EMRFD Message Archive 11943

Message Date From Subject
11943 2015-12-05 14:17:15 Jim Leslie Filter alignment
Many years ago, I used a signal generator and scope to align an IF stage. I'm looking for a much better way to do
that now and trying to figure out the best approach. In doing a lot of reading, it seems to me there are a few ways
to accomplish this. One way I saw described was using a sweep generator, syncing to the start of trace on a scope
and varying the scope trace such that a start/end frequency occupied the screen. Then the screen center would
represent the center frequency (of a bandpass filter under test for example.)
The other two methods were somewhat similar:
In one, a scalar network analyser was used and in the other, a spectrum analyser with tracking generator.
I like either of these methods as it allows a very graphical view on the screen.
Tuning and measurement is more refined as well. So the question is what methods have you used to align
low/high/bandpass and crystal filters? I think I have a good appreciation for the other uses of both
spectrum analysers w/TG, and scalar/vector analysers. But I'm wondering what the first tool/method you would
use for this task. Currently my bench has an 8640B, RMS meter and scope.
11944 2015-12-05 14:30:17 Phil Sittner Re: Filter alignment

My preference is the VNA. I also have a spectrum analyzer with tracking generator but find real value in using the VNA, which also graphs the impedance and phase shift, along with the amplitude variations. There are a number of VNA's currently on the market but if you choose that direction be sure to buy the two-port variety. You also wish to make sure you cover the frequencies you're likely to encounter. I'm very pleased with the DG8SAQ product. Keep up the experiments.


11945 2015-12-05 14:41:12 dave081360 Re: Filter alignment
Hi Jim,

Here's what you're looking for:

 Midnight SNA


Dave Collins - AD7JT

11946 2015-12-05 14:49:41 Tayloe, Dan (Noki... Re: Filter alignment
Once upon a time, I built a fixed frequency VFO
11947 2015-12-05 15:05:34 DuWayne Schmidlko... Re: Filter alignment
I use either my homebrew SNA or an inexpensive Chinese SNA I purchased
on eBay.

The homebrew has its own display, so it is what I usually use for
alignment on the bench. The eBay model needs to connect to a
pc but has several additional features that make it a better option at

11948 2015-12-05 18:53:15 farhanbox@gmail.c... Re: Filter alignment

every method has its limitation.

using a signal generator as a signal source brings all the harmonics and spurs into the plot. for instance an Si570 as a signal generator used to measure a 30 mhz band pass filter will show a phantom response at 10 mhz. that is so because the 3rd harmonic of 10 khz at 30mhz is passing through.

without an rf detector of at least 80 db range, the sweeps will be of little use in measuring the stop band attenuation.

a quick and not so dirty solution is to use a clean, wide range manually tuned oscillator as the signal generator, a home built step attenuator and general coverage superhet receiver. the sdrs would have harmonic responses. 

in such a setup, the step attenuator provides fair resolution of signal's strength if you measure the signal by how much attenaution it takes to push the receiver output back to exactly the same level. you will want to switch off the agc.

- f

11949 2015-12-06 10:41:50 Jim Leslie Re: Filter alignment
Thanks to everyone for your input. While I save for additional equipment I'll go ahead with the below method. My detector
has a range of -70 to 50db & use that along with my 8640B first.
73 jim