Cwmbran and District Amateur Radio Society
Xtal Ladder Filters (I)
I have been playing with filters. From grubby piece of paper with some workings, I transferred it into hardware. The subject is a 4 pole Cohn minimum loss ladder filter, calculated for 4 MHz, as printed on the Xtal cases .
For a first hack I used NPO Ceramics instead of trashing expansive Silver Micas! Leads were left long and, poor practice, the output has direct sight of the input. The values were rounded to the nearest standard value - not far but enough to make a difference. The Xtal cases are grounded, but, did I mention the lack of screening..... and compact it is not; but hey!
So, the response... Well, the slight difference in caps from theoretic shows in the Himalayan effect in the nose! These filters are always LSB by definition, and you can see the shape of the skirts. Approx 1.79 KHz wide at the -3dBm point and about 6.7 KHz at the -60 dBm point:
The in use case. Just to remind ourselves. AM at 4 MHz, 15 % modulation depth at 2 KHz. The sideband are peaks because it is a single freq tone injected, not the square block audio shown in books...
But sticking a Double Balanced Mixer in, we greatly reduce the carrier. Remembering that TRUE suppressed carrier is fiendishly difficult to organise! 4 MHz squirted in from the right and 2 KHz in from the centre. Output on the left:
DBM Response... LSB + USB and the carrier way down:
Now, the filter inserted after the DBM:
And the carrier adjusted up to 4001.5 KHz to the LSB insertion freq:
And down to 3998.5 KHz for the USB insertion freq:
For a first hack, using matched crystals from a slack handful of 99p cheapo computer Xtals, a useful SSB filter has emerged. With more care and a 6 node job, the effort should yield dividends. More investigation will follow!
Thanks to GW0OAJ for this article