Noel, again you pointed out something of real value. When I first got the KC901V I tested a few filters that I had built for Ghz stuff, and I did the same thing, but I tried 3Db and then 6Db on each port hoping to reduce the S21 ripple but it didn't make a big difference. Later I tried one 10 Db pad on port 1 and then moved it to port 2. Again the ripple stayed about the same so I hope the ripple can eventually be fixed with a firmware update. Based on your comment, I will use at least 3 Db on each port to improve the match for S21 or would it be better to use 6 Db on just one port? Thank you again for some great comments. John
It appears from the specs that ports 1 and 2 of the KC901V vary from 50 ohms as indicated by the typical mismatch figures of 2 for VSWR up to 3GHz stated in the KC901V spec sheet with nothing stated for frequencies above that.
The effects are born out in trying to measure cavity filters directly with the stand alone KC901V. To overcome the mismatch effects I put 3 dB attenuators on both ports to normalise the impedance that the cavity sees without sacrificing too much dynamic range. At 400MHz the signal isolation between ports 1
Great information. I have also noticed the high ripple in S21 mode. To be honest, I was tempted to return the unit because it looks so bad, but the other features work so well I will I decided to keep it. It is good to know the fm modulation is spot on even to the point of agreeing with the Bessel null. You have done a nice job finding and explaining problems. I will try to add something useful as well, but for now I'm still learning VNA techniques. I have two HP 8569B spectrum analyzers, but I finally decided I need the phase information so short of buying another old 50 pound piece of equipment for several K, I decided on the KC901V. I am pleased with it, but as you pointed out, there are some flaws that may not be fixed through firmware. Again, Best Regards, John PS: Thanks for explaining how the comments work.
Whatever the most recent comment is on comes to the top of the list. Maximo and I in the past have made comments about the a suitable case protect and carry the VNA and also how to support it in a bench use situation. There has been a lot of other comment and input over time suggesting improvements and pointing out software issues as people use the VNA and see any limitation.
I am still waiting for answers on a number of my posts.
1/ In AF or RF signal mode I suggest that the amplitude figure should be pulsed when the output is not on. This has caught me out a number of times because you have to realize that the stop square is shown rather than the arrow on symbol.
2/ The S21 function seems to have a lot of ripple when used on higher resolution (1dB steps) and I am not sure if this is now a hardware or software limitation.
3/ I noted output level inaccuracies in the RF signal generator levels below 60MHz. This has been fixed however there still may be an issue when you go to adjust the level. Mine seems to jump by about 5dB then return to the correct level when I adjust frequency.
Something for you to check.
A great thing the KC901V does is give FM modulation on the AF generator and also have that output directed to the BNC AF output (1kohm in Volts) or to Ports 1 or 2 (dBm into 50 ohm) to an provide RF. However because you are working with harmonic/mixer products, you must be careful to not overload a receiver or spectrum analyser such as the 141T with the low frequency high level signals also at the output.
The FM generated mathematically by this unit is perfect and can be used to calibrate older equipment such as test sets and modulation meters. You can verify this by setting the modulation frequency to 1kHz and the FM deviation to 5.52 kHz whilst observing the output with you spectrum analyser close in with minimum resolution bandwidth and you will see the carrier is exactly at the second Bessel null point.
Just last week I used it as a transfer source to check a frequency calibration outside my screened room from a GPS locked source with a spectrum analyser in my screened room.
As the KC901V can set frequency down to 0.1Hz steps it was easy to do the transfer (accurate to 1 in 10 to the 9th plus) at 2GHz by beating against the reference source then observing it on the spectrum analyser.
By the way this all came about because the GPS source I had borrowed was stable but in a fault condition. Using the transfer I could verify that it was actually faulty, against my rubidium standard. It was a pain because I had just adjusted some reference oscillators using the GPS standard.