chuckle out of the plots I see on the site from other SAs. But again, if you have to move it you need a chain hoist.Also, if you don't, get a cheap Agilent USB to gpib converter and use Ke5FX's code for plotter emulation and screen capture. Works well. I also have some labview code if you need it.I have to look into those 6201 and 6202 probes you have. They aren't BNC, correct? Not familiar with the 7K scope. I have an active probe, Tek 6205, and about a dozen of the Tek 613x probes. I have both a TDS784a and a 3054B, great scopes but getting a little old.

author: G0HZU
author_rank: Super Contributor
timestamp: December 08, 2015, 11:32:32 pm
content: If you shop around you can find some N type DC blocks that cover 100kHz to 18GHz. I bought mine a few years ago to fit to my HP8566B. Sadly, I can't remember what brand it is but there are bound to be several manufacturers of these things.See below for the insertion loss plot up to 8.5GHz. Above this I have to use a sig gen and power meter to measure the loss.But it seems to work fine right down to about 100kHz. It wasn't cheap to buy but I think it's worth buying a decent DC block for something like the HP8566B. This DC block stays on the analyser pretty much all of the time

author: cncjerry
author_rank: Unknown
timestamp: December 09, 2015, 12:07:06 am
content: G0hzu, yes, that is what I am looking for. Specifically, N-type on both ends, 10k preferred, 100k minimum, to 4Ghz or higher. I've been looking around and if you could find a sticker on yours, that looks ideal. I've seen some cheap ones and others that hit the mark with SMA but would prefer N for a couple of reasons. I'm looking for quality, buy once and all that.Mini circuits has a BLK-89+ that hits the freq's but it is SMA. Their others all start at 10M.

author: G0HZU
author_rank: Super Contributor
timestamp: December 09, 2015, 12:24:02 am
content: Sadly, it has no brand logo on it. All it says isDC Block 18GHzon the milled ring of the N type connector in very good quality lettering.I can take a photo of it if that helps but there is definitely no logo anywhere as I've even looked under a microscope. It looks and feels to have the quality of something from Suhner in terms of materials used and it is quite heavy. But there is no logo so I have no idea who made itI just measured it on the 8566B for low frequency rolloff and it is flat down to 100kHz and drops to -1dB at 50kHz and -3dB at 22.5kHz.See below for an image. If you can find out who makes it I think I would buy another one myself for one of my other analysers

author: MadTux
author_rank: Frequent Contributor
timestamp: December 09, 2015, 12:43:52 am
content: Thank you for cables listP6201/6202 are plain BNC, only a BNC-N adapter and power supply/8568 mod needed.Can you measure capacity of that DC-Block, G0HZU?Just did quick calculation and got a capacity of around 0.30uF

author: G0HZU
author_rank: Super Contributor
timestamp: December 09, 2015, 01:18:27 am
content: From memory, the DC block was advertised as being 0.047uF but I can't be sure.I measured it crudely on a VNA by shorting one end and it measured 0.04uF at 600kHz although the rolloff data from the 8566B tests suggests it should be closer to 0.068uF.I don't have a decent N type cal kit here but the return loss measures better than 30dB up to about 3GHz and about 25dB at 4GHz. It stays at 25dB all the way up to 8.5GHz but this is not a reliable measurement. Below about 500MHz the return loss is >>40dB.

author: G0HZU
author_rank: Super Contributor
timestamp: December 09, 2015, 01:36:46 am
content: Just to clarify things a bit, I remeasured it for low frequency rolloff using a 50R sig gen via a 10dB attenuator to improve the source impedance accuracy. For the receiver I used a Racal 9300 Trms meter and a decent 50R through termination.The -1dB point was about 77kHz and the -3dB point was about 40kHz and this agrees with the VNA measurement of about 0.04uF for the capacitance.I assume that the reason for the discrepancy with the HP8566B measurement is because the source impedance of the generator and the input impedance of the 8566B won't be a perfect 50R at these frequencies

author: cncjerry
author_rank: Unknown
timestamp: December 09, 2015, 01:39:29 am
content: I'll keep looking. The blocks I see that go to 18Ghz are either 10Mhz and up or unspecified. I guess I can just pickup a bunch of the cheap ones and scan them.Come to think about it, all there is in there is a capacitor, probably low ESR type, correct? I have a bunch of 10uf, sm X5R low ESR caps around here, some N connectors, a little lathe work and off I go, no?I have to scan midwest microwave to see what they have and a couple of other companies.Thanks

author: MadTux
author_rank: Frequent Contributor
timestamp: July 10, 2016, 09:06:36 pm
content: On request, the details for the Tek P6201/6202 mod:It's quite simple, you either get a OEM Tek P6201 PSU, sometimes supplied with the probes, or you buy a LEMO connector and wire it to the HP probe power connector. And an BNC to N adaper, of courseLemo connector is the ERA.0S.304, has 4 pins but only 3 are used by Tek P6202:http://www.ebay.de/itm/LEMO-Groesse-0S-Einbau-Buchse-4-polig-ERA-0S-304-/371115274874Was a bit cheaper when I bought it, you probably find cheaper ones in US. BTW, it's not the one found in TM500 temp probes, these have only 2 pins.HP-8568 provides +15V/-14V, but TekP6201 regulates that down to +/- 7V, so it doesn't matter that much.I mounted my connector with the white protruding part on top. Those 2 pins on top are GND.Bottom 2 pins are (from left to right) -14V and +15V directly wired from HP probe power connector.(viewed from connector/front panel front side)The voltages on HP probe connector are: GND on top, +15V on left bottom, -14V on right bottom, but best measure them again to be sure.You can correct the x10/-20dB factor from probe with SHIFT(on display unit)-CENTER FREQ(on RF unit) =>Power Offset +20dB. Or use tracking gen and the A-B mode on display unit (never used that one, so you have to try yourself), if you want really accurate power reading.BTW I really like the 8568/P6202 combo, especially with lower freq stuff up to ~250MHz. You can hook your SA to pretty much any signal without the risk of frying the 1th mixer. The P6202 is quite robust and doesn't load the signal as much as a x10 passive probe (at least on most low frequency signals you find when fixing other test gear)Some pics of my notes and the LEMO connector: