VNA and Phase Angle in Low Pass Filters in Collins KWM-380 - Page 1
Forum section: electronics / rf-microwave

author: NY2KW
author_rank: Regular Contributor
timestamp: September 13, 2019, 09:05:14 pm
content: Apologies in advance for a long story.I am working on a lowpass filter board for my KWM-380 Collins transceiver. One of the relay pairs gave out and I replaced it plus the bad MPSA14 Darlington that switched it. As I worked on this board (3 part board - 2 filter boards jumpered together plus a directional coupler attached on the back) required 4 hands at times. A few times I dropped it on the workbench, so I was worried that some of the large air inductors may have been spread or compressed. Collins service manual shows that testing each filter (terminated into 50 ohm load) with a HP 4815a vector impedance meter - should show a phase angle no more than +/- 10 degrees at specified frequencies below the cutoff. Now to me that means the filter does not cause any significant capacitive or inductive reactance at the test frequencies for each filter. I first checked all the filters with my spectrum analyzer/TG and there is virtually no insertion loss and excellent 40 to 50 dB cutoff just about the specified ranges so I was done BUT I really wanted to use my new SDR-Kits VNA3E to see how phase angle looked for each of the 9 LPF's. The VNA showed SWR near 1.1 and Z across the LPF bandwidth was always very close to 50 ohms with small imaginary reactance values but the the phase angles across the LPF range was all over the place and at the manual's specified test frequency (usually middle of filter) the angle was never remotely close to +/- 10 degrees -- most being somewhere between 70-160 degrees. I am new to using the VNA software and calibrations so maybe I overlooked something. I looked for a crude method so I borrowed a RigExpert 230Pro (measures Z and all parts with sign) and found same phase angles all over but never anywhere close to +/-10 degrees. It got me thinking what did phase angle mean in 19070-80s? I assumed that it means the impedance of the PA final output was very closely matched to each LPFs and at a specific test frequency, PA signal did not undergo a significant capacitive or inductive I-V shift as it went thru the LPF to antenna. I know... some of you will say if it works then why sweat it but I am trying to learn from the situation. Any advice appreciatedI recently got a SDR-Kits VNA to work withealysthattransmission theory and impedances

author: Bud
author_rank: Super Contributor
timestamp: September 13, 2019, 09:39:48 pm
content: It is probably +/-10 degrees of phase deviation from a reference value, not absolute value. E.g. in the bandpasscenter it may be 150 degrees absolute and 140 to 160 at the edges.Also you must observe the calibration plane. Any extension cables added between the VNA calibration plane and the device being measured will introduce phase shift.

author: NY2KW
author_rank: Regular Contributor
timestamp: September 14, 2019, 01:13:35 am
content: The Collins Service manual is comprehensive but no mention of a reference value. I am not familiar with the HP 8415A, but reading the manual it seems like the probe works by "a small test signal is applied to the test circuit through a probe. The RF current through and voltage across the test circuit are converted to two 5 kHz IF signals that retain both amplitude and phase information." I guess without a similar probe any VNA using even short coax will lead to erroneous phase measurement. I am embarrassed to admit that when it comes to impedance and filter circuits I am in deep water. Would I be correct in assuming that between my spectrum analyzer and one-port measurements that the LPF must be in good shape if (1) SWR across LPF upper and lower frequency range is <1.1, (2) the insertion loss across each LPF frequency range is less than 0.5 dB and (3) the cutoff of 40-50 dB is correct right and above the frequency where the desired minimal -20 dB cutoff is expected?

author: Bud
author_rank: Super Contributor
timestamp: September 14, 2019, 02:31:42 am
content: It is all looking good to me.