[Elecraft] Sall bandwidth of the OP1 SSB filter

[Roland CORDESSES]

My K2 #2379 is now completed as well as the KSB2 option board. Thanks to
the fine construction
notice (Heathkit style) and the numerous informations available on the
Elecraft WEB site, I had no
troubles to adjust it and to properly tune the BFO frequencies.

While I still have not made any QSO, I have carried out many
measurements which show the performances
of my K2 (TX and RX) are always as good and often better that the
Elecraft published data.
As a designer and builder of many homebrew high performance ham
receivers, I have been specially
impressed by the measured IP3 and dynamic range reached in such a small
package.

I am now concerned with a problem related to the IF bandwidth of the SSB
filter used in the KSB2.
This BW(-6dB), measured with Spectrogram or in the transmit mode, is
only about 1.75kHz, a small value
compared to the 2.2kHz published BW. In order to confirm my home
measurements I conducted
some tests on the filter alone with an HP Vector Network Analyzer and
obtained the same results.

Before building the KSB2, I measured the crystals parameters and
obtained the following
data (they are the averaged values of the 7 crystals but the dispersion
is rather small):
Series resonance frequency : Fs=4.913866 MHz
Motional inductance # 80.8 mH
Motional capacitance # 0.013 pF
Unloaded Q : Qu # 130000
Parallel capacity : Cp # 4 pF

In the past few years I have build many ladder crystal filters (5 to 12
poles CW, SSB and noise
filter) using the tools and methods provided by Wes Hayward W7ZOI and
others. Thus I simulated the
KSB2 crystal filter using these tools and the ANSOFT Harmonica software
(the "big brother" of the ARRL
Radio Designer program) : I obtained a 1.8kHz BW with the above crystal
parameters.

I then computed the capacitors needed for a 0.5dB 2kHz Chebycheff ladder
filter (1500 ohms I/O
impedances)and get values different from those of the KSB2: moreover, as
the 7 crystal frequencies
have very small dispersion (delta F max=10Hz), I find that I need more
serial capacitors in the meshes than the
KSB2 PCB can accept.
I am now thinking about building a piggyback PCB for an eventual new
filter but before, I would be
glad to receive comments about my problem. A feedback from the Elecraft
K2 designers would be particularly
appreciated.

73's

Roland Cordesses/ F2DC
K2 # 2379