Fitting a Collins Mechanical Filter.

[BillC]

I am experimenting with fitting a 6 kHZ Collins Filter to the 455kHZ stage of my home built Wadley Loop receiver. The filter is fitted in between two back to back IF. transformers after the mixer. I have employed a bit of estimation to try to get a good impedance match between the transformers and the impedance of the mechanical filter. The filters own resonance is not quite centered on 455kHZ, so the IF. strip is adjusted to the filters frequency. The response and shape of the trace looks good. The receiver works well.

 

[VK2RK]

I found this document that could be helpful to you.
Bit concerned that the center frequency is not 455 KHz, wonder if this is due to a loading factor or even temperature.

Note the general specifications on the linked article.

http://bitsavers.trailing-edge.com/communications/collins/brochures/Collins_Mechanical_Filter_Brochure.pdf

Good job Bill.

 

[BillC]

Hi Rob. Thanks for your interest. I have always liked the idea of mechanical filters, but could never afford a new one when they being made. Fortunately due to the generosity of others I have been able to experiment with them in recent times. I am not too worried about the center frequency of the filter, it is a simple matter to adjust to it. One thing for sure is that no external adjustments will alter the resonant frequency of the filter discs. I might have been sweeping a bit too fast, 5oo milli seconds would probably tell a better story Also the filter might have drifted with age. Keeping the connecting leads short helps to prevent bypassing of the filter and keeps the skirts nice and steep, I found out!! Cheers.

 

[BillC]

More on the mechanical filter. I slowed the sweep rate down to 3 seconds on this screen and made some very careful adjustments to the IF. transformer tuning capacitors. The result is a much better picture of what is going on, with eight tuned circuits and all the resonating discs in the Collins filter there would be a considerable propagation delay through the IF. stage with an end to end sweep such as I did. So slowing the sweep down allows time for the circuits to charge and discharge.

The sweep was fed into the mixer grid and output taken at the last IF transformer winding. We are so fortunate these days to have these test instruments with storage screens which hold the image until refreshed at some rate of our choosing,,, not like the old days with phosphor screens.