| |
K0CQ > QRP 23.10.02 03:31l 39 Lines 1880 Bytes #999 (0) @ WW
BID : 14245_W0AK
Read: GUEST
Subj: SSB & CW xtal ladder filters in use
Path: DB0ZKA<DB0GPP<DB0LX<DB0RBS<DB0SEL<DB0WTS<DB0ACH<ON0RAT<ON0LVN<ON0AR<
ON0AR<7M3TJZ<SP7MGD<WB0TAX<N4ZKF<N0IA<N0IA<N7QDN<N7QDN<KA7AGH<W0RLI<
<W0AK
Sent: 021022/1541Z @:W0AK.#CIA.IA.USA.NOAM #:14245 [Des Moines] $:14245_W0AK
From: K0CQ@W0AK.#CIA.IA.USA.NOAM
To : QRP@WW
Ladder filters have been parts of QRP radios for a long time. The Elekraft K2
uses 4.9152 to achieve both SSB and CW bandwidths. Many receiver projects in
the ARRL Handbooks use ladder filters with crystal frequencies between 3.579
and 12 MHz. Its a matter of selecting the capacitors between the crystals to
set the bandwidth. The response curve is reasonable with equal value
capacitors, but better response curve designs are possible. The classic book
on filter synthesis by Zverev has details of the design process. Articles in
QST, QEX, and ham radio magazines have shown some of those details. Surely
RSGB publications have covered the topic some.
The 9 and 6.3 MHz filters from Tentec used in several generations of radios at
least through their Omni VI are ladder filters. Bandwidths are 250 Hz to 2.8
KHz.
Its harder in ladder filters to get truly sharp bandpass shoulders. In my
opinion this is a benefit because those short shoulders lead to filter ringing
that is a fault of mechanical and crystal lattice filters not shared by ladder
filters. I built a ladder filter for adding selectivity to a panadapter first
using equal value capacitors and while it worked when I used a sweep generator
to check it out I had to sweep slowly to keep it from ringing. When I
redesigned it to have a Bessel bandpass shape I could raise the sweep speed by
a factor of ten without it ringing.
This quality of less ringing allows receiver using such filters to receive
through thunderstorm static and power line noise where other receivers using
mechanical or crystal lattice filters can't hear for the ringing from the
static or noise.
73, Jerry, K0CQ @ W0AK.#CIA.IA.USA
Read previous mail | Read next mail
| |
