Classic electronics! Testing a Collins 455 kHz, 500 Hz bandwidth CW filter!

Описание: Hi Folks!

Today I'm going to show you how to do a quick test on a Collins filter for functionality and bandwidth.
Collins filters are highly valued for their ability to pass signals within their bandwidth, and reject unwanted signals at ridiculously close frequencies that simply swamp standard L/C filters.
Typical bandwidths are 250 or 500 Hz for CW (Morse code), 1.8-3 kHz for SSB, and 6 kHz for AM.
Collins stopped making these filters several years ago, so they are getting scarcer every day. The filters are complex and expensive to produce.
Frankly, as excellent as the Collins filters are, with SDR's such as my Icom 7300 I can get variable bandwidth performance down to 50 Hz! I have used it many times to separate same channel VLF NDB CW ID's that drifted maybe 30 Hz off center frequency, so I could tune into the low side of one, copy it, then move to the high side of the slightly higher one and copy it, too!
The star of today's presentation is an Inrad (International Radio Co.) branded #705 filter which has a label placed over a Collins 455 kHz 500 Hz BW (Band Width) filter. This is an excellent filter for CW work!
Supporting it are a HP8656 signal generator, and an old school Tektronix model T922 2 channel 15 MHz CRT oscilloscope.
The procedure is fairly simple: Connect up a signal generator to the filter input, and an o-scope to the output. The testing is done to find the center frequency (very close to 455 kHz. In this case, 30 Hz low at 454.970 kHz). I then increase the generator output to a point where signal strength is enough to show up solid on the scope, but not enough to cause odd effects.
In this case, full scale was about -44 dBm. The cool thing is, that the -6dB end test point is 1/2 the full scale voltage, so when you see that half scale (-50 dBm), you are there! This is real easy to set up!
The filter has multiple poles, so you'll see the amplitude drop off, come back, and drop off for good. Bottom line: The top end 6 dB point was 455.300 kHz, so that's 300 Hz response above 455 kHz.
On the low side, the bottom 6 dB point was 454.770 kHz, about 230 HZ response below 455 kHz. so, 300 + 230 = 530 Hz. Close enough to 500 Hz!
Thanks for watching!

-Tom